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An aircraft flying under IFR in controlled airspace and maintaining FL150 requests a descent. – A second aircraft maintaining FL140 on the same route also requests a descent and receives the clearance "descend to FL70, report passing FL100". – The aircraft at FL150 can expect to receive a clearance for descent to FL140 or below when the aircraft at FL140 has..
  • A
    descended through FL130.
  • B
    reached FL70.
  • C
    left FL140.
  • D
    reported that it has left FL100.

ICAO Doc 4444
5.3.4 Vertical separation during climb or descent

5.3.4.1 An aircraft may be cleared to a level previously occupied by another aircraft after the latter has reported vacating it, except when:

a) severe turbulence is known to exist;
b) the higher aircraft is effecting a cruise climb; or
c) the difference in aircraft performance is such that less than the applicable separation minimum may result;

in which case such clearance shall be withheld until the aircraft vacating the level has reported at or passing another level separated by the required minimum.

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