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Normalization of Deviance

posted 13 Mar 2014, 10:39 by Support SarMobile   [ updated 8 Apr 2014, 04:20 ]

In the last two safety articles we talked about the deficiencies of technical search methods Wing Blanking and Ad hoc Direction Finding Antennas used by air search and rescue units in several locations, but specifically as used by the Civil Air Search and Rescue Association. In this article we will dive deeper into how deficient methods may come to be used, and illustrate the process by examining an attempt to develop and propagate a new technical method by several members of a sub-unit of CASARA (which in their lexicon is known as a Zone). We will examine a paper1 written to document the technique. If needed, a copy of this paper should be available through CASARA; if not contact us at the email address provided at the end and we will try to assist. We will conclude by drawing a similarity the use of these techniques in search and Normalization of Deviance as described by Dr Diane Vaughan.

It is not our intention to embarrass or belittle anyone. In fact this article has been delayed to give those zone members and CASARA leadership an opportunity to communicate any changes of opinion, operations or outlook they may have had. Our attempts to reach out to the members and CASARA were, for the most part, ignored. When not ignored, those responding made no effort to distance themselves or the organization from the paper, nor to enumerate any difficulties they had with the content of the paper. Quite the contrary, in fact.

There is a good amount of material in the paper, we will not be addressing it all. There is a full technical critique of the Cardinal Pass technique that was written by a former member of the same CASARA Zone2. We can provide electronic copies of that paper on request. We will be closely examining four topics discussed in Accuracy of ELT Searches1:
  1. The relationship between the technical knowledge and qualification of the people involved in the development of the Cardinal Pass.
  2. The veracity of the content of the paper and the effect it may have on public confidence in CASARA.
  3. The basic premise of the Cardinal Pass (and the “Traditional Aural Null” presented as a foil for the Cardinal Pass).
  4. What science and engineering tells us about this premise, and some specifics of how a true traditional aural null works.
For the last point we would like the reader to be familiar with our Practical Guide to Aural Searches. You may wish to read that now, if you have not already done so. As an extra benefit we will also be able to show, by using the failures contained in Accuracy of ELT Searches1, to help convince the reader they should be describing Aural Null techniques in terms of the Radio Horizon, not equal signal strength, and using Procedure C instead of Procedure A or B.


The academic qualifications of the people involved are, in general, quite good. One holds a PHD in Physical Oceanography, an other works for the Advanced Cognitive Engineering Laboratory at Carleton University. One might be forgiven for assuming that these two at least would be sensitive to the need for familiarity with the science and engineering involved. And yet, early in the paper they make this statement:

The overall topics of wave propagation and the mathematics of the various aspects of finding maximums and minimums are the subject of a vast literature in the fields of mathematics, physics, and engineering. Nevertheless, in the case of CASARA actually looking for an ELT we have little need to understand either advanced mathematics or complex physics.1

We find it puzzling that they would on one hand acknowledge the vast amount of prior art, but on the other hand dismiss the need to understand. We can accept that for the average CASARA member, following a clear set of instructions, a recipe of sorts, would not need to understand all the science involved. But if CASARA is to design the procedures, and compile the recipe, then someone in CASARA would need to have a comprehensive understanding of the science involved. They attempt to justify this position by continuing: “... we have a small number of transmitters (usually one) and reflectors to deal with.”1 Immediately their lack of knowledge and experience in the field is exposed. First: since the purpose of the method is to find a transmitter with an unknown location, the number of reflectors can not be known until the transmitter is located. Second: even a small number of reflectors can seriously impair the technique. On the other hand it is possible that by choosing the location for the transmitter location they may have improved the performance of the technique by avoiding conditions in which the technique fails. Of course staging a test like this does not prevent failure conditions from occurring during an actual emergency.


The success of scientific discourse as well as search and rescue depend to a large extent in our ability to trust the statements made by scientists and searchers alike. When that trust is betrayed in one part, then trust fails in all parts. Someone who would intentionally misstate facts on one occasion can reasonably be expected to misstate facts on another occasion. This is why it is unusual to report that data has been gathered under one set of circumstances when it was gathered under a different set of circumstances; even if the circumstances are thought to be equivalent.

During the remainder of this article we will be concentrating on the report of an experimental flight documented in Accuracy of ELT Searches1. There may have been several purposes for this flight, but within the context of the paper it appears to have been to collect data on the performance of the Cardinal Pass and compare it to the performance of a different technique. In short the procedure was to place a radio transmitter in a known location, then use each technique to estimate that location. Each estimate was then compared to the known location giving an error distance used as the performance metric:

A practice ELT was set out in a clear field just to the West of Wakefield, Quebec approximately 16 nm NW of Rockcliffe Airport (CYOW). The weather was clear and bright. The position of the ELT was determined by GPS while setting it out to within a couple of metres and the location was chosen so that the radiation pattern of the ELT would not be distorted from circular by any reflectors or obstructions.1 [emphasis added]

Quite straightforward. However, the ELT location was not determined by GPS. An attempt was made to gather GPS coordinates using an iPhone application after the flight when the ELT was retrieved. This attempt failed to produce a usable location. The navigator of the flight resorted to using Google Earth to produce the known coordinates of ELT. These facts were reported by the navigator (Muir L.R.) to the Zone Training Officer shortly after the flight. They were later confirmed by the spotter on the flight (Casey M.).

Perhaps Dr Muir believed that Google Earth was capable of providing a position accurate to within a couple of meters. This is not the case3. Even if it was, we believe reporting a position derived from Google Earth as being derived from GPS is ethically very questionable. For us, the decision to misstate the source of the known location coordinates cast a pall over other decisions the authors made. At times we were torn between believing the authors made a choice because they lacked understanding of the mathematics and physics, and believing they made the decision to intentionally improve their results. It is very troubling to have these thoughts about an organization that one might have to rely on for rescue. The following pictures show image to image and image to GIS data registration errors in the search area from the Google database as it existed in December 2013.

Figure 1: Road GIS data (yellow lines) relative to imagery.

Figure 2: ELT location. Note imagery registration error.


Accuracy of ELT Searches1 describes two airborne electronic search techniques, the Cardinal Pass and what the authors call a Traditional Aural Null. These techniques are in fact almost identical. They both attempt to use received audio volume to measure relative distance to the transmitting ELT. In both cases the receiver frequency is tuned away from the transmitter frequency. This is where the Accuracy of ELT Searches1 so-called Traditional Aural Null differs from what is the canonical Aural Null. The true Aural Null is conducted with the receiver tuned to the actual transmitter frequency. To differentiate these techniques we will call the Aural Null from Accuracy of ELT Searches1 an Off-Tuned Aural Null, and the true traditional Aural Null, an Aural Null.

The Cardinal Pass is based on listening to the received audio volume while the aircraft transits the the search area on one of the four cardinal compass directions North, South, East or West. According to the proposed theory the loudest volume will be heard when the airplane is at its closest point of approach to, and therefore abeam, the transmitter. The navigator then records the Latitude or Longitude as appropriate for the direction of flight. If the aircraft is heading North or South the Latitude is recorded, if the aircraft is heading East or West the longitude is recorded. The Latitude from a North-South pass is combined with the Longitude from an East-West pass to determine the location of the ELT.

The Off-Tuned Aural Null is performed by listening to the received audio volume while the aircraft transits the search area. The receiver frequency is off-tuned from the transmit frequency. When the volume is at some fixed level, often called the minimum received signal strength, the location of the aircraft is recorded. In theory each location where the same volume level is observed, at the same frequency offset is at the same distance from the transmitter. This would put the points on the circumcircle of a simple, convex, cyclic polygon and the circumcentre would be the location of the ELT. Perhaps this is why an effort was made to choose a location “so that the radiation pattern of the ELT would not be distorted from circular by any reflectors or obstructions.1 Unfortunately an airplane crash will not always be able to replicate these desired conditions.

These theories may sound plausible, but are they? Certainly radio signal strength at a receiver is dependent on the distance from the transmitter. Logically the signal strength should have an effect on the volume of the radio audio output. The problem is that distance is not the only factor that affects received signal strength, and received signal strength is not the only factor that affects the audio volume.

Knowing all the Factors

Since we are concerned with measuring distance by using radio waves let’s look at some examples. The simplest way of measuring distance is with a ruler or tape measure. By laying something of known length between the transmitter and receiver we can determine the distance. With a tape measure the distance is a function of how much tape is required to reach between the two antennas. We can simply read this value off the tape. This is impractical for long distances so we might use a surveyor’s wheel. Distance is measured by counting the rotations of the wheel and multiplying by the circumference. This is not as direct as measuring with a tape measure. If the wheel slips during measurement, or is rolled along a curved path, the measure will be in accurate. It will also be inaccurate if the wheel has worn and no longer has the circumference we expect.

Using radio waves there are two well known methods of measuring distance: RADAR and GPS. Pilots may be familiar with a third: Distance Measuring Equipment (DME). RADAR and DME actually function in a very similar way. Primary surveillance RADAR works as we have seen in war movies. A large radio transmitter sends out a pulse of energy that is reflected back off the target. The time the pulse takes to return is measured. By multiplying the speed of the RADAR signal in the atmosphere by one half of the time we can compute the distance. Secondary surveillance RADAR and DME are very similar, except that they don’t depend on the signal bouncing off the target. Instead an airplane will carry a transponder that listens for a signal from the secondary RADAR and replies with a signal of its own. With DME the aircraft has the equipment that sends out the initial signal to which the DME station replies. Again the round trip time can be used to compute distance. A GPS satellite sends out a repeating, but very long signal that is synchronized to its internal atomic clock. Just as we can listen to the lyrics of a song “The Star Spangled Banner” and know if we are listening to the start, middle, or end, the GPS receiver can listen to the signal and know what time the satellite clock read when that signal was sent out. The difference between the clock time taken from the signal and the correct time, multiplied by the speed of the signal is used to calculate the distance from the satellite to the GPS receiver. Several such distances may be used to compute a position.

Unlike direct measurement methods, like the tape measure, indirect methods (the surveyor’s wheel, RADAR, DME, GPS) the primary measurement (number of rotations, or time) is determined by more than just the distance. Usually these factors are small (wear of the wheel, changes in the speed of radio signals in the atmosphere, measurement paths that are curved) and may be correctable if enough information is known. Before these techniques were used to measure distances, these other factors had to be examined, understood and shown to be insignificant, or methods developed to deal with them.

The Cardinal Pass, and Off-Tuned Aural Null use received volume to measure distance or relative distance. In 1945 a Danish-American radio engineer named Harald T. Friis4 working at Bell Labs derived an equation used in telecommunications engineering that computes the power received by an antenna under idealized conditions given another antenna some distance away transmitting a known amount of power. The equation in the form appropriate when the receiving antenna is on a moving airplane specifies the following parameters:
  • the power output of the transmitting antenna;
  • the inverse of the square of the distance from the transmitter to the receiver;
  • the gain of the transmitting antenna in the direction of the receiving antenna;
  • the gain of the receiving antenna in the direction of the transmitting antenna;
  • the polarization vector of the transmit antenna;
  • the polarization vector of the receive antenna;
  • the equation does not account for multipath and other fading effects. 
Some of these values can be assumed to be constant. The power output of an ELT will be relatively constant over the period of a typical search unless the battery is in bad condition. The polarization vector of the ELT will also likely be constant, though there are stories of survivors manipulating the ELT orientation as search aircraft were visible to them. The polarization vector of the receive antenna can be held constant by only measuring volume when the search aircraft is flying straight and level. The remaining factors affecting received signal strength are:
  • the inverse of the square of the distance;
  • the gain of the transmitting antenna in the direction of the receiving antenna;
  • the gain of the receiving antenna in the direction of the transmitting antenna;
  • multipath and other fading effects.
We want the first factor to be able to detect closest approach during the Cardinal Pass, or equal distance during the Off-Tuned Aural Null. However if any of the other three factors, individually or combined, alter the signal strength significantly, and they certainly can, then the volume heard from the radio won’t indicate accurate distance.

There are some other factors introduced at the receiver and its operation. The Friis equation is only concerned up to the receiver antenna. These factors are:
  • The receiver frequency offset from the transmitted frequency. Larger offsets reduce the strength of the signal processed by the receiver.
  • The receiver automatic gain control, which tries to keep the signal being processed by the receiver at a constant level.
  • The volume control.
It is easy to see that when the antenna gain and fading factors are small these techniques will work. In the case of an actual crash or other emergency it is quite possible that the transmitter gain pattern and fading factors could be large resulting in these techniques failing. It can be extremely difficult for a pilot to arrange for a crash that results in a symmetric ELT transmission pattern, and is in an area with few reflectors.This knowledge comes along with an understanding of the appropriate mathematics and physics which the authors of Accuracy of ELT Searches1 decided they did not need to know. Again we wonder why the authors spent so much effort to ensure a circular pattern and limit the number of reflectors when searchers would not be able to do the same in a real emergency. The paper1 does not provide any quantification of these extra factors, nor does it provide any way to deal with them.

Consider the true Aural Null for a moment. If you have read our Practical Guide to Aural Searches, you know that an Aural Null is not performed by looking for locations with a particular signal strength, but by looking for locations on the radio horizon of the search aircraft from the transmitter. An ELT, even on 121.5 MHz has plenty of transmit power to reach LEOSAR (Low Earth Orbit Search and Rescue satellite) platforms in orbit, even if they are no longer listening. Reaching the radio horizon a few dozen nautical miles away is not a problem. Because we are using the radio horizon we won’t use the Friis equation but the line of sight propagation equation. This is much simpler having the following parameters:
  • the radius of the earth;
  • the refraction index of the atmosphere for the signal5;
  • the altitude of the search aircraft above the transmitter (we assume the transmitter is on or very near the surface). 
The radius of the earth is, of course, constant. The search aircraft can be flown at a constant altitude, and this is one of the specifications of the Aural Null. That leaves the refraction parameter. This parameter varies with atmospheric weather conditions. During most search flights the weather will not vary enough to make a significant change. So the radio horizon will be a constant distance from the transmitter while the aircraft is conducting the Aural Null. This is exactly what we want to be the case. The receiver frequency is not offset from the transmitter frequency because we want to receive all available signal strength to make detection of the radio horizon easier. The automatic gain control helps us by ensuring as much receiver gain as is necessary, but no more, is available. The volume control allows us to adjust the volume for comfortable listening to ensure our hearing does not become fatigued, again ensuring accurate detection of the radio horizon. All of this allows us to use the receiver to accurately map points on the radio horizon and use either Procedure A, B or C to determine the transmitter location.

Now that we know the factors, we can examine the results reported in Accuracy of ELT Searches1 and see what they tell us about the Cardinal Pass and the Off-Tuned Aural Null.

Cardinal Pass

Below is a map depicting the flight and computation of the ELT position during the Cardinal Pass.

Map 1: Cardinal Pass1

You can see the path of the airplane and the locations identified as the maximum signal points during first the West to East pass, and then the North to South Pass. It is interesting that the aircraft began to circle to the left immediately after the Max Long position. One would think, given the description of the Cardinal Pass that some amount of travel beyond the point to confirm passage of the max signal point. No matter. They determined the position1 to be 45° 37.79’ N 75° 56.45’ W. The actual position, taken from Google Earth, was reported1 as 45° 37.697’ N 75° 56.333’ W. So the technique was able to fix a location to within 0.125 nautical miles (or 760 feet, or 232 meters) from the known location. This is really quite good. Of course they knew where the ELT was located before the aircraft left on the search, and they had placed the ELT in a location that they thought would allow the technique to produce accurate results.

Off-Tuned Aural Null

Below is a map depicting the flight and computation of the ELT position during the Off-Tuned Aural Null.

Map 2: Off-Tuned Aural Null1

If you believe it looks similar to the Cardinal Pass map, you are correct. In fact it depicts the same flight path. This means that while the paper1 describes a number of benefits that accrue to this technique due to the way it was performed, this technique was, in fact, performed incidentally to the primary Cardinal Pass. And it suffered from that and other problems.

First the benefits that the authors suggest:

Note that using the three waypoints (ELT 1,ELT 2, ELT 3) in the Aural Null B procedure in this example works to the advantage of the Aural Null graphical method since, in general, we would not have these waypoints accurately without the use of a GPS and if we had not been using the Off-Tuning method in the first place.1

So yes, the coordinates of the off-tuned aural null points are more accurate because they used a GPS receiver instead of plotting the positions on the map. But could that not be done anyway, assuming a GPS is available? They continue:

In addition, the graphical construction shown here was done on a computer with a large screen and could not have been done to the same accuracy on the navigator’s lap in a small aircraft.1

If you have read our Practical Guide to Aural Searches - Polygon Methods you would know that an Aural Null computed from three points (a triangle) can result in only one circumcenter location, not a cocked hat as depicted in the image above and described in the paper: “The graphical solution is also shown with the most probable location (graph-ELT) being within the ‘cocked hat’.”1 This branch of mathematics, geometry of triangles, is one that all CASARA members should be familiar with. If the authors had been familiar they would not have confused the Aural Null procedures with triangulation and recognized their error. It seems that even with a computer and a large screen they could not achieve an accurate geometric construction. Their time might be better spent becoming familiar with the mathematics rather than assuming others share their lack of geometric skills. We have members, and have observed many navigators, who can plot an accurate Aural Null pattern while flying in small aircraft.

There is a mathematical formula relating the coordinates of the three vertices of a triangle to the coordinates of the circumcentre of the triangle. We will use that formula later. For now we note that the graphical construction resulted in a position of 45° 37.122’ N 75° 57.020’ W for a position error of 0.750 nautical miles1 (4557 feet, 1389 meters). At fully six times the error of the Cardinal Pass, In spite of the claimed advantages, the Off-Tuned Aural Null seems hopelessly outmatched. But is it?

First, there is a fourth candidate vertex that does not appear in the graphical construction:

An arbitrary decision was made to make a turn to the right to begin the start of chord 2. Since the audible signal did not reappear on 123.25 MHz while the aircraft was proceeding East, the right turn was continued until, upon passing the track on a Westerly heading, it was faintly reacquired for a very short time while passing across chord 1.1

No reason is given for not including this position in the Aural Null computation so we will consider it. Thus we have four points (ELT 1, ELT 2, ELT3 and ELT 4). We also note that, from the GPS track, the aircraft did not roll wings level while proceeding East. Recall from earlier that if the airplane is not wings level the polarity vectors of the transmitter and receiver antennas come into play adding more factors that can obscure the distance measurement. The signal was detected when the airplane rolled wings level to fly chord 2. An even better candidate may have been missed simply because they chose to continue the turn rather than roll wings level. These four points don’t meet the criteria for Procedure A and Procedure B takes only three points. We could use Procedure C, but for now let’s see what can be done with Procedure B.

We need to eliminate one of the points. There are a number of strategies that should work. We could apply the logical requirement that Procedure B is based on a triangle, and that ELT 1, ELT 2 and ELT 4 are all on chord 1. This suggests that ELT 2 should be discarded since it is in the middle of chord 1. We could compute all possible positions from all possible permutations of the four points taken three at a time; then eliminate the point that gives the highest error value. Our software to compute a Procedure C result does this. Again ELT 2 is identified as the point that should be discarded.

Finally we can use the properties of polygons that have circumcircles: simple, convex, cyclic polygons. What we need is a procedure that given a set of vertices, will order the set to form a simple, convex polygon and discard the vertices that do not belong. Luckily such a procedure was developed by Ronald Graham6. Our Procedure C software also uses the Graham Scan. Again ELT 2 is discarded.

The ultimate test however is the distance of each point from the transmitter location. Since we are hoping that all our points are on a single circle with the transmitter at the centre, the distance from each point to the transmitter location will tell us how good the data collection method is. Let’s look at some actual numbers.

Points Used

Position Result

Position Error

ELT 1, ELT 2, ELT 3

45° 37.171’ N 75° 57.028’ W

1327.47 meters

ELT 1, ELT 3, ELT 4

45° 37.643’ N 75° 56.704’ W

491.124 meters

Table 1: Positions computed from two sets of Off-Tuned Aural Null points.

Table 1 shows the results of direct computation of the Off-Tuned Aural Null position using the circumcentre formula for a triangle on the surface of the earth. The top row is using the same three points (ELT 1, ELT 2, ELT 3) as the graphical construction from the paper1. The error is reduced from 1389 meters to 1327 meters. Not a lot. The second row is using the three points selected by the Graham Scan and other methods (ELT 1, ELT 3, ELT 4). The error is now reduced by almost 1 kilometer to 491 meters. Quite substantial. We are puzzled as to why they would not use ELT 4, unless it is to make the Cardinal Pass look even better by comparison.


Distance from Transmitter

RMS delta Range


2398 meters

861 meters


2150 meters

1186 meters


3146 meters

1309 meters


2745 meters

797 meters

Table 2: Distance of each point from the transmitter location.

Table 2 shows the distance of each point from the transmitter location. Remember, if these are perfect Aural Null points they will all be the same distance. For each point we computed range from the transmitter. We then computed the root mean square of the differences between the range of that point and all others. This helps determine which points are better suited to use in an Off-Tuned Aural Null computation.

The point that is farthest from the mean is ELT 3. This would tend to indicate we should drop it, but that would leave us three points in a line which wouldn’t work. The next farthest point is ELT 2, which is also the point discarded by the Graham Scan. It is unclear why they selected ELT 2 and discarded ELT 4. What is clear is that their point selection criteria did not produce a set of points that are anywhere close to equidistant from the transmitter, which is required for the Aural Null procedures to produce an accurate result. There is in fact a difference of 1 kilometer between the nearest and farthest points. This is a substantial difference when the mean is 2600 meters. It appears that the graphic procedure did not benefit at all from off-tuning. In fact it is very difficult to measure, compare or even estimate distances based on received signal strength, even when one has the opportunity to design the equipment from scratch to perform that function. It is even more difficult to do with a receiver designed for a completely different purpose, like an aviation radio telephone transceiver. While in this case, where the transmitter location was carefully chosen for the technique, they were able to get an accurate position from the Cardinal Pass, eventually (if not frequently) an actual accident will place the transmitter in a location that will cause the Cardinal Pass to fail as the Off-Tuned Aural Null did.

This point is worth emphasizing. Aural Null Procedures assume that the positions used to plot the solutions are on a circle with the transmitter at the centre. Procedure A (shown at the left) uses a standard geometric solution to find the centre of the circle, and so the assumed position of the ELT. But what happens if some of the positions are not equidistant from the actual transmitter location. The procedure will still happily give you a position for the ELT, it will just be wrong.

An Aural Null Procedure A with one wrong position will result in an incorrect computed ELT position. Similarly a Procedure B with one wrong position will also give an incorrect computed ELT position. Procedure A has a benefit in that if the navigator computes all the bisectors, they will not intersect over either the true position or the erroneous position. So there is a chance, if the navigator runs this check, to find out that there is a problem. Procedure B provides no such checks.

In Accuracy of ELT Searches1 Barr, Casey and Muir seem to imply that the Aural Null is not as good as the Cardinal Pass even with the advantage of off-tuning. Unfortunately it is actually their method of collecting the positions that results in the poor performance of the Off-Tuned Aural Null.

Other Factors

Discussions between Mr Buckley and Dr Muir that pre-date either paper1 2 are quite illuminating as to how the thinking went astray. Dr Muir was (and may still be) involved in ski patrol activities and attended training on the use of Avalanche Transceivers “The more complex problems are well-known in avalanche searches where you have multiple (up to 4-6) casualties all with beacon antennae pointing in different directions and all radiating on the same frequency leading to multiple maxima, minima, and highly asymmetrical radiation patterns.7 The problem with comparing Avalanche Transceivers with ELT beacons is that Avalanche Transceivers operate on 455 kHz and usually have a range specification of 100 meters. This means that electronic avalanche search operations take place entirely within the near field of the transmitter. In fact within about one sixth of a wavelength. Translating this experience to the frequency of an ELT 121.5 MHz transmission would mean the experience is only applicable when the searcher is within 40 cm of the ELT. It is not practical to operate a search aircraft this close to an emergency beacon. Dr Muir goes on to say “...both Mike and I have tested the methods out at least a couple of times each, so there may be some merit to all this.” Our questions to Barr, Casey and Muir is this: were these tests conducted in similarly contrived circumstances as the one described in Accuracy of ELT Searches1? And: is a handful of tests really enough to discard the sum total of our knowledge of how radio works?

In fact it is quite well known that Avalanche Transceivers suffer very similar limitations to those that plague the Cardinal Pass and Off-Tuned Aural Nulls. Spikes in the signal pattern can cause significant location errors. Some materials surrounding the transceivers, including snow, can cause significant errors in ranging.

That is not to say there is not a method of accomplishing what Barr, Casey and Muir intended. For years multipath interference and fading were problems to be overcome. With the advent of techniques like those used in third generation cellphone technology multipath can actually be used to increase the range and reliability of those links. There is of course a method by which one may use a radiotelephone receiver to reliably determine the distance to a transmitter. Experienced practitioners of the technology should be able to deduce this method, so we leave it as an exercise for the reader.

Normalization of Deviance

Dr Vaughan developed her theory of the normalization of deviance by examining the Challenger Launch Decision. During the development, testing, redesign and retesting of the solid rocket boosters both Morton-Thiokol and NASA observed joint performance that deviated from expected performance. The problem was seen to be recurrent, but had no consequences, so they were able to convince themselves that the flaws were normal and acceptable.

The situation is very similar in the use of the electronic search techniques we have been discussing. Even though the results often deviate from expected or acceptable performance, and there are good scientific reasons for this deviation, there have been no consequences. The deviation has come to be seen as normal, even desirable even when it is actually deleterious to desired results.

In the case of NASA, Normalization of Deviance continued to be a problem playing a role in the Columbia disaster, and more recently the near drowning of an astronaut during a space walk. So unless and until CASARA implements a large and significant change in safety and management culture we can expect more instances where questionable techniques are used routinely. 


We believe Barr, Casey and Muir postulated the Cardinal Pass, and possibly the Off-Tuned Aural Null without considering the applicable mathematics, physics or engineering principles. The flight was carefully staged to showcase the Cardinal Pass technique. No attempt was made to implement either single or double blind protocols. These conclusions result from statements in their paper. In an attempt to show the Cardinal Pass in the best light, a set of points gathered incidental to the Cardinal Pass were cobbled together into an Off-Tuned Aural Null and presented as a Traditional Aural Null, to be a foil for the Cardinal Pass.

All of this makes it seem that the paper was not an honest attempt to present an unbiased investigation of the technique. Rather it seems to be an attempt to justify conclusions already arrived at, possibly from anecdotal observations. It seems reasonable that a similar set of circumstances lead to the adoption of Wing Blanking and Ad-hoc Direction Finding Antenna Placement.

Should volunteer search and rescue organizations use their funding to investigate techniques like this without competent technical oversight? How far have the ideas presented in Accuracy of ELT Searches1 penetrated into the accepted canon of CASARA training? The Advanced Cognitive Engineering Lab, mentioned earlier, was awarded a New Initiative Fund Grant of $2.6 million over three years to develop a training system for CASARA. What are the implications of Accuracy of ELT Searches1 on the quality of that system?

The most important point is not whether the Cardinal Pass or Off-Tuned Aural Null are effective techniques or not. Neither is it whether any of the techniques we have criticized should be used or not. It is quite clear, and professionals in the subject have clearly answered that they are not effective, and should not be used. The most important point is that time after time air search and rescue organizations have come up with techniques that have be used on actual searches without ever being scrutinized for efficacy, or used even after such scrutiny has uncovered problems. The tendency for this to happen puts lives at risk.

We present this as a cautionary tail. Search and Rescue volunteers should not adopt procedures based on conjecture without submitting them to a full and unbiased engineering or scientific study by persons qualified in the appropriate fields. As adults, these volunteers should recognize and accept the boundaries of their own qualifications, no matter how seductive a new idea may seem. Victims of an aircraft accident shouldn't have to depend on the unqualified conjecture or anecdotal experience of volunteers for timely rescue. In a recent article Randall Monroe posed this question “Would you rather bet a million dollars on a spacecraft engineer’s ability to successfully perform eye surgery, or an eye surgeon’s ability to land a probe on a comet?”


1. Barr A. Casey M. Muir L. R. “Improvement in Position Accuracy for ELT and Visual Searches” Civil Air Search and Rescue Association, Ontario Zone 12 (Ottawa), December 2010
2. Buckley H.R. “An Analysis of the Cardinal Pass Electronic Search Technique and Its Effect on Operational Effectiveness” September 2010
3. Potere D. “Horizontal Positional Accuracy of Google Earth’s High-Resolution Imagery Archive” Sensors 2008, 8, 7973-7981; DOI: 10.3390/s8127973
4. Brittain J.E. “Electrical Engineering Hall of Fame: Harald T. Friis [Scanning our Past]” Proceedings of the IEEE (Volume: 97, Issue: 9) pages 1651-1654
5. Haslett C. “Essentials of Radio Wave Propagation” Cambridge University Press, 2008 052187565X pages 119-120
6. Graham R.L. “An Efficient Algorithm for Determining the Convex Hull of a Finite Planar Set” Information Processing Letters 1 Pages 132-133 (1972)
7. Email from Dr Muir to Mr Buckley et al, March 2, 2010 4:41 PM EST

A Wing and a Prayer

posted 3 Sep 2013, 15:08 by Support SarMobile   [ updated 26 Sep 2013, 14:54 ]

A Wing

We haven't been able to determine exactly how the Wing Blanking technique came about, but it may have happened like this.

Many years ago, a pilot flying an ELT search, listening to the beacon signal on the airplane communications radio, executed a 360° circle but noticed at one point in the circle the ELT signal dipped momentarily in strength so that it could not be heard. It turned out that this dip occurred when the bottom of the airplane pointed towards the ELT location. The pilot thought perhaps this effect could be used on searches, but what could be causing it. Again we don't know for sure, but we suspect this happened with a fairly common high wing airplane, a Cessna 172, with the communications antenna mounted on top of the fuselage but aligned with the wings. Perhaps with the 'line-of-sight' propagation of ELT radio signals in mind the pilot realized that with the airplane banked for the turn, at one point the wing would block the line-of-sight from the beacon to the airplane antenna. It seems obvious that the wing must be blocking the signal. The wing is constructed from aluminium, which conducts electricity and can reflect radio signals, right? Unfortunately it isn't that straight forward.

In order to block, or reflect electromagnetic signals such as radio wave or light, an object must be made of the right material, and be large enough. We compare the size of the object to the wavelength of the signal. Objects that are much smaller than one wavelength will have essentially no effect on the signal. Objects much larger than one wavelength will reflect, absorb or pass the signal depending on the materials it is made of. Objects that are neither much smaller, nor much larger, may have some effect of the signal, again depending on what it is made of. So far we haven't assigned any specific values to these sizes, so let's start. In professional radio technology circles it is widely accepted that much smaller is around one tenth of a wavelength and much larger is ten wavelengths. So to reliably and effectively block the beacon signal the wing would have to be as large as, if not larger than ten wavelengths. The ELT beacon frequency is 121.5 MHz and has a wavelength of 2.47 meters. According to Wikipedia, a Cessna 172 has a wingspan of 11 meters and a chord (the distance of from the front of the wing to the back of the wing) of 1.5 meters. So can an object measuring 1.5 by 11 meters effectively block a radio signal with a wavelength of 2.47 meters? According to generally accepted principles no it could not. But we have an experiment that you may be able to try to convince yourself.

A consumer GPS receiver, or the GPS chip in a modern smartphone, receives the GPS data signals on 1575.42 MHz. Those signals would have a wavelength of 19 cm. We have access to a car that has a steel roof that is 1.7 meters by 2.3 meters. If an airplane wing that is 0.6 wavelengths by 4.4 wavelengths can block the ELT signal, the car roof that is 8.9 wavelengths by 12.1 wavelengths should be able to block th
e GPS signal. Most consumer GPS receivers have a display that shows the positions of all satellites in view, and the strength of the signal from each satellite. If you have a smartphone you may have to install an application. By taking your GPS receiver into a car and holding it under the roof you can measure the strength of the satellite signals entering the car. With good timing you can perform this experiment while a satellite is directly overhead so that the roof is between the satellite and the receiver. Sometimes you will notice that the signal from that satellite is week, or not detectable. But often enough you will find that you can receive a strong signal even under the roof which, according to wing blanking, should be able to block it. On the left you can see a picture of GPS reception in a car. You will notice that satellite 18 is nearly directly overhead, but still the signal is quite strong. You may click on the picture to see a larger version. If you are still skeptical, to the right is a similar image taken while inside a passenger jet completely surrounded by the aluminium fuselage. Signals are still able to get to the receiver from all directions.

We have had some questions about our signal strength displays above. Luckily we have signals analysts and programmers that enjoy working on these types of problems. Ordinarily one would place an antenna in the situation to be tested in an anechoic chamber. One would then either transmit from the antenna under test and measure the signal in all directions, or transmit from all directions and measure the signal at the antenna under test. Anechoic chambers large enough to hold airplanes are very rare and expensive. For large aircraft one would normally mount the aircraft on a pole in a very remote area without any reflectors for a long distance in all directions. This was done to test the stealth characteristics of the Horten Ho 229. This is also expensive and beyond our means. However there are systems that, over time, transmits a microwave signal from almost all directions that can be received by commercially available receiving equipment that will report signal strength, the Global Positioning System (GPS) and the Global Navigation Satellite System (GLONASS).  With some fairly simple software one can produce signal strength and standard deviation maps.

First we will start with a stationary test point. These two pictures are the signal strength (left) and standard deviation (right) of GPS and GLONASS signals recorded in a single story wood frame structure. Signal strength is from 0 to 45 color coded from red through yellow to green. Standard deviation is from 0 to 20 coded from green through yellow to red. Black represents segments directions from which no signals were received. Direction in azimuth is 360° around the circle, North at the top. Elevation is 0° at the circumference of the circle to 90° at the center. These maps are made up of 15,523,503 individual signal strength measurements. Even with so many samples there are gaps due to the parameters of the satellite orbits relative to the earth. North is located at the top. Clearly signal strength is weaker near the horizon, which is to be expected, but there is also an area in the SSE direction that is week up to about 40° elevation. We arranged to have something actually block the signal from that direction. These images clearly show what to expect if something is blocking or reducing signal strength.

These two images are taken from a car with a hard top, steel roof. Again these images represent signal strength and standard deviation. Because the car is mobile we have the opportunity to move it around to get signals arriving from almost all directions in fewer samples. These images represent 1,792,983 individual signal strength measurements. Because we know the orientation of the car we can compute the direction, relative to the front of the car, the signals are coming from. In these images the front of the car is up. We can see there is no clear signal blockers, even though according to the Wing Blanking theory, and the Strut Mount Direction Finding Theory the roof, doors and other metal parts of the car should be blocking the signal.

To understand why this happens you really need to understand Quantum Electrodynamics, but we can find a simpler analogy. This is all about wave propagation. While the physics of radio wave propagation are very different from ocean wave propagation the macro effects are similar. The main point of confusion seems to surround the behavior of visible light, so let's start there. The visible light spectrum, from violet through red, covers wavelengths from 390 to 700 nano meters. There are one billion nano meters (abbreviated nm) in a meter. For simplicity let's use 500 nm as the wavelength for visible light. That means the wavelength of the GPS signal is 380,000 times (19×10-2÷500×10-9) as long as visible light. We can translate these numbers into ocean wave that are more familiar. Let's use a 50 meter wavelength ocean swell to represent visible light. At that scale the GPS signal would have a wavelength of 19,000,000 meters. The circumference of the earth is 40,075,000 meters. So on this scale the GPS signal is well represented by ocean tides. A break water may be constructed to prevent the ocean swell and other waves from damaging boats moored in a harbor, the tide however will be hardly effected by a break water. Scaling 2.47 m ELT signal wavelength to this model would result in a wavelength of 247 million meters, or 247 thousand kilometers. To put this is perspective the moon orbits with a radius of 385 thousand kilometers. A wave with that length would propagate around the entire earth without much effect. The lesson here is that you can't assume a radio signal, even one that propagates by 'line-of-sight' will behave in any way similar to the way visible light does.

Another way to test theories according to scientific principle is through testing predictions. Once a theory is formulated, one can use it to make predictions. If the predictions turn out to be false, it is usually an indication that the theory has problems. We are examining a theory that the wing of an airplane can block radio signals. If true then any geometry that places the antenna behind the wing from the transmitter should result in a blocked signal. A VHF air-band communications antenna is 19 inches in height. A Cessna 172 has a wingspan of 36 feet 1 inch. This puts a top mounted antenna about 17 feet 3 inches from the nearest wing tip. Simple trigonometry shows us that any signal arriving at an angle greater than 5.3° below the wing should be blocked and not received by that antenna. So the theory allows us to predict that wing blanking should work with any angle of bank over 6°, or say 15° to give a comfortable margin. Why then does the CASARA training manual call for a 30° angle of bank, and the Civil Air Patrol documentation call for a 40° angle of bank? Perhaps there is another explanation. We might also ask why high wing airplanes, with antennas mounted above the wings, don't loose contact with any control tower that is directly left or right of them. 

There is another property of a radio antenna which is probably responsible for the observations that lead to the Wing Blanking technique; that is the antenna radiation pattern. Some antennas, when transmitting, send more energy in some directions than in others. The amount of energy sent in each and every direction is know as the radiation pattern and is normally presented in graphic form. Antennas work in reception in the same way they do in transmission, just in reverse. So if an antenna transmits more energy in a particular direction, it will receive correspondingly more energy from that direction.

There has not been a lot of work put into measuring the radiation patterns of antennas on general aviation aircraft, but there has been for commercial transport and military aircraft. These studies show that, as expected, an all metal fuselage acts as the ground plane or counter poise for the mono-pole communication antennas. The shape and conductivity of the ground plane does affect the radiation pattern. An ideal ground plane will restrict all energy to one side (the same side as the mono-pole) of the ground plane. An airplane, it turns out, is not an ideal ground plane. It is not uncommon to have significant energy radiated to (or received from) as much as 30° or 40° below the lateral axis of an airplane with a top mounted antenna. Here are two diagrams of such radiation patterns superimposed on a Cessna 172.

This is a much more satisfying explanation for this phenomenon. You will, no doubt, have noticed that changing the explanation of how the phenomenon works does not show the technique to be flawed. But we are just starting our examination of the technique in light of how radio signals actually work. We can't pick and choose which properties of radio signals we want to be in effect when we are using a particular technique. We must accept and account for all. Even though airplanes are able to maneuver in quite extreme ways, most of the time they are flying straight and level. This results in the communications antennas being orientated vertically. During a Wing Blanking maneuver the airplane will be banked over to 30° or 40° wich will result in the antennas be orientated off vertical by the same angle. We need to examine what effect this will have on the technique. Keep in mind that there is nothing about any particular high wing general aviation airplane that would lead to the conclusion that either of those two radiation patters are actually correct. The cone of silence below the airplane may be larger and require lesser bank angles, or smaller and render the Wing Blank technique completely useless. To be sure each airplane would have to be examined.

A Prayer

The property of radios signals that finally makes Wing Blanking a flawed technique is polarization. Polarization is a measurement of the orientation of the radio wave that is imposed by the transmitting antenna. For maximum signal reception, the receiving antenna must be oriented such that its polarization matches the transmitting antenna. As the polarity of the receiving antenna differs from the polarity of the transmitting antenna the received signal strength will be reduced until the polarity differs by 90°. Common examples of polarization are some types of sunglasses and flat panel computer displays. By placing polarized sunglasses lens in front of a flat panel monitor or LCD TV and rotating the lens you can see the effect that polarization mismatch has on electromagnetic radiation. The display emits light polarized a a particular angle, the sunglasses lens only passes light which matches its polarization. Here are three images showing the lens orientated at 0°, 45° and 90° relative to the polarization of the display:
If the search airplane is flying straight and level, its antenna will be oriented and polarized vertically. Even if the ELT antenna is not oriented vertically the relative orientation will remain the same regardless of the heading of the search airplane. This is good if the airplane is using proper direction finding antennas, or performing an Aural Search Technique. Similarly if the ELT antenna is oriented vertically (as it likely is during training or false alarms) then even when the search airplane is banked to turn the polarization will not change the signal strength since the relative polarization will be the angle of bank. It is when the ELT antenna is not vertical and the airplane is banked to turn, as in the Wing Blank technique, the relative polarization will be different for each heading the airplane takes while turning around in a circle. 

To understand this picture a toy top. When it first starts to spin its axis of rotation will normally be vertical. As it slows down it will begin to precess. This is similar to the motion of the polarity of the airplane communications antennas as it turns a full circle. If the angle of bank of the airplane is the same as the polarization angle of the ELT antenna, at one point in the turn the relative polarization will be 0°. When the airplane is 180° around the circle from that point the relative polarization will be the sum of the ELT antenna polarization and the airplane angle of bank. If the some of these angles approaches 90° then the signal strength will be greatly reduced, just as the crew would expect during a Wing Blank turn. This could result in two signal dips indicating two potential bearing to the ELT. If the airplane antenna radiation pattern does not give a dip, the plane is banking a 30° when a bank of 40° or even 50° is needed, this could result in a completely erroneous bearing. If the sum of the angle of bank and the ELT antenna polarization is greater than 90° the relative polarization will go through the 90° dip twice for each complete circle.

So there are two significant problems with Wing Blanking:
  1. Crews have been taught that Wing Blanking works because the wing blocks the radio signal. This implies, and some documents actually state, that one can determine if an airplane is suitable by visually examining the antenna placement. Here we have shown that one must actually perform a radiation pattern measurement on the airplane to determine suitability. Since there is no exhaustive body of work done on small general aviation airplanes, once can not be sure what changes to the airplane may change the radiation pattern making a previously suitable airplane unsuitable. 
  2. There is no way to know the polarization of the ELT antenna on a crashed airplane without first finding it. To be sure of effectiveness, electronic search techniques should be polarization independent. Wing Blanking is not polarization independent. 
We are not religious, and don't apeal to a deity during technical searches; but unless the Search and Rescue organizations that might come looking for you are willing to issue public assurance that they won't be using the Wing Blank technique, you may want to pray that if you do crash, your ELT finishes up with vertical polarization.

Is 'Highly Variable' or 'Seems to Work to Some Extent' Good Enough?

posted 26 May 2013, 05:01 by Support SarMobile   [ updated 14 Aug 2013, 18:47 ]

In our last article we saw how a simple misunderstanding in executing an electronic search technique can cause delays in locating a crash site. In this article we will look at how misunderstanding the technology involved can result in the widespread use of ineffective techniques. In the last article both search aircraft dispatched by the Joint Rescue Control Centre were equipped with Radio Frequency Direction Finding (RFDF) equipment. In this article we will see an attempt to make deployment of RFDF equipment to volunteer search aircraft more convenient that actually sacrifices effectiveness.

First let's look at the RFDF equipment used by the Civil Air Search and Rescue Association (CASARA), the volunteer group. In November of 2011 the Canadian Armed Forces announced that it was expanding search and rescue coverage in the arctic by using private contractors and civilian volunteers (CASARA). Effective radio direction finding capability will be important when conducting searches is the large and mostly empty Canadian Arctic.

According to statements we received from the National President of CASARA the primary electronic homing method used by CASARA is the L‑Tronics Little L‑Per portable direction finder. This device is designed to operate in a hand held mode on the ground. The operator scans the device in azimuth while monitoring a display that indicates if the radio signal is being received from the left, from the right or from directly ahead or behind. Using this equipment and simple ground navigation techniques a search team is quickly able to locate an Emergency Locator Transmitter (ELT) once they are within reception range. The Little L‑Per works by using two dipole antennas spaced at a distance close to one quarter of the wavelength of the radio signal. The electronics inside the Little L‑Per alternately switches each dipole from an antenna to a reflector forming a two‑element Yagi antenna that is pointed alternatively left and right. A Yagi antenna is directional, receiving more signal from one direction, less signal from all other directions. With the antenna switched in this manner the Little L‑Per can measure the strength of the radio signal coming from the left, then from the right. If the signal from one side is stronger the ELT is presumed to be on that side. If the signal is of equal strength on both sides the ELT is presumed to be directly in front or behind. If you are feeling a little lost by this description don't be concerned. There is an excellent demonstration of how a Yagi antenna works given by Diana Eng KC2UHB hosted by Make Magazine. We suggest you go watch this video before continuing with this article.

So you're back. Wasn't that well done? 

L‑Tronics also provides antennas to mount on a vehicle so that search crews may drive and hunt for the ELT location at the same time. They also make available antennas and plans to install directional antenna arrays on aircraft so search crews may take to the skies. This additional flexibility can significantly reduce the time to find an emergency beacon, especially one associated with an airplane crash which may have a large search area. There is a small problem though. With a ground vehicle antennas may be simply installed in the correct locations with magnetic bases that hold the antennas firmly in place at the relatively low speeds that ground vehicles reach. Airplanes are normally made out of aluminium alloys which are non‑magnetic and have few places where an antenna may be clamped onto the structure. To have the antennas located at the proper spacing to form a two‑element Yagi they usually must be permanently attached. To the left are diagrams of a typical antenna placement (red) viewed from the front and side. This requires holes be drilled through the airplane skin. Not all aircraft owners want to have extra holes drilled in their airplanes, so it is not possible for all volunteer search units to always have access airplanes with the antennas installed. It is the solution to this dilemma that give rise to the problem we will discuss in this article.

As a quick aside, you may have noticed that the antennas, viewed from the side, are rather curiously bent. Normally whip antennas are used, but there is not sufficient ground clearance beneath the airplane for the size of whip antenna that would be needed. The engineers at L‑Tronics recognize this and provide instructions for converting the whip antennas into bent whip antennas. There are also commercial versions may be used. Bent whips function in very much the same way as whips. Of course normal whip antennas could be used and mounted to the top of the wing where it crosses the fuselage, but this is the usual mounting location for the aircraft communications antennas. Mounting the direction finding antennas near the communications antennas would cause each to interfere with the operation of the other. 

So a solution was sought to temporarily mount the direction finding antennas so that they may be removed leaving no permanent changes to the aircraft structure. We are not certain where the idea of mounting the antennas on the struts came from, but we think it likely that it was adapted from a common practice in wildlife tracking. Here we see a picture of a 3‑element Yagi antenna mounted to the strut of a Cessna aircraft. This particular installation was used to track cranes, but the practice is common world wide for various fauna. A Yagi of the correct size to receive the emergency beacon signal on 121.5 MHz would be too large to safely mount on a strut. Even the drag of the two whip antennas mounted on one side could unbalance the airplane and make it uncontrollable in some flight regimes. So a decision was made to mount one antenna on each strut, far enough out from the fuselage for adequate ground clearance. It is interesting however, to read what a professional wildlife tracking organization has to say about mounting antennas on aircraft:

To obtain the greatest lateral range, and thus maximize reception range and minimize search times, a vertically-polarized Yagi antenna should be mounted under each wing, at right-angles to the plane's fuselage and pointing 15°-30° downwards from horizontal (Gilmer et al. 19812). Because wing struts provide the best antenna mounts, the ideal aircraft for radio-tracking are high-winged monoplanes, such as the Cessna 150 or 172, the Piper Cub, and the Maule.1

It is completely bewildering to us that the task of tracking wildlife with radio tags gets this level of though and scientific information consideration, but the task of tracking persons in distress with Emergency Locator Transmitters doesn't.

This diagram on the left is what was finally arrived at, there is a photo from an on-line document on the right. Clearly the antennas are spaced much further apart than in the belly mount example. If you were paying attention to what Diana Eng was saying in her video you know that the size and spacing are not correct to form a Yagi antenna. The theory of this approach as explained to one of our team is this: Since the fuselage is between the two antennas the left antenna receives signals from the left side of the airplane louder than signals from the right side of the airplane, and visa verse. The switching performed by the L‑Per then compares the signal strength from the left antenna with that from the right antenna (instead of comparing the strength from directional antennas pointed left and right). That sounds plausible unless you know, as our team mate did, the wave length of the emergency beacon signal, about 2.5 meters, is longer than the fuselage is wide, about 1 meter (just over 3 feet). Radio signals quite handily propagate around obstacles that are smaller than their wavelength. Think of waves passing under a pier. As the wave passes each pile supporting the pier you may observe a small drop in the wave size, but by the time the wave is a very short distance away all evidence of the pile acting on the wave will be gone. There will be some loss of signal strength as the radio wave passes the fuselage but will it be enough to determine an accurate direction? All the time? When your life, or the life of someone you love depends on it? A spokesperson for L‑Tronics had this to say: "We have experimented with configurations like this and have heard of results from others as well. The results are best described as highly variable." One has to wonder why CASARA would use such an installation. Why does convenience trump effectiveness in search and rescue?

Unfortunately this is not the only questionable technique in use. We were recently reviewing a CASARA training presentation that contains images of antenna installations similar to ours above. The presenter's notes for the slide with those images includes the following statements:

Antennas mounted on struts ... Not recommended by Ltronics, however it seems to work for us to some extent.

The best system is permanently mounted belly antennae which are much more effective and accurate due to proper spacing and better ground plane.

Also used are internal antennas which work; but must be handled with some care as they are blocked by the airframe.

As you may imagine to us as scientists, engineers and technologists it is horrifying that a search and rescue group, even a volunteer group, would use and train their members in the use of techniques which seem to work to some extent. We asked CASARA why they were using this technique given the significant technical problems with them. Their only response was that CASARA members only use techniques that have been reviewed and approved by the Royal Canadian Air Force. So we asked the Royal Canadian Air Force for the documentation of their review and approval. After an exhaustive search they were unable to find any such documentation. We are left with many questions and no answers.

On a final note we went back to the L-Tronics website to see what we could learn about the internal antennas described in the training presentation. They have this to say:

Internally mounted antennas, such as wires taped to windows of a metal aircraft, generally give unsatisfactory results. The major problems are ambiguity and false courses, particularly to the rear of the aircraft, and sensitivity to the presence and movement of cabin occupants. Thus, such an antenna may seem to work on a limited test but have major problems on a real search.

We know of no temporary or "inside the cabin" antennas that are satisfactory for regular use. A portable DF set with its antennas inside the cabin of a light plane is worse than using the plane’s own communications receiver and wing shadowing.

This is interesting because the next technique we are going to be discussing is wing shadowing. It seems that L-Tronics does not think very much of that technique either.

[1] Philip J. Seddon; Richard F. Maloney: Tracking wildlife radio-tag signals by light fixed-wing aircraft. Department of Conservation Technical Series 30. Department of Conservation Wellington, New Zealand.

[2] Gilmer, D.S; Cowardin, L.M.; Duval, R.L.; Mechlin, L.M.; Shaiffer, C.W.; Kuechie, V.B. 1981: Procedures for the use of aircraft in wildlife biotelemetry studies. Fish and Wildlife Service Resource Publication 140. United States Department of the Interior, Washington DC, USA.

Electronic Seduction Delays Rescue

posted 12 Mar 2013, 06:07 by Support SarMobile   [ updated 21 Jul 2013, 18:50 ]

Electronic seduction occurs when an electronic signal is generated or modified, either naturally or artificially, such that the signal guides a recipient to a location other than that intended by the recipient. The recipient is seduced away from the intended target to another location. 

Mid October 2000 A Piper PA-31 Navajo Chieftain, C-GIPB, departed Yellowknife, Northwest Territories, on a night charter flight to Fort Liard1. One pilot and five passengers were on board. At Fort Liard they encountered moderate to heavy snow. The pilot attempted a non-precision approach with a circling procedure to land on Runway 02. The aircraft struck a gravel bar on the west shoreline of the Liard River, 1.3 nautical miles short of the runway, and 0.3 nautical mile left of course. The aircraft sustained substantial damage, but there was no fire. Three passengers were fatally injured, the pilot and two passengers were seriously injured. The emergency locator transmitter activated and was received by the search and rescue satellite system. Canadian Forces dispatched two aircraft to conduct a search. The wreckage was located by homing the beacon the following morning. Help arrived at the accident site approximately 10 hours after the crash.

Even though the crash was located using the signal from the emergency locator transmitter (ELT), confusion about what the electronic signal was telling the search crews would delay the arrival of help by at least 45 minutes. The search and rescue satellite system (COSPAS/SARSAT) first identified the crash site as being approximately 40 miles south of Fort Liard. A second satellite pass indicated the emergency locator transmitter position was approximately 19 miles south east of Fort Liard. The Rescue Coordination Centre dispatched a Hercules aircraft from Winnipeg, Manitoba and a Twin Otter from Yellowknife. Both aircraft equipped with electronic direction finding equipment that would allow them to home the emergency signal. 

The Twin Otter arrived on scene first and overflew the COSPAS/SARSAT provided location at 3,400 feet above ground level. Due to weather conditions in the area, the Twin Otter was being flown on instruments, Fuel considerations forced it to continue to Fort Nelson, British Columbia before attempting to home in on the emergency transmitter signal. Initial communication with the Twin Otter indicated the ELT was transmitting within two miles of the SARSAT predicted position, based on aural null indications with the very high frequency (VHF) squelch off.1 Here we will have to engage in some educated speculation as to what aural null indications are. If they did not have fuel to begin homing the signal, a process much more efficient than the aural null procedure, they certainly didn't have fuel to complete a canonical aural null. We suspect that what is meant here is this: As the Twin Otter approached the location provided by COSPAS/SARSAT they tuned a radio to the emergency transmitter frequency, 121.5 MHz, and selected squelch off to increase their chances of picking up the signal. As they neared the COSPAS/SARSAT location they began to hear the emergency signal then, some time and distance later, the signal disappeared. Some geometric analysis of these two locations provided a fix within two miles of the COSPAS/SARSAT location.

The Hercules arrived in the same area, near Lake Bovie, NWT, a short time later but was unable to pick up a signal from the emergency transmitter. The COSPAS/SARSAT system had allocated a high confidence to the predicted position. With this in mind, and maybe the report from the Twin Otter, it was believed that the Hercules was in the right area. Rather than fly a procedure to acquire the ELT signal, the crew contemplated performing a descent to visual conditions. This is a procedure that takes the skill and dedication of a professional flight crew, and not likely one that most civilian organizations would even consider. Aircraft approach to land through clouds and poor visibility all the time, but they have electronic navigation systems to guide them along paths that have been surveyed and are known to be safe. This crew was deciding if conditions were right for them to descend through bad weather, without any of the normal guidance, in the hope that they would find good visibility before they came too close to the ground. We are very lucky to have these people available to come rescue us when we need it. They dropped flares in the area for about 45 minutes to see if the cloud base was high enough for the descent. In the end the descent procedure was not carried out. The Hercules then flew further to the west where a stronger signal from the ELT was encountered. The Hercules was able to home the signal to determine the crash location was approximately 1 mile south of Fort Laird. A local civilian helicopter, was able to fly under the clouds to the crash site and render aid. 

So, what happened here? Why was the Twin Otter able to pick up a signal, but the Hercules was not? Why was the COSPAS/SARSAT calculated location so far from the actual crash site? Why should we care? Let's take the last question first.

We care because by trying to understand what happened in this case, we will understand more about the complex nature of performing electronic searches. With this understanding we may be able to avoid similar problems in the future; this is an important aspect of flight safety. It is all too easy to be a Monday morning quarterback, but we don't fault the Hercules crew for taking the actions they did. We know something of what is generally believed about emergency locator transmitter signals, and while these beliefs are broadly true they also contain some subtle misdirection which came to the fore in this case because of the circumstances. One of these beliefs is that the ELT will be located within the confidence circle of the COSPAS/SARSAT calculated location. It is likely this belief, combined with the twin otter crew's indication that the ELT was within two miles of the COSPAS/SARSAT location, that lead the Hercules crew to remain in that area rather than follow a procedure that would have them cover more ground in an attempt to acquire the ELT signal.

Why was the Twin Otter able to pick up the signal, but the Hercules was not? First let's set the stage. C-GIPB was eventually found 1.3 nautical miles from the threshold of the Fort Liard runway, 0.3 nautical miles left of the approach path to the airport. This put it on a gravel bar near the left bank of the Liard River. Here we have a topographic map2 with the crash site marked with a red dot (as in all images on this page you may click on this map to see a larger version):

C-GIPB Crash Site

The river valley and other terrain features provide a complex environment in which to home a signal. With the two aircraft operating in the area at different times and altitudes, and constrained by normal en route navigation accuracy one could easily receive the ELT signal while the other did not. 
To visualize this we need a larger map. Below we see a second topographic map2 of the search area. Fort Liard and the crash site are located just up from the center, Lake Bovie (where the Hercules concentrated initial efforts) towards the bottom right.


You will notice the terrain, indicated by the brown contour lines each representing an elevation change of 20 meters, forms a fairly narrow valley. In the same way that the river may be hidden from view at a distance because it is down in the valley, the crash site and ELT may be 'hidden' from the view of an aircraft receiver. Between Lake Bovie and the crash site the terrain rises to 480m before plunging down to below 200m at the river edge. Even if the Hercules was flying high enough to be well clear of the hills around Lake Bovie, the crash and the ELT could have been, and probably were, obscured by terrain closer to Fort Liard. To the left is a photograph [source: Wikipedia] that shows how the Fort Liard runway looks on final. You can see the gravel bar where C-GIPB came to rest in the middle distance at the right edge of the picture. You can also see the rising terrain to the south (left) of the airport.

So why was the COSPAS/SARSAT location so far away from the actual crash site? The accident investigation report points to magnetic interference and previous anomalies in ELT locations but makes no determination as to the cause. The report goes on to discuss how ELT frequency stability affects the quality of the position generated. This is quite correct, but the stability of the ELT on board C-GIPB must have been quite good to generate a high confidence position. Not as good as a TSO-C126 (also known as 406 MHz) beacon, but good enough to convince the Hercules crew the crash was very close to Lake Bovie. The documentation for COSPAS/SARSAT positions we have does not specify a high confidence value but gives a numeric rating from 1 to 4:

 Confidence   Accuracy 95% Confidence Interval
 1  >50 Miles
 2  20 to 50 Miles
 3  5 to 20 Miles
 4  <5 Miles

We assume that the high confidence level from the accident report corresponds to a confidence level of 4 from the chart above; any other level should not have lead the Hercules crew to believe that the crash site was so close to Lake Bovie and limit the search area as they did. All other levels of confidence would have included the actual crash site in the probable location area. To understand this we need to take a detour through refraction and scintillation.

refraction Gritty Refraction Refraction is the change in direction of a wave due to a change in the propagation medium. It doesn't matter if the wave is in a body of water, an acoustic wave in the air, a light wave or a radio wave. As the wave moves from one medium to another the waves are refracted at the interface between the two media causing the direction of propagation to change. In the photograph4 to the left you can see what happens as light propagating from the diagonal lines, through the glass, into the water and back through the glass to the air and finally to the camera is refracted causing a distortion of the lines. A more familiar effect can be seen in the photograph5 on the right. The pencil appears to be discontinuous because the light waves travelling from the pencil to the camera are refracted when passing from the water to the glass and to the air. In order to determine where the lines, or the pencil actually are located we need to know how much refraction the various media are imposing on the light.

This is similar in principle to a COSPAS/SARSAT satellite locating an ELT. As the satellite passes within range of the ELT it receives the radio wave carrying the distress signal. If the radio wave is subjected to refraction, the ELT will 'appear' to be in a location some distance from where it really is. The atmosphere and local magnetic interference can contribute to refraction of radio waves; but the most significant factor is the ionosphere. The ionosphere is a layer of the upper atmosphere that is ionized by solar radiation. The level of ionization varies by location, time of day and solar activity which causes the solar wind. In the arctic and antarctic the level of ionization is further complicated by the same process that gives us the aurora. The earth's magnetic field deflects particles of the solar wind toward the poles where they interact with the molecules of the atmosphere to create the northern and southern lights, and contribute to greater ionospheric refraction and scintillation. With only one satellite measuring the ELT signal from an unknown location, there is no way to find out how much the radio wave is refracted.

For a more detailed look at refraction you can check out this web site.

In these two previous photographic examples, and our discussion of ionospheric refraction we have only considered stable propagation media. The water has been still. Of course the ionosphere is rarely still. Just like the rest of the atmosphere it can be chaotic, vary in density and composition. If you have watched the aurora you know that they are very fluid and constantly changing shape and color. Atmospheric turbulence causes the stars to twinkle when we look at them at night. The phenomenon we call twinkling arises when variations in the atmosphere cause the star to shift apparent location and brightness due to refraction and diffraction. Astronomers call this scintillation. Ionospheric scintillation also affects radio waves transmitted from the earth into space. This causes the apparent location and strength of the ELT signal to vary as received by the COSPAS/SARSAT satellite. Just like a twinkling star, or coins in the bottom of a water fountain (photograph6 to the left) when the surface of the water is disturbed by spray, the apparent position will vary randomly around a central position. This random variation over time contributes to the COSPAS/SARSAT computation of the confidence level. Other things also contribute to this random variation. One important contributor is the frequency stability of the radio signal. That is why a 406 MHz beacons, which have better frequency control, can usually provide better location accuracy than a 121.5 MHz beacon. But, again with only one satellite making the measurements, even a 406 MHz beacon does not give us the ability to eliminate all refraction and scintillation.

To compute an ELT location the COSPAS/SARSAT system makes several position measurements of the signal over time during a pass. Each position will have error introduced by refraction, scintillation, frequency instability, etc. The position measurements are used to compute an 'average' position. Then the distance from each of the measurements to the average position is used to compute confidence value of the location. The formula used to compute the confidence value is designed so that the actual ELT location should be within the confidence distance of the computed location 90-95% of the time. Any error that is random and varies above and below zero will average out. Some errors that are not random can be accounted for in the computation and removed. If there are errors that are not random and can not be removed by the computation they will introduce an offset. Depending on the source of the error this offset can be quite large, in this case 19 nautical miles. The accident report refers to other ELT anomalies in the area between the Yukon border and Great Slave Lake. This area is within what is known as the auroral oval. This is a good indication that the source of this error is probably related to the effect of space weather on the ionosphere.

So after all this what have we learned? This is a case where, with the best of intentions at heart, a team working to resolve a missing aircraft assumed that a COSPAS/SARSAT location accurately locates an ELT. Even when they could not hear the ELT signal, they assumed the COSPAS/SARSAT location and confidence area were valid. In retrospect, these assumptions are not supported by what is known about radio propagation in the arctic. There are procedures to follow when a search resource arrives at a presumed ELT location and can not hear the ELT. The procedures were not followed in this instance because of the earlier assumptions. This is a case where naivete and assumptions trumped procedure. Mistakes will occasionally be made and we assume the RCAF has learned from this and promulgated those lessons. Never the less this incident stands as an example of why electronic search procedures must have a grounding in a good and thorough understanding of the science and technology. We don't expect search and rescue crews to master this knowledge. Indeed we know their time is better spent mastering the specific aspects of their craft, like the descent to visual conditions. Those who would design search techniques and procedures do require a mastery of the science and technology. Unfortunately this is not always the case. In our next article we will look at what happens when naivete and incorrect assumptions are used to create a procedure; and oversight is unable to detect and correct the problems that arise.


[1] Transportation Safety Board of Canada, Aviation Report A01W0261
[2] Natural Resources Canada, Atlas of Canada (
[4] Refraction pattern (some rights reserved
[5] Pencil in glass refraction (some rights reserved
[6] Fountain coins stock 4 by caliconcept-stock (

What is Wrong with Search and Rescue in Canada?

posted 10 Feb 2013, 07:03 by Micha @SARMobile

It has been some months since we have posted any updates to our site. We haven't been idle. Our developers have been busy building great features on the new BlackBerry 10 platform. But what I want to talk about here is the research we have been doing into the question posed above. What is wrong with search and rescue in Canada?

First let me be clear on one point. At we have the utmost respect for the abilities, training, dedication and bravery of Search and Rescue workers in Canada and around the world. What ever problems exist, they don't originate with the front line workers, search and rescue technicians or volunteers, who are just trying to learn and apply the skills to save those in need.

It is just as clear that there are problems. The memory of Burton Winters remains fresh among those who were close to him, or who keenly felt his loss. Those people perceive that there are problems in the system. Our own research has turned up documentation of problems on the search and rescue technology front; our specialty. Perhaps it is appropriate that our research has come to fruition so near the anniversary of Burton Winter's death. Perhaps it is also appropriate that this coincides with the tenth anniversary of the loss of NASA shuttle
Columbia. What we have discovered about the Canadian search and rescue system indicates problems that are very similar to those that plagued NASA and led to that loss; and similar as well to the problems that led to the loss of Challenger.

Over the coming weeks we will publish articles describing how misunderstanding, or misusing technology during search and rescue operations has resulted the delay of rescuers arriving at the scene of a least one airplane crash in Canada. We will examine two electronic search techniques that have been included in search and rescue training, and used to search for active Emergency Locator Transmitters even though these techniques are of questionable usefulness. We will examine how questionable techniques find their way into the repertoire of search and rescue units through first hand accounts. And finally we will examine what managers and leaders do when informed that they are using questionable techniques. Through these articles we will delve into radio communications, sun spot activity, the aurora borealis, antennas, and how all of that interacts with airborne search platforms. I am confident that once you have read these articles you will have a clear idea of one type of problem that confronts search and rescue in Canada.

Safety is no accident.

What is a Safety Incident?

posted 22 Jul 2012, 11:39 by Micha @SARMobile   [ updated 1 Dec 2013, 16:45 by Support SarMobile ]

safety: noun
  1. The condition of being protected from or unlikely to cause danger, risk or injury
  2. Denoting something designed to prevent injury or damage
So a safety program is a program designed to prevent injury or damage. A safety program does this by promoting activities and behaviours that reduce the likelihood of danger, risk or injury and discourages activities and behaviours that increase the likelihood of danger, risk or injury. One of the first things we must do then is categorize activities and behaviours according to impact on safety. In aviation this is not as easy as one might first expect. Obviously when we see news footage of the broken and smoking wreckage of an air plane crash, we know something has happened to negatively affect safety. Unfortunately it is too late for the victims of that crash. To be effective a safety program must identify behaviours and activities before injury and damage result. Ideally before there is danger or risk. This can be difficult, especially with the tools in common use by aviation safety analysts today, particularly as used by those who don't invest the time to learn how to use them. One example is the Swiss Cheese model by James T. Reason at the University of Manchester in 1990. Many people focus on the imagery of the cheese slices as a metaphor for an organization's defensive safety barriers, neglecting the other more salient aspects of the model. For example this image from the ASTRA Project makes the author's point very well but is not really the model presented by Reason with three layers of latent error followed by one layer of active errors. 

For this reason I use the concept of risk surface when advising clients how to decide if a particular event is implicated in safety or not. The shape of the risk surface is determined by factors that could be considered contributory or direct causes of an accident, had one occurred. If two incidents or events have congruent risk surfaces, and one is implicated in safety, then the other must also be implicated in safety. A case study may help here.

Risk Surface Congruence - A Case Study

I was searching the internet for information on a program of interest to members of the SARMobile team, myself included, when I came upon the web site of a volunteer air search and rescue group. Their web site contains a great deal of information, obviously intended for members, but available to anyone. This shows a degree of openness which I find refreshing, and is a touchstone of a good safety culture.

About a year ago they had an incident which was unambiguously implicated in safety. One of their search aircraft had a loss of separation (a near miss) with an aircraft not involved with the group activity in the vicinity of an aerodrome listed in the national Air Information Publication (AIP). I don't have all the details, but reading between the lines it appears that the group was conducting search training near the aerodrome. In most jurisdictions aircraft may not operate in the vicinity of an aerodrome, at altitudes set aside for aircraft arriving or departing that aerodrome, unless the intention is to land. An investigation was conducted, meetings held and new protective policies promulgated.

Then just last month, during a debriefing, feedback indicated that the communications frequency in use for an aerodrome not listed in the AIP was not presented in the pre-operations briefing. Seeing this I dashed off a quick email to their published contact address with a recommendation of one way to implement the solution they proposed for this safety issue. I did not expect to receive a reply, but imagined that any reply would be along the the lines of "thanks for the suggestion". Instead the reply I did receive sought to assure me that no safety issues arose from this incident. Actually the wording was strange under the circumstances, but very telling as well. But more on that later. For now let's confine ourselves to the question: is the second event implicated in safety or not?

Let's have a closer look at the the shape of the risk surface. First, whether or not the aerodromes are listed is immaterial. The pertinent regulations do not make that distinction. This is wise since the laws of physics will not be abated in the event that a mid air collision happens near an unlisted aerodrome. So the new protective policy enacted in response, that aircraft would avoid aerodrome airspace listed in the AIP, was doomed to be ineffective. In the first incident a loss of separation apparently occurred because a pilot did not act appropriately in the vicinity of an aerodrome. There could be many factors that contributed to this incident. A failure to brief the location or other data of this aerodrome with respect to the training task the aircraft was flying would certainly be contributory. In the second incident important information about an unlisted aerodrome was not briefed. If the second event is seen as a safety incident then they have an opportunity to improve their protective policy. But since the group believes "no safety issues arose" from this event their barriers remain porous. These two events have congruent risk surfaces because they both involve search training operations in the vicinity of an aerodrome. Over time other events with more or less similar risk surfaces will occur, until a sufficiently alarming outcome refocuses the attention of the safety program on the issue. We are left to hope that the alarm does not come in the form of injury or death.

I mentioned earlier that the wording of the reply I received took me aback. So much in fact that I did a little digging, the same thing I would do before taking on a new client. I searched news reports, publicly accessible government databases, the group's publicly published information and the like. There are limits to what I can learn through this, but it can be surprisingly informative. I was looking for indications of other activities with similar risk surface shapes to the two I have already covered here, except I took one more level of abstraction. I wasn't looking for incidents involving operations near aerodromes. Since the first incident likely involved an infringement of regulations I was looking for events that likewise involved transgressions. I very quickly found two.

The person who replied to me appears to be the ultimate owner (through a numbered company) of an air plane that was involved in a hand starting that resulted in substantial damage to two aircraft and minor injuries to one person. Applicable regulations require that when an aircraft is started either a person qualified to operate the aircraft is situated at the controls ready to act, or the aircraft must be restrained so that it can not get out of control. According to accident investigation records neither of these preventative measures were taken. I was not able to establish a definitive link between the aircraft, pilot and the group in question; but the proximity to the group makes this an item of interest that would be on my list for investigation if I was taking the group on as a client.

The second incident seems to have been a case of continued operation under Visual Flight Rules into Instrument Meteorological Conditions. Pilots are licensed to fly in two broad categories of flight depending on weather conditions. They are often referred to as visual flight and instrument flight. The regulations that apply in each case are called Visual Flight Rules (VFR) and Instrument Flight Rules (IFR), and the weather conditions Visual Meteorological Conditions (VMC) and Instrument Meteorological Conditions (IMC) respectively. So, when a pilot who is only authorized to fly under Visual Flight Rules operates an aircraft into an area of Instrument Meteorological Conditions it is called continued VFR into IMC. This is a very dangerous activity often resulting, as in this case, fatalities and destruction of aircraft. This particular accident does not directly involve the group I am talking about, but another group that falls under the same overarching national body. So again the proximity makes this an item of interest.

This is as far as the information I can gather from public sources can take me. What follows is a description of what I would do if I had access to the people and records of the group, not a description of deficiencies I have found. However, if you notice a similarity between what I describe below and your group, you should take a good long look at your safety program before someone like me or a government agency is doing that for you.

We now have three incidents involving the breaking of a regulation that resulted in either death, destruction of property or a close call. What do we do with this? If I was advising this group on their safety program I would conduct interviews with the members trying to see if these are isolated events or not. It is unlikely that they would be isolated. It is quite rare for a pilot to actually come to grief during the first incident of continued VFR into IMC, or first hand start, for example. It is reasonable to assume that there have been multiple events similar to these three, or that involve other regulatory infractions. I am also always interested in the level of authority given to the people involved. To understand why I feel this is important we have to take a short detour through some safety theory.

Human Error

Most accidents involve some sort of human error. Human errors can be either errors of commission or errors of omission. Errors of commission typically involve failure to follow regulations or procedures, taking short cuts or making incorrect assumptions. Errors of omission usually occur during the evolution of an accident. Failing to execute the appropriate check list during an engine failure for example. Human errors, as discussed by Mostia (2003)1, may be divided into two categories:
  • Errors of Intent
  • Errors of Action
Errors of intent occur when someone exercises authority to intentionally override or ignore a policy, procedure or regulation; or violate the intent of a policy. Clearly the three events I am interested in are all errors of intent (another area of risk surface congruence). The pilot must intentionally decide to operate according to the search training task rather than the rules governing operations near aerodromes. One might claim to not be aware of an aerodrome, but then one is not following the regulation requiring the pilot to be familiar with all data pertinent to the flight. Aviation can be a harsh mistress. A pilot may encounter IMC accidentally at night, but continuing on is an intentional error of omission. The appropriate action for a VFR pilot encountering IMC is to turn about and return to where visual conditions were available. Finally there is no possible excuse for not following regulations for starting aircraft. Errors of intent usually occur when the person with authority does not believe in the purpose or rational behind the policy or regulation; or because they are willing to override the undesirable implications of the policy or regulation.

Returning to  James T. Reason but going beyond the cheese metaphor, we find that systemic errors of intent fall into the category of latent failures. They can lay dormant for long periods of time without contributing to an accident. A safety program may wait for an accident or incident to implicate these systemic errors then deal with them. Unfortunately that is often too late for the victims of the accident. It is much better to recognize and deal with latent failures when they are only an inconvenience. An organization with a truly effective safety program will spend as much time and effort, if not more, at this stage as they do when they have a close call or a loss. Often the fact that they invest the time and effort earlier in the process means they never have to deal will a close call or a loss. 

So, back to our search and rescue group. With Mostia and Reason in mind, my next step would be to examine non-operational areas, human resources and finance for example. If there is a culture of latent failure involving errors of intent, they will often show up in short cutting or abuse of fiscal and personnel policies. Again, using the idea of risk surface congruence, if an infraction under a policy or regulation in operations is a safety issue, then an infraction under a financial or personnel policy must also have safety implications. If nothing else a culture of subverting non-operational policies and regulations will eventually bleed over into operations.

So, what is a safety incident? Any occurrence that is similar to (has a risk surface congruent to that of) any other safety incident. This will include those events that might not seem to be so at first look. A safety program should be about preventing accidents, not explaining the ones that have already happened. To do that takes constant vigilance and a willingness to see what other dismiss as not safety related. 

Safety is no accident.

[1] Mostia, B. September 2003. Avoid Error. Chemical Processing.

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