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Over in this answer there's a description of Hobbs Time versus Tach Time, and how the Hobbs counts hours while the tach time is equivalent to "hours at cruise RPM".

How do these two "clocks" work, and what do these numbers mean in practical terms for flying and maintaining an aircraft?

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Hobbs Time

In most planes, the Hobbs clock is started and stopped based on an oil pressure switch, so it starts when the engine starts, and stops when the engine is shut-down. While it's running, it just ticks off a tenth of an hour every 6 minutes, based on "regular wall clock time". So a tenth of idling on the ramp is the same as a tenth at cruise.

Tach Time

The tach clock isn't really a clock at all, it doesn't actually measure time, it really measures engine revolutions. But it's calibrated such that a tenth of an hour of tach time is clicked off when the engine is at cruise RPM for 6 minutes. In other words, if the plane is at cruise RPM, the tach clock will be clicking off tenths of an hour at the same rate as the Hobbs clock, and the same as the watch on your wrist. But if the engine is idling at an RPM speed that's half of what cruise RPM is, then the tach clock will be running at half the speed of the Hobbs clock.

Relation

So, for the tach clock, less "time" is clocked when the plane is idling on the ramp, or flying at low RPM. For short flights (where ramp idling time is a significant percentage of total time), and flights where you're doing a lot of pattern work (and thus operating at low RPM), tach time will be significantly less than Hobbs time.

Maintenance

When an engine is replaced or overhauled, that engine is considered to be "zero time". Whether they actually reset the tach gauge to 0 when the engine is overhauled, or if there's a way to put a time on a brand new tach or Hobbs meter when the old one dies and is replaced, I dunno, but in general, I'd assume that the aircraft's engine and airframe log books are really the official recording of the progression of airframe/engine times, so I don't know that it's particularly important whether the tach is set to the engine's time, or if it's really just used as an incremental meter.


AOPA has a nice (and long) article about this.

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  • $\begingroup$ "When an engine is replaced or overhauled, that engine is considered to be "zero time"." This is not true. Only certain overhaulers can zero-time an engine. And to do so, the overhaul quality must be to certain exacting standards. For example, a top overhaul (usually just changing one or more cylinders out for new or refurbished cylinders) is not a zero time overhaul. $\endgroup$ – Skip Miller Jun 22 '14 at 15:59
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    $\begingroup$ In a real zero time overhaul, neither the Hobbs nor the tach revolution counter is reset to zero, although they can be if the owner prefers. A log book entry is made to record the time the engine is put back into service and engine is computed as time on the meter minus the return-to-service time. $\endgroup$ – Skip Miller Jun 22 '14 at 16:00
  • $\begingroup$ Is a 2000 hour engine overhaul based on time measured on the Hobbs or on the tach meter? Same question for the the 50 and 100 hourly. I am asking because flight schools charge on Hobbs time which often entails long taxis and I'd like to work out what the real costs of operating the aircraft are. $\endgroup$ – VH-NZZ Apr 12 '18 at 9:18
  • $\begingroup$ So when you're running an engine at max power for takeoff/climb, the tach time will run faster than Hobbs time? Or is tach time really calibrated to max power, and even cruise would run slower than Hobbs? If it really is cruise power, which one? My POH lists three cruise RPM settings. $\endgroup$ – StephenS Jan 11 at 5:09
  • $\begingroup$ @StephenS Answer 1: yes; Answer 2: it is calibrated to cruise RPM, not max power; Answer 3: you've to find out in your POH which cruise RPM is calibrated to tach. $\endgroup$ – Farhan Jan 11 at 12:24

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