Communication FailureClearances for departing aircraft may specify an initial or intermediate level other than that indicated in the filed flight plan for the en-route phase of flight, without a time or geographical limit for the initial level. Such clearances will normally be used to facilitate the application of tactical control methods by ATC, normally through the use of an ATS surveillance system. Where clearances for departing aircraft containing no time or geographical limit for an initial or intermediate level are utilized, action to be taken by an aircraft experiencing air-ground communication failure in the event the aircraft has been radar vectored away from the route specified in its current flight plan should be prescribed on the basis of a regional air navigation agreement and included in the SID description or published in AIPs.
Source: ICAO doc 4444
See also:
Air-Ground Communications Failure
Communication Failure
Failure of Equipment