HANKE-Aviation GmbH - Flight Crew Training
CHAPTER 12 - PROCEDURES IN THE EVENT OF NAVIGATION SYSTEM DEGRADATION OR FAILURE
12.1.1 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 minimize 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.
12.1.2 For unrestricted operation in MNPS Airspace an approved aircraft must be equipped with a minimum of two fully serviceable LRNSs. MNPS approved aircraft 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).
12.1.3 If after take-off, abnormal navigation indications relating to INS or IRS systems occur, they should be analyzed 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.
12.1.4 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
12.1.5 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
12.1.6 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
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
12.1.7 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
12.1.8 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 generalization, 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
12.1.9 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.
12.1.10 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
12.1.11 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 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
12.1.12 If the GPS receiver displays a fault detection alert (i.e. a failed satellite), the pilot may choose to continue to operate using the GPS-generated position if the current estimate of position uncertainty displayed on the GPS from the FDE algorithm is actively monitored. If this exceeds 10 nm, the pilot should immediately begin using the following navigation system failure procedures, until the exclusion function or navigation integrity is regained, and should report degraded navigation capability to ATC.
12.2 LOSS OF NAVIGATION/FMS CAPABILITY
12.2.1 Some aircraft carry triplex equipment (3 LRNSs) and hence if one system fails, even before take-off, the two basic requirements for MNPS Airspace operations may still be met and the flight can proceed normally. The following guidance is offered for aircraft having state approval for unrestricted operations in MNPS airspace and which are equipped with only two operational LRNSs:
One System Fails Before Take-Off
12.2.2 The pilot must consider:
a) delaying departure until repair is possible;
b) obtaining a clearance above or below MNPS Airspace;
c) planning on the special routes known as the ‘Blue Spruce’ Routes, which have been established for use by aircraft suffering partial loss of navigation capability (Note: As indicated in Chapter 1, these routes may also be flown by aircraft approved for NAT MNPSA operations but equipped with only a single LRNS). These Blue Spruce Routes are as follows:
- MOXAL – RATSU (for flights departing Reykjavik Airport)
(VHF coverage exists. Non HF equipped aircraft can use this route)
- OSKUM – RATSU (for flights departing Keflavik Airport)
(VHF coverage exists. Non HF equipped aircraft can use this route)
- RATSU – ALDAN – KEF (Keflavik)
(VHF coverage exists. Non HF equipped aircraft can use this route)
- ATSIX – 61°N 12°34'W – ALDAN – KEF
(HF is required on this route)
- GOMUP – 60°N 15°W – 61°N 16°30'W – BREKI – KEF
(HF is required on this route) NORTH ATLANTIC OPERATIONS AND AIRSPACE MANUAL CHAPTER 12
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- KEF – EMBLA – 63°N 30°W – 61°N 40°W – OZN
- KEF – GIMLI – DA (Kulusuk) – SF (Kangerlussuaq) – YFB
- SF (Kangerlussuaq) - 67°N 60°W - YXP
- OZN – 59°N 50°W – PRAWN – YDP
- OZN – 59°N 50°W – PORGY – HO
- OZN – 58°N 50°W – LOACH – YYR
d) The following special routes may also be flown without an LRNS (i.e. with only short-range navigation equipment such as VOR, DME, ADF), but it must be noted that State approval for operation within MNPS Airspace via these routes is still necessary:
- VALDI - MY (Myggenes) - ING – KEF (G3)
- GONUT - MY (Myggenes) (G11)
12.2.3 Such use of the foregoing routes is subject to the following conditions:
a) sufficient navigation capability remains to ensure that MNPS accuracy and the ICAO Annex 6 (Part I para 7.2.9 and Part II para 184.108.40.206) requirements for redundancy can be met by relying on short-range navaids;
b) a revised flight plan is filed with the appropriate ATS unit;
c) an appropriate ATC clearance is obtained.
(Further information on the requisite procedures to follow can be obtained from Section ENR 220.127.116.11.3 in AIP Iceland and in Section RAC 11.22 in AIP Canada.)
Note: detailed information (including route definitions and operating procedures), which enables flight along other special routes within MNPS Airspace, may be found in relevant AIPs. This is specifically so, for aircraft operating without 2 LRNSs between Iceland and Greenland and between Greenland and Canada.
One System Fails Before the OCA Boundary is Reached
12.2.4 The pilot must consider:
a) landing at a suitable aerodrome before the boundary or returning to the aerodrome of departure;
b) diverting via one of the special routes described previously;
c) obtaining a re-clearance above or below MNPS Airspace.
One System Fails After the OCA Boundary is Crossed
12.2.5 Once the aircraft has entered oceanic airspace, the pilot should normally continue to operate the aircraft in accordance with the Oceanic Clearance already received, appreciating that the reliability of the total navigation system has been significantly reduced.
12.2.6 The pilot should however,
a) assess the prevailing circumstances (e.g. performance of the remaining system, remaining portion of the flight in MNPS Airspace, etc.);
b) prepare a proposal to ATC with respect to the prevailing circumstances (e.g. request clearance above or below MNPS Airspace, turn-back, obtain clearance to fly along one of the special routes, etc.);
c) advise and consult with ATC as to the most suitable action;
d) obtain appropriate re-clearance prior to any deviation from the last acknowledged Oceanic Clearance.
12.2.7 When the flight continues in accordance with its original clearance (especially if the distance ahead within MNPS Airspace is significant), the pilot should begin a careful monitoring programme:
a) to take special care in the operation of the remaining system bearing in mind that routine methods of error checking are no longer available;
b) to check the main and standby compass systems frequently against the information which is still available;
c) to check the performance record of the remaining equipment and if doubt arises regarding its performance and/or reliability, the following procedures should be considered:
attempting visual sighting of other aircraft or their contrails, which may provide a track indication;
calling the appropriate OAC for information on other aircraft adjacent to the aircraft’s estimated position and/or calling on VHF to establish contact with such aircraft (preferably same track/level) to obtain from them information which could be useful.
e.g. drift, groundspeed, wind details.
The Remaining System Fails After Entering MNPS Airspace
12.2.8 The pilot should:
a) immediately notify ATC;
b) make best use of procedures specified above relating to attempting visual sightings and establishing contact on VHF with adjacent aircraft for useful information;
c) keep a special look-out for possible conflicting aircraft, and make maximum use of exterior lights;
d) if no instructions are received from ATC within a reasonable period consider climbing or descending 500 feet, broadcasting action on 121.5 MHz and advising ATC as soon as possible.
Note: This procedure also applies when a single remaining system gives an indication of degradation of performance, or neither system fails completely but the system indications diverge widely and the defective system cannot be determined. NORTH ATLANTIC OPERATIONS AND AIRSPACE MANUAL CHAPTER 12
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Complete Failure of Navigation Systems Computers
12.2.9 A characteristic of the navigation computer system is that the computer element might fail, and thus deprive the aircraft of steering guidance and the indication of position relative to cleared track, but the basic outputs of the IRS (LAT/LONG, Drift and Groundspeed) are left unimpaired. A typical drill to minimise the effects of a total navigation computer system failure is suggested below. It requires comprehensive use of the plotting chart.
a) use the basic IRS/GPS outputs to adjust heading to maintain mean track and to calculate ETAs.
b) draw the cleared route on a chart and extract mean true tracks between waypoints.
c) at intervals of not more than 15 minutes plot position (LAT/LONG) on the chart and adjust heading to regain track.
Note: EAG Chart AT (H) 1; No 1 AIDU (MOD) Charts AT(H)1, 2, 3 & 4; the Jeppesen North/Mid Atlantic Plotting Charts and the NOAA/FAA North Atlantic Route Chart are considered suitable for this purpose.