BmPilot21 - The definition of having left an assigned level is when the aircraft`s altimeter indicates that the aircraft has actually departed from that level and is maintaining a positive rate of climb or descent in accordance with published procedures.
I take published procedures to refer to the requirement to inform ATC if the rate of climb or descent is or will be less than 500fpm when above the Transition Altitude.
I would echo Standard Speed`s warning because CAP 413 (RTF Manual ) is rather vague on this matter. It seems to imply that the words "leaving" and "left" have the same meaning which clearly they do not.
I would like the phraseology to be amended along the lines of;
ATC- "XXX descend flight level 100 report flight level 110 vacated."
