Hobbs

That is interesting. Does the FAA specify tach vs. oil pressure Hobbs,
vs. electric Hobbs?

Think of how many pilot logbooks are based on electric Hobbs time.


Neither is correct to the letter of the regulation, both are acceptable
to the FAA.

Pilot time is the real time from the time that the aircraft first moves
under its own power for the purpose of flight until it comes to rest at
the destination. The oil pressure time is pretty darned close (I
assert the aircraft moves forward a tiny bit as soon as the engine
starts and isn't really at "rest" until it stops). Unless you leave
the master on for a long time in a prolonged preflight, the difference
is probably within the tenth of an our accuracy of the unit.

For maintenance, it's time in service. Frankly, even putting the hobbs
on a gear switch is acceptable to the FAA (saves you some time if you
operate out of places like Dulles where you can wait / taxi for a long
time before taking off).