I've heard that 1 tach hour = 1 Hobb's hour at maximum RPM. I've also heard that tach equivalency is at "cruise" RPM, although that seems vague.

I believe I've also heard that the accuracy of tachometers is regulated (I think to within 50 RPM).

Because engine time, and often TTSN, is measured by tach hour, there surely must be a strict standard for each aircraft for what RPM the tach counter should be set to.

What is the standard for tachometer calibration? Max RPM? Cruise RPM? Something else?

  • 1
    $\begingroup$ FWIW, I didn't notice the faa-regulations tag and that means I closed your question immediately. I still think it's probably a dupe but if you disagree please let us know what isn't already answered in the other question and we can re-open. $\endgroup$
    – Pondlife
    Jan 11, 2019 at 6:07
  • $\begingroup$ FAA regulations only have Air Time and Flight Time. We all have a clock or watch so we can accurately record time for the logbook. We are all lazy though, and that is why there is such a reliance on Tach Time or Hobbs Time. I voteted to reopen as this does pertain to FAA Regulations. $\endgroup$ Jan 11, 2019 at 7:50
  • $\begingroup$ FAA Advisory Circular AC 43-9C points out the legalities of using Tach Time or Hobbs Time. faa.gov/documentLibrary/media/Advisory_Circular/… $\endgroup$ Jan 11, 2019 at 8:11
  • $\begingroup$ @MikeSowsun: Thanks for the link. I couldn't see anything about how it must be calibrated; did I miss something or do you know of any regulations that specify this? $\endgroup$
    – Zaz
    Jan 11, 2019 at 18:14
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    $\begingroup$ Voting to reopen. The "dupe" question doesn't provide an answer beyond "at cruise RPM" which the OP already acknowledged. The question is asking for more definition & depth than that, along with perhaps a source for that "at cruise RPM" specification. Seems like a legit new question, definitely more specifically focused than the other one. $\endgroup$
    – Ralph J
    Jan 11, 2019 at 21:27

1 Answer 1


Hobbs time is most useful for tracking log time (and billable time in rentals.)

Tach time is simply a counter of revolutions of the engine, displayed as "hours." Calibration is basically at normal cruise RPM, which can vary for different engine/prop/desired speed. The tach counter is approximate at best, but it is what we have.


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