Type of Time Used for Inspection

[Stephan]

I tried searching to no avail ..

Is "in service" time used to determine when an inspection is due?

[mhoran]

Yes, we use the total time in service on the G1000 engine screen, which was annoyingly reset to 0 a few years ago during an MFD swap. Some users on the forum don't even have Hobbs meters!

[Rich]

Traditionally "tach time" has been used, rather than Hobbs. Largely this has been because Hobbs meters were not commonly installed in aircraft and older tach time recording really was a count of engine revolutions. This is not really true of our newer planes. The practice continues, however, as tach time is almost always at least somewhat lower than Hobbs, so those hour-based maintenance items are a bit further apart in calendar time. For example, for most flights I make, the Hobbs elapsed time will be .2 hour more than tach time. Since I make lots of fairly short flights, overall my total tach time is now 76% of the Hobbs value. Conversely my Hobbs reads 30% higher than the tach.

[Karl]

Most aircraft I have worked on the Hobbs is connected to an oil pressure switch so should record same a tach hours.

Diamond maintenance is in flight hours, so take off to touch down.
Engine run hours are not generally used for maintenance events in GA.

[Rich]

Most aircraft I have worked on the Hobbs is connected to an oil pressure switch so should record same a tach hours.

Diamond maintenance is in flight hours, so take off to touch down.
Engine run hours are not generally used for maintenance events in GA.

The first statement is incorrect, though the remainder is accurate. Mechanical techs really are like odometers, they really record engine revolutions and are calibrated to record real time at a specific RPM, not far from redline. So when you’re sitting there at 1000 RPM or taxiing at maybe 1200 they do record, but at a rate slower than real time.

The VM1000 records real time at or above 1500 RPM regardless of higher RPM. Lower than that it records no time at all. So startup, taxi, waiting for release and even on final approach and landing it records nothing. Meanwhile the Hobbs ticks clock time as long as there is oil pressure. That is why on a flight where I’m in the air for an hour the Hobbs will tick off 1.2 hours and the tach 1.0. Almost every single flight is like this. The difference is startup, taxi out, any holding for take off and the landing and taxi to tie down or hangar. It is likely the G1000 has analogous behavior to the VM1000, perhaps based on airspeed or groundspeed.

Flight time logging in the US for purposes of pilot experience was changed some years ago such that it is closely consistent with Hobbs time. ForeFlight behavior goes to some lengths to reflect the US flight logging requirements and winds up in exact agreement with the Hobbs roughly 80 percent of the time that I make an actual flight. If they disagree it’s almost always 0.1 hour. Again, tach time will lag almost always exactly 0.2 hours each flight.

[Davestation]

Mechanical tachs are basically a measurement of duty cycle, this makes a lot of sense. A flight school runs its planes a lot harder than a private owner fir example, and this is not captured on the hourmeter. Cop car odometers have the opposite problem, as much of their engine run time is at a standstill, so they may install hourmeters to plan maintenance.

In the DA40, the hobbs is based off of oil pressure (engine hours) and the tach runs off an airspeed switch (approx. air time). If you used the hobbs, you’d be doing a lot more maintenance than someone with a mechanical tach because their tach is running A LOT slower during taxi.

In the twins, the hobbs is whatever the installer wanted it to be, because none were installed at the factory. The tach is simply a squat switch.

Tach time is imperfect, but for all intents and purposes, it’s “close enough”. Doing maintenance off the hobbs would be just plain cruel.