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necessary for the crew to make frequent comparison checks. When an aircraft is fitted with three
independent systems, the identification of a defective system should be straightforward.
Methods of Determining which System is Faulty
10.1.6 With only two systems on board, identifying the defective unit can be difficult. If such a
situation does arise in oceanic airspace any or all of the following actions should be considered:
a) checking malfunction codes for indication of unserviceability
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b) 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).
c) contacting a nearby aircraft on VHF, and comparing information on spot wind, or ground
speed and drift.
d) 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
10.1.7 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
10.1.8 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
10.1.9 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.
GPS Failures
10.1.10 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.
Satellite Fault Detection Outage
10.1.11 If the GPS receiver displays an indication of a fault detection function outage (i.e. RAIM is
not available), navigation integrity must be provided by comparing the GPS position with the position
indicated by another LRNS sensor (i.e. other than GPS), if the aircraft is so equipped. However, if the only
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sensor for the approved LRNS is GPS, then comparison should be made with a position computed by
extrapolating the last verified position with airspeed, heading and estimated winds. If the positions do not
agree within 10 NM, the pilot should adopt navigation system failure procedures as subsequently described,
until the exclusion function or navigation integrity is regained, and should report degraded navigation
capability to ATC.
Fault Detection Alert
10.1.12 If the GPS receiver displays a fault detection alert (i.e. a failed satellite), the pilot
　
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