North Atlantic Operations - Contingency

North Atlantic Operations - Contingency


The airspace of the North Atlantic (NAT), which links Europe and North America, is the busiest oceanic airspace in the world. In 2012 approximately 460,000 flights crossed the North Atlantic and that volume of traffic continues to increase. Direct Controller Pilot Communications (DCPC) and ATS Surveillance are unavailable in most parts of the NAT Region. Aircraft separation, and hence safety, are ensured by demanding the highest standards of horizontal and vertical navigation performance/accuracy and of operating discipline.

Operating discipline not only includes procedure and protocols for normal operations but also encompasses specific profiles and protocols for abnormal operations such as equipment failure, weather avoidance, rapid descent, enroute diversion and turn back procedures. Non-compliance with these protocols and procedures could result in a Loss of Separation situation.

This article is intended to provide an overview of the Contingency procedures, applicable to individual aircraft, whilst in North Atlantic (NAT) airspace.

NOTE: Some of the contingency procedures, as outlined below, were changed effective 28 March 2019 in accordance with the information contained in NAT Ops Bulletin 2018_005. For the full text of the Ops Bulletin, refer to the Further Reading section below.

Loss of Navigation Capability

Some aircraft carry 3 Long Range Navigation Systems (LRNS) and if one system fails, even before take-off, the requirements for NAT HLA 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 NAT HLA and which are equipped with two operational LRNS.

One LRNS Fails Prior to Takeoff

If one LRNS fails prior to departure, the pilot has the following options:

  • delaying departure until repair is carried out
  • obtaining a clearance above or below NAT HLA
  • completing the flight on a "Blue Spruce" route which have been established for use by aircraft with a single LRNS or suffering partial loss of navigation capability. A re-filing of the flight plan will be required

One LRNS Fails Prior to Oceanic Entry

If one LRNS fails while airborne but prior to entering the OCA, the pilot has the following options:

  • landing at a suitable aerodrome before the boundary or returning to the aerodrome of departure for repair
  • diverting via one of the "Blue Spruce" routes
  • obtaining a re-clearance above or below NAT HLA

One LRNS Fails After Oceanic Entry

If one LRNS fails after entering NAT HLA, the pilot should continue to operate the aircraft in accordance with their Oceanic clearance and advise ATC of their degraded navigation capability status. The pilot should also:

  • assess the prevailing circumstances including:
    • the performance of the remaining system
    • the extent of the remaining portion of the flight in NAT HLA
  • if appropriate, prepare a proposal to ATC with respect to the prevailing circumstances. Options include:
    • continuing as cleared
    • request clearance above or below NAT HLA
    • turn-back
    • obtain clearance to fly along one of the "Blue Spruce" routes
  • if appropriate, consult with ATC as to the most suitable course of action
  • obtain a revised clearance prior to any deviation from the current Oceanic clearance

If the flight continues in accordance with its original clearance, and especially if the distance to go within NAT HLA is significant, the pilot should begin a careful monitoring programme. The facets of this programme include:

  • taking special care in the operation of the remaining system understanding that routine methods of error checking are no longer available
  • crosschecking the main and standby compass systems frequently against the LRNS information
  • checking the performance record of the remaining LRNS in the Aircraft Technical Log. If there is doubt regarding its performance and/or reliability, the following procedures should be considered:
    • attempt to establish a visual track indication by the sighting of other aircraft or their contrails
    • request ATC provide information on other aircraft close to the aircraft’s estimated position
    • establish VHF contact with nearby aircraft (preferably same track/level) to obtain potentially useful information such as:
      • drift
      • groundspeed
      • wind details

The Remaining LRNS Fails While in Oceanic Airspace

Should the remaining LRNS also fail or become unreliable, the pilot should:

  • immediately notify ATC
  • make optimum use of the procedures specified above related to visual sightings and establishing VHF contact with other aircraft
  • look-out visually and electronically (ACAS/TCAS) for conflicting aircraft
  • make maximum use of exterior lights

If no instructions are received from ATC within a reasonable period of time, consider climbing or descending 500 feet, broadcasting that action on 121.5 MHz and advising ATC of the altitude change as soon as possible.

Complete Failure of Navigation Systems Computers

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.

Severe Weather Avoidance

If a deviation from track to avoid weather, such as thunderstorms, is required, the pilot should request a revised clearance from ATC - even when the expected deviation is relatively small. The pilot should initiate communications with ATC via voice or CPDLC. A rapid response may be obtained by either:

  • stating “WEATHER DEVIATION REQUIRED” to indicate that priority is desired on the frequency and for ATC response; or
  • requesting a weather deviation using a CPDLC lateral downlink message

Pilots must understand that ATC can only issue a clearance that will assure minimum separation standards from all other traffic. If this is not possible, ATC will advise "UNABLE" in response to the request for a revised clearance and request the pilot's intentions. The pilot should then indicate the direction, anticipated track displacement and any expected altitude change of their intended deviation.

Contingency procedures, as described below, should only be applied if, for any reason (including "UNABLE"), a clearance CANNOT be obtained. If a clearance to deviate is received when requested or is received after initiating the contingency procedures, the revised clearance should be followed in all respects and will supersede any and all provisions of the contingency procedures.

ONLY in the event that a revised ATC clearance has not been obtained, the following contingency deviation procedures should be adopted in their entirety:

  • If possible, deviate away from the organized track or route system
  • Establish communications with and alert nearby aircraft by broadcasting, at suitable intervals, on the ATC frequency in use (if in VHF coverage) and on frequency 121.5 MHz (or, as a back-up, on 123.45 MHz):
    • aircraft identification
    • flight level
    • aircraft position (including ATS route designator or the track code) and intentions
  • Watch for conflicting traffic both visually and by reference to ACAS/TCAS (if equipped)
  • Turn on all aircraft exterior lights
  • For deviations of less than 5 NM, aircraft should remain at the level assigned by ATC
  • For deviations of greater than 5 NM, when the aircraft is approximately 5 NM from track, initiate a level change of 300 ft in accordance with the following table:
Route Centreline Track Direction of Deviation Level Change
EAST (000° to 179° Magnetic) LEFT (north)

RIGHT (south)


CLIMB 300'

WEST (180° to 359° Magnetic) LEFT (south)

RIGHT (north)

CLIMB 300'


  • When returning to track, regain the last assigned flight level when the aircraft is within approximately 5 NM of centre line.
  • The pilot should inform ATC when weather deviation is no longer required, or when a weather deviation has been completed, and the aircraft has returned to the centre line (or previously adopted SLOP offset) of its cleared route.

Special Procedures for In-Flight Contingencies

Although all possible contingencies cannot be envisioned, the following procedures provide for cases such as:

These procedures are applicable primarily when rapid descent, turn-back, or diversion to an alternate aerodrome is required. With due regard to the circumstances at hand, the pilot's judgement will determine the specific sequence of actions taken.


If an aircraft is not able to continue in accordance with its ATC clearance, a revised clearance should be obtained, if possible, prior to initiating any deviation from the cleared route or altitude. Emergency Communications protocols using the distress (MAYDAY) signal or urgency (PAN PAN) signal should be used as appropriate. Aircraft forced to manoeuvre without receipt of a revised clearance should broadcast position and intentions, at frequent intervals, on 121.5 MHz (123.45 MHz back-up). If ATC communications are being conducted on VHF, pending receipt of any revised clearance, these broadcasts should instead be made on the current control frequency.

Until a revised clearance is obtained, the NAT in-flight contingency procedures, as specified in the ICAO PANS ATM (Doc 4444) and the ICAO NAT Regional Supplementary Procedures (Doc 7030), should be carefully followed. In general terms, these procedures result in the aircraft being flown at a flight level and/or on a track where other aircraft are least likely to be encountered. The procedures are outlined below.


The basic concept of the in-flight contingency procedures is for the aircraft in distress to offset from the assigned route by 5nm and to climb or descend to an altitude that differs (by 500' when below FL410 and 1000' when above that altitude) from those normally used.

Initial Actions

The aircraft should leave its assigned route or track by initially turning at least 30° to the right or left. Direction of turn should be based on the position of the aircraft relative to the OTS (whether the aircraft is outside, at the edge of, or within the system). The direction of turn may also be affected by:

  • direction to the diversion airport
  • terrain clearance
  • allocated levels on adjacent routes or tracks
  • known traffic

Subsequent Action

If the aircraft is able to maintain its assigned flight level, once it is 5nm from the centreline of its cleared track (laterally clear of any higher or lower traffic on the same track), it should:

  • climb or descend 1000' if above FL410
  • climb or descend 500' when below FL410
  • climb 1000' or descend 500' if at FL410

If the aircraft is unable to maintain its assigned flight level (engine failure, power loss, loss of pressurization, etc) it should, whenever possible, initially minimise its rate of descent when leaving its original track centreline. Once clear of any potential traffic following the same track at lower levels, it should then expedite descent to an operationally feasible flight level, which differs from those normally used by 500' or 1000' as appropriate to the altitude.

Before commencing any diversion across the flow of adjacent traffic, aircraft should, whilst maintaining the 5 NM offset track, expedite climb to above or descent to below the vast majority of NAT traffic; that is, to a level above FL410 or below FL280. Once there, they should maintain a flight level which differs from those normally used by 1000' if above FL410 or by 500' if below FL410. In the event that the pilot is unable or unwilling to carry out a major climb or descent, any diversion should be carried out at a level which is 500' different from those in use within NAT HLA airspace, until such time that a new ATC clearance is obtained.

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