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There is a revolution counter built into the face of the Tachometer in most piston powered GA planes. This counter is calibrated in hours. Most planes also have a Hobbs meter, which is a clock.

Only at a certain RPM are tach hours equal to clock hours. This equivalence rpm is different among the various manufacturers and models, and it is published somewhere. For a Piper Archer II with the standard Lycoming O-360-A4M engine, what is the equivalence RPM, and in what document is it found? It is not in the Type Certificate Data Sheet (TCDS) for the Archer II (PA-28-181).

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As far as I know the only place to get this information is the tachometer specifications (or at least I know Mitchell provides it for their tachometers).
As you noted, it's not on the TCDS, nor have I ever seen it in the (Piper or Lycoming) maintenance manuals. (It is of course possible I've missed it in the maintenance manuals - they're pretty thick books.)

If you are flying around in a plane with a working tachometer and don't know the hour-meter's equivalence speed you can always determine it with a watch: every 6 minutes is 0.1 hours, and when the tach is recording at that rate you've found the equivalence RPM.
(For the Archer I assume it's the same as my Cherokee 180 - around 2300 RPM.)

It's worth noting that while you would think the tach's hour meter would be "accurate" (1:1 correspondence) at typical cruise powers that is often not the case!
Using the Archer as an example, 65% power cruise at 7,000 feet is around 2500 RPM, so the tach hour meter will be running slightly faster than 1:1 in a 65% power cruise at 7,000 feet.

The watch-check I described above doesn't help you if the reason you need to know this information is your tach has died and you need a replacement - fortunately tachometer manufacturers all know this and publish recommended applications for their tachometers.
Mitchell lists their D1-112-5023 (2300RPM) tach as the appropriate replacement for the PA28 family, which matches up with the watch-test on my Cherokee.

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  • $\begingroup$ @Vortaq7: Thanks for the answer. I am not ready to believe that the airplane manufacturers or the engine manufacturers do not specify what the equivalence rpm should be, and leave this up to the Tach manufacturers. In this highly regulated industry, that doesn't seem likely. Basically, that's why I asked the question... So again, thanks for the answer and let's see if somebody has better detail on this one. $\endgroup$
    – Skip Miller
    Jul 29, 2014 at 13:12
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    $\begingroup$ @SkipMiller I agree this seems like something that should logically be in either the airframe or engine service manual (and I was pretty baffled when my mechanic and I couldn't find it except by looking at the specs of "approved replacement" tachs). I'm hoping someone comes along and says "You can find it over here!" - it would be useful to know where to look for a general reference! $\endgroup$
    – voretaq7
    Jul 29, 2014 at 15:45

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