Group tacho flying
In our Group we pay by tacho time. I think our tacho records one hour for full-throttle running for an hour, so 70% rpm will cost 70% of an hour. We record these tacho times and chock-to-chock on our booking in/out sheet.
Many years ago we had CAA approval to use the tacho times as equivalent to airborne times for the purposes of the aircraft log book, but when we changed maintenance organisations a while back they wanted to see a separate tech log with actual airborne times recorded in it, so we keep the two systems running side by side; a bit fiddly but with only one a/c, not a disaster. In fact, looking long-term, with a full-power tacho and sensible cruise power settings, the two still run pretty close.
We also like this tacho arrangement because it encourages people to set a reasonable cruise power; I always felt that the Hobbs encouraged people to thrash the engine. Most of our members want to enjoy watching the countrysidee slip by, not see how quickly they can get from A to B, so the tacho as we operate it suits their inclination and their wallet.
The other organisation where I work use neither system; one records the airborne time, then add 6 mins either side for the cost of the flight. This is regardless of having spent 30 mins explaining how the checklist works after engine start and before taking off. Quite a good deal for the customer, really. I've become adept on trial lessons at being airborne for EXACTLY 48 mins!
TheOddOne