Aural Null Procedure C

Flying Procedure C

Procedure C is not as simple as either Procedure A or B. There are more decisions that the navigator must make as the flight unfolds. These decisions are not difficult to make, and there is more information available to make the decisions.

Not surprisingly Procedure C starts in the same way that Procedure A and B did when the aircraft first enters the signal area and the first point is marked. The aircraft continues into the area for a few miles then turns about 90°. Again left or right doesn't matter. The aircraft continues on the new course for some distance. How far is up to the navigator. The further the aircraft flies the longer the chord lines will be, and the more accurate the eventual fix will be. However, making the chords too long takes extra time, and if they are too large relative to the signal area fix accuracy will actually start to degrade. In this simulation we have aimed for chords about 10 nm in length. We do not want to create a regular polygon so it is not important that the sides be the same length, in fact we would prefer them to be different lengths. If the aircraft flies out of the signal area sooner than the navigator wishes it is a simple matter to turn 27
0° (in the same direction as the first turn), re-enter the signal area and continue util the navigator is satisfied. Once the navigator is satisfied, the aircraft is turned 90° (again in the same direction as the first turn) and flown util it leaves the signal area. In the simulation we again use the green square call around marker to keep track of this location. The aircraft flies well out of the signal area, reverses direction and flies back into the signal area to mark the second point. 

With the first two points marked the navigator can estimate a heading that will take the aircraft another 10 nm or so before it leaves the signal area. The aircraft follows the same steps outlined above if it leaves the signal area early, or if the leg is getting too long. When the aircraft does leave the signal area, the orientation of the circumcircle will be apparent. The aircraft is manoeuvred to bring it back into the signal area as near to perpendicular to the circumference as possible to get the best quality point possible. Once this third point is marked there are three points which will form a triangle. By drawing the perpendicular bisectors one can get a good idea of the location of the transmitter. It is only necessary to draw two bisectors, the third is redundant, then the circumcircle may be drawn.

At this point the software will be able to compute a fix as with Procedure B, no
 2dRMS values. The navigator may draw two perpendicular bisectors at this point, and doing so will give some idea of the transmitter location. The pilot will be wanting a new heading though, and the two chords are enough for a good navigator to estimate what the next heading should be.

This is one point where the software really makes this procedure easier. With the circumcircle displayed on the compass rose it is quite easy to select the next heading. You can see on the left that the aircraft leaves the signal area some distance outside the circumcircle. This is one indication of the quality of the fix computed based on the triangle. The aircraft is again manoeuvred to bring it back into the signal are to collect the fourth point. The manually drawn bisectors will form a cocked hat that will give an indication how accurate the fix is. If one of the points has a large error, the bisectors formed with that point will diverge from the others, or the cocked hat may be very large. In this case the aircraft may continue around the circumcircle collecting more points until a sufficient level of accuracy is achieved.

At this point the aircraft flies directly to the fix and begins a visual search. This simulation used the same transmitter location as Procedure A and B from before. The final fix computation based on four points is on the right. The 
2dRMS value of 1.3 nm is quite good enough to proceed to a visual search. This procedure took forty minutes of simulated flight, just under the time required for the Procedure B, but the search crew can be confident that the fix quality is good, or alternatively know that something introduced errors before they commit to a visual search. So this procedure is more complex, and requires more knowledge to perform effectively, but we believe the benefits far outweigh these meagre costs. 

Altitude Considerations

Many descriptions of aural search techniques (Procedure A and B) will recommend that the search aircraft descend to the lowest safe altitude at which the signal may be heard. This reduces the diameter of the signal area by reducing the line of sight distance, and reduces the time that it takes to fly those procedures. This is because the path of the aircraft through the signal area is greatly dependent on chance. The aircraft will likely encounter the signal at essentially a random location relative to the emergency transmitter. The crew does not know where the ELT lies in relation to the aircraft. Unfortunately descending is not without cost. At low altitude the search aircraft may lose radio contact with base or air traffic services. Flying at low altitudes also affects safety. The lower the altitude the greater chance of controlled flight into terrain or obstacles. Lower altitude also reduces the time available to react to an in flight emergency such as an engine failure, which would be compounded by being out of communications with base or ATS.

There is also a practical consideration of altitude with respect to the accurate determination of Aural Null points. In the section on Radio Propagation we discussed Fresnel Zones and how their cross section area affects radio propagation near obstacles. At a higher altitude the aircraft will be further away from the transmitter when the Aural Null points are reached. The greater distance will make the cross section area of the Fresnel Zone larger reducing the effect of obstacles making the Aural Null points more indicative of the horizon rather than of horizon clutter like trees, building and small hills.

With Procedure C the search crew takes an active role in determining the flight path, and optimizes the path as each aural null point is collected and more data on the location of the transmitter is available. This allows an efficient pattern to be flown at a higher altitude at a greater distance from the transmitter. This also allows the crew to determine the location of the transmitter before closing the distance to it, reducing the probability that the aircraft will fly in the wrong direction, or close on an inefficient track.

On the left we see the start of a simulated aural null procedure with the signal just detected and the first aural null point marked. The simulated aircraft is 'flying' at 1900 ft above ground elevation. The aircraft proceeds as before collecting two more points when the first indication of the transmitter location is computed and an estimated circumcircle is plotted (image on the right). Not surprisingly given the altitude, the aircraft is still a considerable distance from the transmitter so an extended leg is used for the final point to help improve the fix accuracy.

Now that four points have been collected a fix with an indication of accuracy may be computed. Because the fix location may be estimated by eye from the collection of points, the aircraft is already heading in the general direction of the transmitter. This gives the navigator plenty of time to draw the bisectors and evaluate the cocked hat formed by them. At this time an appropriate visual search pattern should be selected and plotted. The bisectors may be seen on the left, though the range makes the cocked hat difficult to see. On the right are the results of the fix computation. Because this is a simulation, the location of the simulated ELT is also shown.

A Sector Search is appropriate in this situation, to keep the image clutter down only a single cycle is drawn. Returning to the navigation display we can see the route for the Sector Search, and the Course Deviation Indicator is now displayed. From first detection of the signal until a four point fix can be computed only thirty minutes pass. Transit from the final aural null point to the Commence Search Point takes a further twenty minutes, but this distance would have to be traversed in order to perform a pattern a a lower altitude anyway.

Our final image is passing the Commence Search Point for the Sector Search. We have seen that with the appropriate background information and tools, a search crew can take charge of an aural search. The search may be performed efficiently at higher altitudes where it may be possible to maintain communications, and where safety is improved. The crew will also get an indication of the quality of the fix. There is no significant time penalty paid to realize all these benefits. 

And finally while simulations are well and good, a real demonstration is much better. We don't have access to training ELT beacons, Search and Rescue officials would probably take a dim view of using a real one for a test so we used a major airport ATIS broadcast instead and remained one thousand feet above ground. We had to get much further away because the ATIS transmitter is probably located on a tower, or indeed the tower. The 
2dRMS value is quite high at 7 nm, but then the range was quite large at 70 nm. The computed fix is only 2.6 nm from the official location of the airport (which is only a couple thousand feet from current the location of the control tower) and well within a 5 nm radius of a standard Sector Search, though with a 2dRMS of 7 nm a 10 nm radius would be appropriate.

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