NORTH ATLANTIC OPERATIONS AND AIRSPACE MANUAL
13.2 GENERAL PROCEDURES

13.2.1 If an aircraft is unable to continue the flight in accordance with its ATC clearance, a revised clearance shall be obtained, whenever possible, prior to initiating any action. If prior clearance cannot be obtained, the following contingency procedures should be employed until a revised clearance is received:

a) leave the cleared route or track by initially turning at least 30 degrees to the right or to the left, in order to intercept and maintain a parallel, same direction track or route offset 9.3 km (5.0 NM). The direction of the turn should be based on one or more of the following:

1) aircraft position relative to any organized track or route system,
2) the direction of flights and flight levels allocated on adjacent tracks,
3) the direction to an alternate airport;
4) any strategic lateral offset being flown, and
5) terrain clearance;

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 ft or 1000 ft as appropriate to the altitude.