Below are extracts from both the AIP and JEPPS. They both refer to calls made associated with a frequency change when descent has already commenced. Some people now take this to mean that any time you leave an assigned level, the correct terminology is "leaving". When I am on centre maintaining one level whilst assigned another, should I say "leaving" or "left" when I start descent? I am after specific references either in the AIP or JEPPS for commencing descent in controlled airspace. I am not after opinions or English lessons, only specific references for this single case whilst still on the centre frequency that assigned the lower level.
'After any frequency change, pilots must advise the last assigned level and, if not maintaining the assigned level, the level maintaining or last vacated level: eg, "MELBOURNE CENTRE (CALLSIGN) CLEARED FLIGHT LEVEL TWO ONE ZERO, LEAVING FLIGHT LEVEL TWO NINER ZERO".'
Jeppesen
Air Traffic Control (page AU-705)
DESCENT AND ENTRY
1.9.1.4
eg: "MELBOURNE CENTER (CALL-SIGN) CLEARED FLIGHT LEVEL TWO ONE ZERO, LEAVING FLIGHT LEVEL TWO NINER ZERO"