logging
The CAA accept three systems for maintenance logging.
a) Actual airborne times. 'Wheels off - Wheels on.
b) Start up - shut down, minus 10 minutes (0.2hr) for ground time ( more correctly from the moment of taxiing to take off to the brakes on following a landing.
c) Actual tacho readings. Start to finish of each flight.
Take your pick.
We find that in the way that we operate b) makes us about 10% better off over the year. Tacho would be the best and simplest but for schools would appear to be too expensive when setting charges. Most groups that I know use tacho hours. For more complex types, particularly if turbocharged, tacho is used by ourselves to avoid rushing the warm up and to encourage cooling time when closing down.