I guess if I was doing it and was trying to include everything I could and be as fair as possible I would use last years scheduled mx bill / hours flown. It’s not going to be perfect but it’s a pretty reasonable attempt. Obviously If I flew 50 hours last year and this year I am doing 600 hours of cost sharing flights then that won’t be right, but I am talking about trying to COMPLY with the spirit of the law, not trying to twist it to my advantage.
So you pay your maintenance fee after each flight?
trying to COMPLY with the spirit of the law
You need to comply with the law as it is written, no more.
Another source is the CAA doc discussed before CAP1590
Same non exhaustive list.
These authorities do have lawyers available. OK; their calibre will vary, but they all know how to write an exhaustive list. Yet all have chosen to not do that.
I can’t find it for some reason, but last time read about what can be included if the owners let’s you fly his/her aircraft:
- Fuel – yes
- engine/prop o/h hourly cost – maybe
- 50/100h labour costs – maybe
- cost of annual, any capital costs, etc – no.
Yes; that’s my view also.
It’s not hard to work out what costs are related to the current flight.
All the rest of the discussion is centred on complying with somebody else’s interpretation of somebody else’s interpretation of some guidance publication 
For example a “50hr check” is a 50hr check i.e. you do it every 50hrs, so it is an hourly cost.
Same with the engine fund – in any jurisdiction where there isn’t an engine life limit. That’s a clear hourly cost also.
What if your engine is operated on condition (over TBO limit)?
I am convinced direct cost is fuel, oil and airfield charges. Projecting future maintenance or engine overhaul costs is not part of them.
How this is applied in reality is a completely different matter. My answer is really only for establishing in principle how the law is directed, as it personally is of interest to me.
