tach vs. hobbs

[FastEddieB]

Hobbs = 1.2 x tach will yeild an incorrect Hobbs time. Why? Because Hobbs is "time the aircraft is in use." Taking a ratio of tach time will NOT yield the total time the aircraft is in use. No simpler way to state it.

Just curious...

...what do you fly and what means do you use to log time?

 

[flyingron]

The Hobbs won't give you the "time the aircraft is in use" either in most cases, just an approximation.
Both the Tach and the Hobbs are approximations.
As far as time-in-service goes, the FAA is happy with either as long as you're consistent.
As far as flight time (for yoru pilot log book), You need something reasonably accurate to the definition (clock time from the time the aircraft first moves under its own power for the purpose of flight until it comes to rest at the destination). 1.2X may work for you. 1.4X may work for you (depends what your power settings end up vis-a-vis the nominal RPM on the tach). The hobbs probably works well for you. Your watch will also work.

 

[SkyDog58]

Hobbs = 1.2 x tach will yeild an incorrect Hobbs time. Why? Because Hobbs is "time the aircraft is in use." Taking a ratio of tach time will NOT yield the total time the aircraft is in use. No simpler way to state it.

Sigh. You really are thick aren't you?

Would you have been happier if he said that is tach time is typically .83 * Hobbs?

I take him at his word that his Hobbs time registers 1.2 hours for each hour his tach reads. Might his tach be inaccurate? Yes. May his Hobbs be inaccurate? Yes. But if he makes a flight and at the end of the flight, his tach has accrued 2 hours and his Hobbs has accrued 2.4 hours than the formula he gave would be correct.

I'm sorry if the math is beyond your level of comprehension. Funny though how everyone else gets it.