Oceanic Loss of Long Range Navigation
The ICAO procedures for "loss of, or significant reduction in, the required navigation capability when operating in an airspace where the navigation performance accuracy is prerequisite to the safe conduct of flight operations" are contained in ICAO Document 4444, Amendment 2 § 15.2.1 and summarized in Special Procedures for In-flight Contingencies in Oceanic Airspace.
ICAO Operation of Aircraft Rules
7.2.1 An aeroplane shall be provided with navigation equipment which will enable it to proceed:
- in accordance with its operational flight plan; and
- in accordance with the requirements of air traffic services;
except when, if not so precluded by the appropriate authority, navigation for flights under the visual flight rules is accomplished by visual reference to landmarks.
7.2.2 For operations where a navigation specification for performance-based navigation has been prescribed, an aeroplane shall, in addition to the requirements specified in 7.2.1:
- be provided with navigation equipment which will enable it to operate in accordance with the prescribed navigation specification(s); and
- be authorized by the State of the Operator for such operations.
Note.— Information on performance-based navigation, and guidance concerning the implementation and operational approval process, are contained in the Performance-based Navigation (PBN) Manual (Doc 9613). This document also contains a comprehensive list of references to other documents produced by States and international bodies concerning navigation systems.
7.2.3 For flights in defined portions of airspace where, based on Regional Air Navigation Agreement, minimum navigation performance specifications (MNPS) are prescribed, an aeroplane shall be provided with navigation equipment which:
- continuously provides indications to the flight crew of adherence to or departure from track to the required degree of accuracy at any point along that track; and
- has been authorized by the State of the Operator for the MNPS operations concerned.
Note.— The prescribed minimum navigation performance specifications and the procedures governing their application are published in the Regional Supplementary Procedures (Doc 7030).
7.2.4 For flights in defined portions of airspace where, based on Regional Air Navigation Agreement, a reduced vertical separation minimum (RVSM) of 300 m (1,000 ft) is applied between FL 290 and FL 410 inclusive, an aeroplane:
- shall be provided with equipment which is capable of:
- indicating to the flight crew the flight level being flown;
- automatically maintaining a selected flight level;
- providing an alert to the flight crew when a deviation occurs from the selected flight level. The threshold for the alert shall not exceed ± 90 m (300 ft); and
- automatically reporting pressure-altitude;
- shall be authorized by the State of the Operator for operation in the airspace concerned; and
- shall demonstrate a vertical navigation performance in accordance with Appendix 4.
7.2.9 The aeroplane shall be sufficiently provided with navigation equipment to ensure that, in the event of the failure of one item of equipment at any stage of the flight, the remaining equipment will enable the aeroplane to navigate in accordance with 7.2.1 and, where applicable, 7.2.2, 7.2.3 and 7.2.4.
Note.— Guidance material relating to aircraft equipment necessary for flight in airspace where RVSM is applied is contained in the Manual on Implementation of a 300 m (1,000 ft) Vertical Separation Minimum Between FL 290 and FL 410 Inclusive (Doc 9574).
Exceptions: Minimum Navigation Performance Specification (MNPS) Airspace
[ICAO NAT Doc 007, ¶12.1.2] For unrestricted operation in the NAT HLA an approved aircraft must be equipped with a minimum of two fully serviceable LRNSs. NAT HLA 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 NAT HLA but only on specified routes established for this purpose. Aircraft may be approved for NAT HLA operations with only a single LRNS. However, such aircraft are only permitted to plan and fly on these same specified routes and on other particular routings serving individual traffic axes e.g. the Tango Routes, Routings between the Iberian Peninsula and the Azores/Madeira and Routes between Iceland and Greenland.
See Chapter 3 of ICAO NAT Doc 007 for more about this.
[ICAO NAT Doc 007, ¶12.1.3] 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 (NAT HLA) airspace with unresolved navigation system errors, or with errors which have been established to have been caused by inertial platform misalignment or initial position insertion error.
Methods of Determining which System is Faulty
[ICAO NAT Doc 007, ¶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:
- 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
[ICAO NAT Doc 007, ¶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
[ICAO NAT Doc 007, ¶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 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.
LOSS OF NAVIGATION/FMS CAPABILITY
[ICAO NAT Doc 007, ¶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.
The reason North Atlantic procedures are different is that the North Atlantic can become incredibly crowded and they would rather you stay on the tracks if you can safely (and accurately) do so. The procedures that follow assume you only started with two LRNS. If you had three and lose one, you are good to go. If you had three and lose two, the following also applies to you.
One System Fails Before Take-Off [With only one system remaining]
[ICAO NAT Doc 007, ¶12.2.2] The pilot must consider:
- delaying departure until repair is possible;
- obtaining a clearance above or below MNPS Airspace;
- 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).
- 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)
- Such use of the foregoing routes is subject to the following conditions:
- sufficient navigation capability remains to ensure that MNPS accuracy and the ICAO Annex 6 (Part I para 7.2.9 and Part II para 126.96.36.199) requirements for redundancy can be met by relying on short-range navaids;
- a revised flight plan is filed with the appropriate ATS unit;
- an appropriate ATC clearance is obtained.
For more about this, see: Blue Spruce Routes (by Jason Herman).
These Blue Spruce Routes are listed in ICAO NAT Doc 007 and your Jeppesen Airway Manual Atlantic Planning Charts.
One System Fails Before the OCA Boundary is Reached [With only one system remaining]
[ICAO NAT Doc 007, ¶12.2.4] The pilot must consider:
- landing at a suitable aerodrome before the boundary or returning to the aerodrome of departure;
- diverting via one of the special routes described previously; obtaining a re-clearance above or below MNPS Airspace.
- obtaining a re-clearance above or below MNPS airspace.
One System Fails After the OCA Boundary is Crossed [With only one system remaining]
[ICAO NAT Doc 007, ¶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.
- The pilot should however,
- assess the prevailing circumstances (e.g. performance of the remaining system, remaining portion of the flight in MNPS Airspace, etc.);
- 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.);
- advise and consult with ATC as to the most suitable action;
- obtain appropriate re-clearance prior to any deviation from the last acknowledged Oceanic Clearance.
- to take special care in the operation of the remaining system bearing in mind that routine methods of error checking are no longer available;
- to check the main and standby compass systems frequently against the information which is still available;
- 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
[ICAO NAT Doc 007, ¶12.2.8 The pilot should:
- immediately notify ATC;
- make best use of procedures specified above relating to attempting visual sightings and establishing contact on VHF with adjacent aircraft for useful information;
- keep a special look-out for possible conflicting aircraft, and make maximum use of exterior lights;
- 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.
Complete Failure of Navigation Systems Computers
[ICAO NAT Doc 007, ¶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.
- use the basic IRS/GPS outputs to adjust heading to maintain mean track and to calculate ETAs.
- draw the cleared route on a chart and extract mean true tracks between waypoints.
- at intervals of not more than 15 minutes plot position (LAT/LONG) on the chart and adjust heading to regain track.
Theoretically, you should be able to examine ICAO Doc 7030 to come up with regional differences and call it good. Unfortunately this document is rarely up to date and you should closely study the state pages in your Jeppesen Airway Manual as well as any NOTAMS. If you do examine "Chapter 9. Special Procedures" for each region in ICAO Doc 7030 as of 2015, here is a summary of what you will find:
- AFI (Africa) — No navigation entries.
- CAR (Caribbean) — No navigation entries.
- EUR (European) — If the aircraft cannot meet the navigation route or procedure requirements, a revised clearance should be obtained using conventional navigation aids or radar vectors. If still on the ground, the aircraft may be permitted to fly to the nearest suitable aerodrome where repairs can be made.
- MID/ASIA (Middle East/Asia) — If the aircraft has a failure to below RNP 5 or is unable to continue in accordance with its current clearance, a revised clearance shall be requested.
- NAM (North America) — No navigation entries.
- NAT (North Atlantic) — An aircraft compelled to descend through MNPS airspace should plan on descending below FL280, should descend through the tracks on a course midway between parallel tracks, and contact ATC as soon as practicable.
- PAC (Pacific) — No navigation entries.
- SAM (South American) — No navigation entries.
Portions of this page can be found in the book International Flight Operations, Part VI, Chapter 3.
ICAO Annex 6 - Operation of Aircraft - Part 1 Commercial Aircraft, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part I, July 2010
ICAO Annex 6 - Operation of Aircraft - Part 2 General Aviation, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part II, July 2008
ICAO Doc 7030 - Regional Supplementary Procedures, International Civil Aviation Organization, 2008
ICAO NAT Doc 007, North Atlantic Operations and Airspace Manual, v 2018-1