As a radar controller, can you clear an aircraft to a level previously occupied by another one in a way that you don't have 1000ft vertical separation between target Mode C indications as long as the following is fulfilled?

The question stems from the following paragraph in ICAO Doc

"Aircraft vacating a level. An aircraft cleared to leave a level is considered to have commenced its manoeuvre and vacated the previously occupied level when the pressure-altitude-derived level information indicates a change of more than 90 m (300 ft) in the anticipated direction from its previously assigned level."

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    $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    Jan 20, 2023 at 13:18
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    $\begingroup$ This question is way too broad. $\endgroup$ Jan 20, 2023 at 15:55
  • $\begingroup$ Good edit - that narrows the scope of the question very nicely. Thank you for focusing your question! $\endgroup$
    – Ralph J
    Jan 22, 2023 at 3:10

1 Answer 1


No, you still need vertical separation: 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.

  • $\begingroup$ What if the aircraft performances are similar and the reason is that the other one climbs/descend faster? To my understanding, in a non-radar procedural environment, there is no way of knowing if the actual 1000ft separation exists but you can clear the other one to the level that the other one reports vacating. This would mean that you can be more flexible without a radar than with a radar. $\endgroup$ Jan 22, 2023 at 8:21

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