FAA Code of regulations
Flight time means:
(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing; or
(2) For a glider without self-launch capability, pilot time that commences when the glider is towed for the purpose of flight and ends when the glider comes to rest after landing.
This is exactly the same as the UK. The manufacturer stipulates the the maintenance cycle and how it should be calculated.
The Aircraft Tachometer
The mechanical 'tacho' measures the rotational speed of the engine drive shaft. It can be optimised how you require it to be. Speed = distance/time. It is usual for this calibration to be optimised circa a speed of 2300rpm (rotations per
minute). With most light aeroplane operations the optimum cruise rpm is commonly operated around this figure. Of course, for the time spent on the ground the rpm will be much less. For flights of 1-2 hours the tacho hour will only equate to around 50 minutes or 0.8 of an hour owing to the ground time much lower rpm. For longer flights the tacho hour and clock time become very close to being matched. If you operate at much higher RPM then this match will also be true even for shorter flights.
The rpm also = engine care. Over speeding the engine can cause severe damage and for this reason monitoring the engine speed in accordance with the engine limitations is crucial. Too low and this can lead to plugs fouling which can also be a cause of engine failure.
Some electronic tachometers accumulate elapsed hours from engine start to engine stop with no option to base hours on rpm.
There are units that allow you to chose the clock or switch to the tacho source. RPM is always critical. The above is not a tachometer.