I look to our maintainers for this. With our Lycoming powered aircraft, they require airborne time for the airframe and engine, which is recorded in the airframe and engine log books and counts towards the Time before Overhaul for the engine.
Curiously, our microlight maintainer (for the C42) insists that engine and airframe wear should be counted from startup to shutdown, so quite different!
Now we come to the thorny issue of what do we charge against? With the C42, it's easy, start to stop, as recorded by the timer powered by the master switch.
I've been in groups where charge is made against the tacho, which encourages everyone to use sensible power settings. The problem here is that the price is apparently higher per hour, which requires the group members to understand that for every hour in their log book (chock to chock) they'll only be paying .8 or so. With a school or bigger Club, people won't understand that, so we have a chock to chock price. We also need a mechanism to stop people claiming that they flew for less time than they did so they don't have to pay as much. Thus, we have a timer in the Lycoming aircraft, NOT the tacho, which is activated by the master switch. They pay against this, whatever they might say about 'I was only 50 mins' when the timer shows an hour. We use the tacho time for maintenance records, acceptable to our maintainers.
Nothing is ideal here. In the States, I've heard of commercial pilots only being paid for when the undercarriage is retracted, leading to the inevitable late extension and expensive and noisy scrape along the runway.
TOO