Guidance concerning Air Navigation in and above the NAT MNPSA
checking malfunction codes for indication of unserviceability
obtaining a fix. It may be possible to use the following:
the weather radar (range marks and relative bearing lines) to determine the position relative to an identifiable landmark such as an island; or
the ADF to obtain bearings from a suitable long-range NDB, in which case magnetic variation at the position of the aircraft should be used to convert the RMI bearings to true; or
if within range, a VOR, in which case the magnetic variation at the VOR location should be used to convert the radial to a true bearing (except when flying in the Canadian Northern Domestic Airspace where VOR bearings may be oriented with reference to true as opposed to magnetic north).
contacting a nearby aircraft on VHF, and comparing information on spot wind, or ground speed and drift.
if such assistance is not available, and as a last resort, the flight plan wind speed and direction for the current DR position of the aircraft, can be compared with that from navigation system outputs.
Action if the Faulty System Cannot be Identified
Occasions may still arise when distance or cross track differences develop between systems,
but the crew cannot determine which system is at fault. The majority of operators feel that the procedure most likely to limit gross tracking errors under such circumstances is to fly the aircraft half way between the cross track differences as long as the uncertainty exists. In such instances, ATC should be advised that the flight is experiencing navigation difficulties so that appropriate separation can be effected if necessary.
Guidance on What Constitutes a Failed System
Operations or navigation manuals should include guidelines on how to decide when a
navigation system should be considered to have failed, e.g. failures may be indicated by a red warning light, or by self diagnosis indications, or by an error over a known position exceeding the value agreed between an operator and its certifying authority. As a generalisation, if there is a difference greater than 15 NM between two aircraft navigation systems (or between the three systems if it is not possible to detect which are the most reliable) it is advisable to split the difference between the readings when determining the aircraft's position. However, if the disparity exceeds 25 NM one or more of the navigation systems should be regarded as
having failed, in which case ATC should be notified.
Inertial System Failures
INSs have proved to be highly accurate and very reliable in service. Manufacturers claim a
drift rate of less than 2 NM per hour; however in practice IRSs with laser gyros are proving to be capable of maintaining accuracy to better than 1NM per hour. This in itself can lead to complacency, although failures do still occur. Close monitoring of divergence of output between individual systems is essential if errors are
to be avoided and faulty units identified.
If the GPS displays a “loss of navigation function alert”, the pilot should immediately revert
to other available means of navigation, including DR procedures if necessary, until GPS navigation is
regained. The pilot must report the degraded navigation capability to ATC.
NAT Doc 007