(6) Cross-check heading, groundspeed, track and wind information between systems and compare to best known information.
(7) Attempt to contact nearby aircraft to obtain wind or groundspeed and drift information which may identify the malfunctioning system.
The compass deviation check discussed in paragraph 6b(4) may provide a clue as to which system 1s faulty for systems such as INS.
NOTE: Even though these steps are taken, the occasion may still arise where a divergence between systems 13 observed, but the crew cannot determine which system is at fault. When this occurs, the practices in paragraph 10d should be used.
Recommended Actions Following System Failure. After a system malfunction or failure has been detected, ATC should be informed that the flight is experiencing navigation difficulties so that separation criteria can be adjusted, if necessary. Peporting malfunctions to ATC is an ICAO requirement and compliance is required by FAR Part 91. If the failed system can be identified with a high degree of confidence and the other system appears normal, the best course of action may be to fly the normal system and carefully monitor its performance using any additional navigation aids available, including DR. In the unlikely event that a total navigation failure occurs and other aids are unavailable, the only action may be to dead reckon using the flight plan headings and times. Under these circumstances, continue to use all means available to obtain and use as much navigational information as possible. Be alert for visual sightings of other aircraft since a potential hazard may exist due to inadverent deviation from your assigned track. Also, in some cases, it may be possibie to establish and maintain visual contact with another aircraft on the same track.
Recommended Actions Following a Divergence Between Systems. Since a small divergence between systems may be normal, the significance of the divergence should be evaluated. In general terms, if the divergence is less than 10 nm, the best course of action may be to closely monitor system performance and continue to steer the system considered most accurate. If the divergence between systems is greater than 10 nm, one of the systems may be degraded. Therefore, attempts should be made to determine which system may be faulty. If the faulty system cannot be determined using the practices in paragraph 10b and both systems appear normal, the action most likely to limit gross tracking errors may be to position the aircraft so that the actual track is midway between the crosstrack differences for as long as the position uncertainty exists. ATC should be advised that navigation difficulties are being experienced so that separation criteria Par 10 13
AC 90- ?9 7/14/80 may be adjusted as necessary. Consideration should be given to abandoning this "split-the-difference" practice if the divergence exceeds the separation criteria currently in effect. If a divergence of this magnitude occurs and the faulty system cannot be isolated, the best course of action may be to DR using the best known wind information. In some cases, the best known information may be flight plan headings and times.
KENNETH D. HUNT Director of Flight Operations 14 Par 10
Appendix 1 APPENDIX 1• RELATED READING MATERIAL Additional information concerning navigation practices, procedures and equipment may be found in the following documents:
1. Federal Aviation Administration (FAA) Advisory Circulars.
a. Advisory Circular 90-76, Flight Operations in Oceanic Airspace.
b. Advisory Circular 91-49, General Aviation Procedures for Flight in North Atlantic Minimum Navigation Performance Specifications Airspace.
c. Advisory Circular 120-31A, Operational and Airworthiness Approval of Airborne Omega Radio Navigation Systems as a Means of Updating Self-Contained Navigation Systems.
d. Advisory Circular 120-33, Operational Approval of Airtorne Long-Fange Navigation Systems for Flight Within the North Atlantic Minimum Navigation Performance Specifications Airspace.
e. Advisory Circular 120-37, Operational and Airworthiness Approval of Airborne Omega Radio Navigation Systems as a Sole Means of Overwater Long Range Navigation.
f. Advisory Circular 121-13, Self-Contained Navigation Systems (Long Range) .
Copies of these documents may be obtained free of charge from the U.S. Department of Transportation, Publications Section M 443.1, Washington, D.C. 20590.
2. Other Federal Aviation Administration Documents.
a. Airman's Information Manual, Basic Flight Information and AIC Procedures.
b. International Flight Information Manual.
c. international Notices to Airmen.
Copies of these documents may be obtained on a subscription basis from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.
3. International Civil Aviation Organization (ICAO) Documents.
a. Annex 2 to the Convention on International Civil Aviation -- Rules of the Air.
Appendix l
b. Procedures for Air Navigation Services - Rules of the Air and Air Traffic Services (Doc. 4444-RAC/501).
c. Regional Supplementary Procedures (Doc. 7030/2).
d. Aviation Use of Omega (Circular 139-An/95).
Copies of these documents may be purchased from the International Civil Aviation Organization (attn: Distribution Officer), P.O. Box 400, Succursale: Place de I'Aviation Internationale, 1000 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2R2.
4. Guidance and Information Material Concerning Air Navigation in the NAT Region, T 13/5N-854.
Copies of this document may be obtained from the International Civil Aviation Organization, European Office, 3 Bis, Villa Emile Bergerat, 92200 - Neuilly-Sur-Seine, France.
2 • U 8. GOVERNMENT PRINTING OFFICE 1980 621-996/2520