Originally Posted by
Shaft109
Engines -
Sorry for the intrusion here - I'm having real trouble understanding the logic to buy less engines than airframes. OK a small flying school wouldn't have complete spare engines knocking around, and an airline will have alternative arrangements for a large podded engine or 3.
But surely the slight expense of having say 35 complete engines (not withstanding the fact a specific weakness as mentioned across all of those engines) stored ready to swap would be a nice hedge. Especially for Military flying training where the usage profile is much more aggressive.
My mental hourglass icon is on overdrive. It just doesn't compute.
In VERY simple terms, there is a formula that is used to calculate Depot Stock; called, imaginatively, STOCKCAL.
It uses Number of Aircraft, Fit Policy, Flying Hours, Repair Pipeline Times, Recovery Rate at 1st Line, Recovery Rate at 2nd Line, and Mean Time Between Removals (not Failure, as it is the act of removal that places the demand on the spares) to determine the Depot Stock level. This, in addition to laid down spares levels at each unit, which is a far simpler calculation based on aircraft numbers, role, and MTBR.
Numbers/ Fit Policy/Flying Hours/Times are dictated by the Centre for each year. Last time I looked, Recovery Rates were required to be at least 8% and 68% respectively; the latter determining the scale and scope of 3/4 Line contracts. It can be seen that maintaining the integrity of STOCKCAL (the primary role of any Support Authority), for any given inventory item, is utterly crucial, as it is inextricably linked to, for example, manning and funding in almost every area of aircraft operations. Almost any In-Service Support problem can be resolved by applying it.
It would be interesting to know if this mandated policy is reflected in the outsourced support contracts that now dominate military support. I somehow doubt it. Lacking knowledgeable Service input, the figures are too easy to manipulate. That is why scrutiny of all requirements must be carried out from the User's point of view; another widely ignored mandate.
As I said, a deliberately simplistic overview.
Edit: Sorry, I really should have said that if the Fit Policy is not Full Fleet Fit, and there is only one item per aircraft, and a single unit/squadron at one location, then it will not be unusual for the total number of items to be less than the number of aircraft.