The ICAO cleaned up the disparate procedures for oceanic contingencies nicely about 2007, so what follows below, word-for-word, is ICAO Doc 4444, Procedures for Air Navigations Services, Amendment 2 §15.2.
[ICAO Doc 4444, ¶126.96.36.199] Although all possible contingencies cannot be covered, the procedures in 15.2.2 and 15.2.3 provide the more frequent cases such as:
- inability to comply with assigned clearance due to meteorological conditions, aircraft performance, or pressurization failure;
- en route diversion across the prevailing traffic flow; and
- loss of, or significant reduction in, the required navigation capability when operating in an airspace where the navigation performance accuracy is a prerequisite to the safe conduct of flight operations.
ICAO Doc 4444, ¶188.8.131.52] With regard to 184.108.40.206 a) and b), the procedures are applicable primarily when descent and/or turn back or diversion is required. The pilot shall take action as necessary to ensure the safety of the aircraft, and the pilot's judgement shall determine the sequence of actions to be taken, having regard to the prevailing circumstances. Air traffic control shall render all possible assistance.
Declaring an emergency is a game changer just about anywhere in the world and using "Mayday" or "Pan Pan" is the only way to change the rules of the game in some parts of the world. You shouldn't be shy about using it if you need traffic priority. I've done this eighteen times in my short life and there has never been anything negative as a result.
[ICAO Doc 4444, ¶220.127.116.11] If an aircraft is unable to continue the flight in accordance with its ATC clearance, and/or an aircraft is unable to maintain the navigation performance accuracy specified for the airspace, a revised clearance shall be obtained, whenever possible, prior to initiating any action.
[ICAO Doc 4444, ¶18.104.22.168] The radiotelephony distress signal (MAYDAY) or urgency signal (PAN PAN) preferably spoken three times shall be used as appropriate. Subsequent ATC action with respect to that aircraft shall be based on the intentions of the pilot and the overall air traffic situation.
More about this: Declaring an Emergency.
[ICAO Doc 4444, ¶22.214.171.124] If prior clearance cannot be obtained, until a revised clearance is received the following contingency procedures should be employed and the pilot shall advise air traffic control as soon as practicable, reminding them of the type of aircraft involved and the nature of the problem. In general terms, the aircraft should be flown at a flight level and on an offset track where other aircraft are least likely to be encountered. Specifically, the pilot shall:
- leave the assigned route or track by initially turning at least 45 degrees to the right or to the left, in order to acquire a same or opposite direction track offset 15 NM (28 km) from the assigned track centreline. When possible, the direction of the turn should be determined by the position of the aircraft relative to any organized route or track system. Other factors which may affect the direction of the turn are:
- the direction to an alternate airport;
- terrain clearance;
- any strategic lateral offset being flown; and
- the flight levels allocated on adjacent routes or tracks;
- having initiated the turn:
- if unable to maintain the assigned flight level, initially minimize the rate of descent to the extent that is operationally feasible (pilots should take into account the possibility that aircraft below on the same track may be flying a 1 or 2 NM strategic lateral offset procedure (SLOP)) and select a final altitude which differs from those normally used by 150 m (500 ft) if at or below FL 410, or by 300 m (1,000 ft) if above FL 410; or
- if able to maintain the assigned flight level, once the aircraft has deviated 19 km (10 NM) from the assigned track centreline, climb or descend to select a flight level which differs from those normally used by by 150 m (500 ft) if at or below FL 410, or by 300 m (1,000 ft) if above FL 410;
- establish communications with and alert nearby aircraft by broadcasting, at suitable intervals on 121.5 MHz (or, as a backup, on the inter-pilot air-to-air frequency 123.45 MHz) and where appropriate on the frequency in use: aircraft identification, flight level, position (including the ATS route designator or the track code, as appropriate) and intentions;
- maintain a watch for conflicting traffic both visually and by reference to ACAS (if equipped);
- turn on all aircraft exterior lights (commensurate with appropriate operating limitations); and
- keep the SSR transponder on at all times.
ICAO Doc 4444, ¶126.96.36.199.1] When leaving the assigned track:
- if the intention is to acquire a same direction offset track, the pilot should consider limiting the turn to a 45 degree heading change, in order not to overshoot the offset contingency track; or
- if the intention is to acquire and maintain an opposite direction offset track, then:
- operational limitations on bank angles at cruising altitudes will normally result in overshooting the track to be acquired. In such cases a continuous turn should be extended beyond 180 degrees heading change, in order to re-intercept the offset contingency track as soon as operationally feasible; and
- furthermore, if executing such a turn back in a 56 km (30 NM) lateral separation route structure, extreme caution pertaining to opposite direction traffic on adjacent routes must be exercised and any climb or descent, as specified in 188.8.131.52 b) 2), should be completed preferably before approaching within 19 km (10 NM) of any adjacent ATS route.
North Atlantic Exception for 180° Turnback
You may find exceptions to the ICAO procedures here and there. Perhaps most notable of these is the North Atlantic procedure for a 180° turnback. The ICAO advises that you execute a "continuous 180° turn" beyond 180° so as to acquire and maintain the proper offset. The NAT procedure is to maintain a "same direction" offset until completing a climb or descent before making the turn.
[NAT Doc 007, ¶13.3.5] Before commencing any diversion across the flow of adjacent traffic or before initiating any turn-back (180°), aircraft should, while subsequently maintaining a same direction 15 NM offset track, expedite climb above or descent below the vast majority of NAT traffic (i.e. to a level above FL410 or below FL280), and then maintain a flight level which differs from those normally used: by 1000 ft if above FL410, or by 500 ft if below FL410. However, if the pilot is unable or unwilling to carry out a major climb or descent, then any diversion or turn-back manoeuvre should be carried out
Portions of this page can be found in the book International Flight Operations, Part VI, Chapter 1.
ICAO Doc 4444 - Air Traffic Management, 16th Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, October 2016
NAT Doc 006 Part I - ATM Contingency Plan, Amendment 11, July 2018, Published on behalf of the North Atlantic Systems Planning Group (NAT SPG) by the European and North Atlantic Office of ICAO
NAT Doc 007, North Atlantic Operations and Airspace Manual, v 2018-1