Guidance concerning Air Navigation in and above the NAT MNPSA
CHAPTER 11: PROCEDURES IN THE EVENT OF NAVIGATION SYSTEM DEGRADATION OR FAILURE
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.
For unrestricted operation in MNPS Airspace an approved aircraft must be equipped with a
minimum of two fully serviceable LRNSs. MNPS approved aircraft which have suffered any equipment failures prior to NAT entry 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. Aircraft may be approved for NAT MNPS operations with only a single LRNS. However, such aircraft are only permitted to plan and fly on these same specified routes and on certain other routes serving individual traffic axes e.g. the Tango Routes, Routes between the Iberian Peninsular and the Azores/Madeira and
Routes between Iceland and Greenland (See Chapter 3 of this Manual).
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.
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
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
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:
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