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or Failure
10.1 GENERAL
10.1.1 The navigation systems fitted to MNPS approved aircraft are generally very accurate and
very reliable and GNEs as a result of system technical failures are rare in NAT MNPS Airspace.
Nevertheless, the risks that such errors pose can be significant and crews must employ rigorous procedures to
ensure early detection of any possible errors and hence mitigation of the ensuing risk. The NAT CMA
thoroughly investigates the circumstances of all reported GNEs in the MNPS Airspace. The majority are the
result of human error, and diligent application by crews of operating procedures such as those described in
Chapter 8 should help to minimise the frequency of such errors. As previously stated, actual failures of
navigation systems or equipment in MNPS approved aircraft occur very rarely. However, when they do
occur, their potential effects on the aircraft’s navigation capability can be subtle or progressive, resulting in a
gradual and perhaps not immediately discernible degradation of performance. ‘Vigilance’ must be the
watchword when navigating in NAT MNPS Airspace. ‘Complacency’ has no place here.
10.1.2 For unrestricted operation in MNPS Airspace an approved aircraft must be equipped with a
minimum of two fully serviceable LRNSs. MNPS approved aircraft that have suffered any equipment
failures that result in only a single LRNS remaining serviceable may still be flight planned and flown
through the MNPS Airspace but only on specified routes established for this purpose.
10.1.3 If after take-off, abnormal navigation indications relating to INS or IRS systems occur, they
should be analysed to discover their cause. Unless the flight can proceed safely using alternative approved
navigation sources only, the pilot should consider landing at the nearest appropriate airfield to allow the
problem to be fully investigated, using technical assistance if necessary. Under no circumstances should a
flight continue into oceanic (MNPS) Airspace with unresolved navigation system errors, or with errors which
have been established to have been caused by inertial platform misalignment or initial data input error.
10.1.4 Crew training and consequent approval for MNPS operations should include instruction on
what actions are to be considered in the event of navigation system failures. This Chapter provides guidance
on the detection of failures and what crew action should be considered, together with details of the routes
that may be used when the aircraft’s navigation capability is degraded below that required for unrestricted
operations in NAT MNPS Airspace.
Detection of Failures
10.1.5 Normally, navigation installations include comparator and/or warning devices, but it is still
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:
• checking malfunction codes for indication of unserviceability
• obtaining a fix. It may be possible to use the following:
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 10
NAT MNPS 58 Edition 2005
- 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
10.1.7 Occasions may still arise when distance or across 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
across track differences as long as the uncertainty exists. In such instances, ATC should be advised that the
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