I know that a lot of PPLs use engine on/engine off instead of off blocks/on blocks for their personal logbook.
Reason? You've got to record engine on/engine off and true flight time in any case for aircraft logbook purposes.
Furthermore, the operator may charge you for any of the following, depending on the operators policy:
- Engine on/engine off time, usually measured with a hobbs meter connected to the engine master (if FADEC equipped) or oil pressure sensor.
- Engine tacho time. Note that a tacho runs proportional to your RPMs, so taxi time is cheap.
- Flight time, usually measured with a hobbs meter connected to the pitot/static system or a pressure-on-wheels sensor.
Or any of the above plus or minus a taxi allowance.
So what goes into your personal logbook, what goes into the aircraft logbook and what the bill is based on, is normally different. Only if you have a hobbs meter connected to the engine master or oil pressure sensor can you use this as a reasonable number to put in your personal logbook.