Dangers of Flying Home-made GPS Approaches

by Pierre Duchaine, SatNav Program Office, NAV CANADA

Isn’t the global position system (GPS) neat? The latest avionics, the GPS receiver, has brought very accurate area navigation (RNAV) guidance within reach of all aviators. Just enter the runway threshold and GPS accuracy will line you up safely for an approach to any runway, or does it?

This article aims to destroy this myth and explain why pilots must use instrument flight rules (IFR)–certified receivers, follow a published approach, and use a current database to safely line up with the runway while in cloud. Doing it any other way is like playing Russian roulette with the lives of all on board.

TSO C129 GPS receivers — To fly GPS under IFR in Canada, pilots must use a GPS receiver that meets Technical Standard Order (TSO) C129 (commercial pilots must also receive approved training and certification). The C129 standard ensures that the receiver will have, among other things, proper course deviation indicator (CDI) sensitivity and receiver autonomous integrity monitoring (RAIM). The RAIM function ensures that the position displayed to the pilot is trustworthy for the receiver’s perceived phase of flight. The CDI sensitivity helps the pilot stay right on track — essential on short final. The following table shows the modes for each phase of flight used by the receiver:

Strict regulations oblige the pilot to retrieve the approach (waypoints and sequences) from a current database. This is how the receiver knows to change the RAIM alert threshold to 0.3 NM. The pilot must also verify the position (usually bearing and distance) of the waypoints against the approach chart.

RAIM — RAIM works by comparing the position solutions from different groups of four satellites in view. If there aren’t enough satellites in good positions to make a comparison, the GPS integrity light will come on. If there are enough satellites in view for RAIM to work and a satellite is transmitting faulty signals, the position solutions using that satellite might exceed the alert threshold for that phase of flight, triggering the GPS integrity light. Upon seeing the light, the pilot must revert to using traditional aids.

The catch is that, by default, most receivers think the aircraft is en route unless an approach has been loaded from the database. In other words, the receiver might show that you are right on track (without an alarm), but you could be almost 2 NM off. This is OK if you are actually en route, but not if you are executing an approach. The only way to get the RAIM in approach mode is by flying an approach procedure loaded from the database.

Instrument approach procedure design process — Flying an approach with the wrong CDI sensitivity is also asking for trouble. To understand this, one must grasp the rigorous process for developing a published instrument approach procedure (IAP). Presently in Canada, public approaches are designed, verified and published by NAV CANADA in accordance to standards prescribed by Transport Canada.

Approach designers have received certified training and undergone on-going on-job assessment on this very technical task. In most cases, they are current airline-rated pilots with varied flying experience.

All GPS approaches are based on very accurately surveyed runway reference points. These are used as anchor points for the whole transition and comprise about six waypoints. The paths between two consecutive waypoints are called segments, for which particular requirements, such as minimum altitudes, are prescribed based on depicted obstacles on the topographical map and a current national obstacle database. Segments are narrower near the runway threshold as this is where there is less manoeuvring expected by the pilot. The prescribed dimension of each segment is the result of adding the inherent maximum GPS signal and receiver error (called a navigation system error) to the potential pilot tracking error (called a flight technical error). The latter has been determined through extensive test flights by pilots flying using terminal and approach CDI sensitivities.

Once an approach has been designed, it is reviewed locally and test-flown to verify the accuracy of the waypoints, the flyability, the local signal interference and, most important, the actual location of the obstacles used to determine minima for each segment. Quality assurance designers carry out a further review on a national level to check for compliance with standards. These steps follow a traceable documented ISO 9000 process.

After approval, the approach is distributed to Natural Resources Canada for inclusion in the Canada Air Pilot (CAP) and to database suppliers, such as Jeppesen, for the inclusion in the global database from which subscribers receive their regional databases every 28 days.

Why do I need a database? Besides the risk of misentering the co-ordinates, manually entering waypoints, especially for an instrument approach, means danger. We saw earlier that in the case of a manually entered approach, the receiver does not know that the pilot is intending an approach; it thinks that the aircraft is still in the en-route mode. Therefore, the pilot operates with an integrity alert (RAIM alert threshold) of two nautical miles and a full-CDI deflection, which represents five nautical miles off course.

At five nautical miles full deflection, the width of the needle can represent a large error in navigation, which is outside the flight technical error assumed during the design of the approach. Although some receivers can be forced into increased CDI sensitivity, the only safe way to ensure proper CDI sensitivity is again to load an approach from a current database. Manually entering co-ordinates of a published approach will not do.

Conclusion — Without a proper approved receiver and an IAP retrieved from a current database, flying a GPS approach is simply taking risks, serious risks.

For more information — Aeronautical Information Circular 1/00 contains all the terms and conditions for the use of GPS under IFR in Canada.

The SatNav Program Office (SNPO) is the focal point for the introduction of SatNav in Canada. This group of pilots and engineers teams with experts in Transport Canada and in a wide range of international and national organizations to sort out the operational and technical issues involved in bringing the benefits of SatNav technology to Canadian aircraft operators safely and expeditiously. SatNav information can be found on their Web page on NAV CANADA’s Web site http://www.navcanada.ca. The SNPO e-mail address is SatNav@navcanada.ca; their fax number is (613) 563-5602.