Well, lets ask the controllers here (rather than AIM, which we all have access
to). On a cruise clearance, for example Cruise 6000, you observe (mode C) an
aircraft has descended from 6000 to 4000. There is another aircraft which
could use the 6000 altitude. You've received no communications from the
aircraft on a cruise (except the acknowlegement of the clearance). Do you
consider the 6000 foot alititude vacant?

Same question, but the cruise aircraft has reported descending to 4000. Has
his report of a descent vacated the 6000 foot altitude? (no bobbing back up)

Same question, but instead of a cruise question, it's a "descend at pilot's
discretion to 2000". The pilot acknowleges the clearance, and later is
observed at 4000 feet, having not reported the descent.

Same question, same airplane, but in this case the pilot reported descending
from 6000 to 4000.

So, in which cases does 6000 no longer belong to the pilot, in your eyes and
actions?

Jose

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