Engine Reserve -- Hobbs vs Tach?

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brandonpalmer
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Engine Reserve -- Hobbs vs Tach?

Post by brandonpalmer »

Hi Diamond community.

Do plane owners generally account for engine / propeller reserve based upon Tach or Hobbs? I know that the actual overhauls are based upon tach but do people generally use the higher hobbs number in their calculators to cover for the "oh crap" issues vs tach for the actual overhaul need? I've been reserving based upon Hobbs but that's overkill by almost a third for my real-world hobbs to tach ratio.

Opinions?

Thanks everyone!
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Colin
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Re: Engine Reserve -- Hobbs vs Tach?

Post by Colin »

Tach, for me at least.
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Gasser
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Re: Engine Reserve -- Hobbs vs Tach?

Post by Gasser »

Tach/flight not Hobbs.
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rwtucker
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Re: Engine Reserve -- Hobbs vs Tach?

Post by rwtucker »

I quit using Hobbs. For one thing, there are too many ways on most aircraft for the Hobbs to pick up 100 hours when your engine isn't running. On the Diamond, all that takes is leaving your map left light on until the battery runs down to a certain voltage.

Separately, it sounds like a part of your question goes to finances. If so, you could use either one. There aren't that many mandatory exact hours requirements. An engine rebuild, the biggest expense, has no hours requirements. It is based on the engine's health (at annual or any time you get an indication). I know some O-360 engines that are still passing compression checks at 2,900 hours.
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Re: Engine Reserve -- Hobbs vs Tach?

Post by Lance Murray »

HOBBS is for flight school charges, rental charges, Pilot log entry etc.

TACH is for Airframe, Engine and Propeller maintenance time tracking.

There have been situations with a low battery that allow the HOBBS to run without the engine running.

One nice feature of the older pre-G1000 aircraft is that
TACH does not start running until RPM is above 1300. That means taxi and warm up is free if you are careful with rpm. I don't remember if G-1000 is similar. My Hobbs has something like 500+ hours more than my TACH time. This was a previous flight school airplane that likely spent a lot of time sitting in the run-up area training students and holding for take off. Over the engine life that is 500 hours of extra revenue with little cost associated.
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