Hobbs vs Tach time

[Roger Lee]

Tach time on many planes is affected by rpm. It records less time at low rpm and more time at higher rpm. We spend more time in higher rims flying than taxi time. Most Mfgs ant Hobbs because that is engine run time.  Mak s no difference n what you're doing during that time. There is engine wear, burning of fossil fuel causing build ups of carbon (especially during warm up and taxi low rims),  leading if using 100ll. It is heating and cooling cycles, wear on hoses, and ect... Hobbs is just a more accurate and Mfg accepted point of use on the engine.

[Doug G.]

My Dynon has Hobbs which does not agree with the physical meter.

[Tom Baker]

There are different things that trigger the hour meters to start keeping time. For example the Dynon starts when you have oil pressure. The panel mounted meter can be connected to the master, or some other source to turn it on. For my CT it is tied to the alternator switch. If you don't have it turned on it doesn't run. Oddly enough it is wired through the alternator light, and if the bulb is burnt out the hour meter doesn't run.

[WmInce]

11 hours ago, Doug G. said:

My Dynon has Hobbs which does not agree with the physical meter.

Dynon Skyview's can be edited to agree with external Hobbs meters. That withstanding, they won't stay that way though. Depending on what triggers Hobbs running, there will be some drift between the two.

[Anticept]

I would debate the remark about most manufacturers want hobbs, unless you mean LSA manufacturers. Rotax is the oddball of the bunch. Hell, we don't even track car age by run time, we track by time in use, measured in miles.

Anyways, the fact is, the thing that does the most damage is start up. The oil is thick, there's no oil flow initially to keep the film built up, and the RAPID heating of components. That rapid heatind is why airliners don't care about engine hours for most components. They care about start cycles. (They also track takeoffs and landings, pressurizations, and such individually too).

There is no wear or stress on the airframe either until after the wings generate enough lift. That's why the FAA says TIS is when the aircraft leaves the ground.

I agree to follow Rotax's guidelines about maintaining to run time, mainly because even at idle, the combination of high compression, gearbox, and firing impulses wear out parts faster than traditional engines, but I will never agree to tracking my aircraft hours by it. That's beyond stupid. And if am aircraft manufacturer's airplane is so fragile that the engine running wears out their airframe, then they can keep it.

Roger: I reiterate what I have said before: if your aircraft's tach is running faster than your hobbs, something is wrong. Tach should be set to run 1:1 at your cruise speed. If it's running faster, it's either configured too low or you fly firewalled.