SHAR
I recognise what Engines talks of, although use different terminology. I don't know about the RAF, but the RN broke fleets down into FAE (Front Line Aircraft Establishment), TAE (Training), IUR (In Use Reserve) and IR6 (held at 6 months readiness - some argued 6 weeks). Some fleets had Half-TAE. Each category was allocated an average annual flying rate (say, 300 for FAE, 150 for TAE, and so on) from which one calculated support requirements. As Fleet Manager, Engines will recall numbers better than I, but I do know there was a lot of flex in the SHAR fleet because these Establishments were actually met; whereas, for example, Sea King AEW Mk2 only had FAE and TAE (so were always 6 cabs down; partially corrected by Mk7). The practical difficulty (for me) was that the ratio of FAE to Whole Fleet meant a full fleet fit of all avionics was very expensive, so I'd always use the highest number Active Fleet number the FM could come up with. (His best day in the past year). When the attrition buys were made post-Falklands, we weren't allowed radars for example. The remaining 54 had to do the job of 70 for most of the FRS1's life. A 6 month deployment at sea left precious little ashore. Just different ways of looking at it.