NORTH ATLANTIC OPERATIONS AND AIRSPACE MANUAL
CHAPTER 5: OCEANIC ATC CLEARANCES
5.1.8 If any of the route, flight level or Mach Number in the clearance differs from that flight planned, requested or previously cleared, attention may be drawn to such changes when the clearance is delivered (whether by voice or by data link). Pilots should pay particular attention when the issued clearance differs from the Flight Plan. (N.B. a significant proportion of navigation errors investigated in the NAT involve an aircraft which has followed its Flight Plan rather than its differing clearance).
5.1.9 Furthermore it must be recognised that if the entry point of the oceanic route on which the flight is cleared differs from that originally requested and/or the oceanic flight level differs from the current flight level, the pilot is responsible for requesting and obtaining the necessary domestic re-clearance to ensure that the flight is in compliance with its Oceanic Clearance when entering oceanic airspace.
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