What is an engine hour?

[Radioflyer]

To my way of thinking, an engine hour is Tach Time of the bottom end or heart of the engine. The top end, typically cylinders, are more fungible. I think tach time is ideally more representative of actual engine wear and use. One can also consider Hobbs Time, which is usually some variable percentage (normally ~ 15% ?) higher, but doesn’t reflect relative idle vs high speed use. An engine hour is really not an absolute thing, right? Under ideal service conditions, engine hours can be a useful metric, but in real life…often not. Nevertheless, people often talk about engine hours (especially published TBO) as an absolute thing. Incidentally, I’ve looked at logs with tach and Hobbs used interchangeably.

A 300 tach hour engine has sat for 5 years with only an occasional brief run. That good sounding time is irrelevant, except for hope, with respect to condition. If an engine is opened for inspection after a prop strike, no defects found, and returned to service, it still retains the original service time… legally. To my mind, because of infant mortality or enhanced plastic strain, service time is now not so helpful. Simpler case - if an X time engine has all cylinders overhauled or replaced with (new or used) some hours later, time is still just X, but the number doesn’t reflect the improvement unless you study the logs.

Sorry for the rant, but I’m out of time.

 

[Ironflight]

Back when I started flying and it was common to fly trainers with no electrical systems, we paid for flight time by multiplying recording tach time by 1.3. Of course, there were lots of touch and goes in that time, so it probably did work out close to what a Hobbs would have measured had there been a Hobbs…

 

[PhatRV]

I was listening to one of Mike Busch webinars and I remember he said only the manufacturer can zero time the engine. If the engine is overhauled, cylinder(s) replaced, etc, then the hours continue to count on the same engine. There is no rolling back the hours from the engine log book. I suppose from your example, a 300 hours engine, after a complete overhaul, maybe with 1 cylinder replaced, it will still has 300 hours but in a much better shape than before.

 

[AZ Cactus]

An engine hour is really not an absolute thing, right?

Well it is according to 14 CFR 1.1 definitions -

"Time in service - with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing."

Neither Hobbs nor tachometer record that time but a Garmin G3X Touch system can be configured to do so.

I'm sure this has been debated ad nauseam but you did ask the question.

With respect to engine hours - who cares? If it's running fine leave it alone. People get so excited about 2,000 hour TBO on Lycoming engines but conveniently forget, or never knew, that TBO is 12 years or 2,000 hours, which ever happens first.

 

[Radioflyer]

The question was specifically about engine time, not flight time. Obviously, the engine is logging time even before the plane flies. Yeah, my flight school also charged by Hobb time but used tach times multiplied by some factor when the Hobbs was non-functional. The idea being that since the engine was not running fast in the pattern that the tachometer under-reporting.

“…who cares?” Really?

 

[Rdog420]

What TBO are you trying to get out of? Your experimental. Tach time/engine time TBO is merely a suggestion for you lol

 

[AZ Cactus]

The question was specifically about engine time, not flight time.

The answer was about engine time not flight time. The FAA definition of flight time is very different

edited to add "FAA".

 

[HFS]

In the pre-dinosaur days, we (EABer’s) had different ways of slowing down accumulated engine hours logged. One (sneaky) way was to install a mechanical tach geared for a fixed pitch propeller (2500 RPM cruise), and then with a constant speed propeller you could operate at a lower RPM - say in my Rocket’s case, 2200 RPM. So do the math - for every hour flown at 2200 RPM cruise, would net 22/25ths (.88 hrs). So … over time you accumulated “free” engine hours toward your TBO.

Kinda like the shell game - “Where’s the Pea”.

My FSDO inspector never asked about at the certification inspection, and I sure as h**l didn’t offer it …

HFS

 

[Radioflyer]

The answer was about engine time not flight time. The FAA definition of flight time is very different

edited to add "FAA".

Hmmm, maybe you're right, but I don't think so. There is no specific mention of "engine", but there is reference to "aircraft". My interpretation of that CFR definition that you mentioned is that it refers to the maintenance logs of the aircraft, not the engine specifically. (Most people keep separate airframe and powerplant logs, right?) But by mentioning that CFR definition, you raise one of the points I'm trying to make i.e., there is no absolute meaning to "engine time", at the very least because people use different time standards.

All those start-up times (especially in winters), maintenance ground testing, and taxi times add up meaningfully in the time history of an engine. My brand new engine had at least 1 hour of run time before it was delivered and so my engine logbook starts with 1 hour, actually more before the plane actually flew. I would argue that time and all the other low speed run times (flying or not) are cumulative and count toward engine condition. Another point I was questioning was whether there was agreement that tach time is more representative of "engine time" than say, Hobb time.

BTW, my post was for academic discussion of the relative meaning of engine time. My engine is healthy, running well, and I'm not trying to solve or justify any mechanical issue. However, insofar as I do care about things like oil changes, magneto overhaul times, intake gasket deterioration, hoses, etc, I do care about what is the best representation of engine time. (Admittedly, other maintenance items are perhaps better monitored "on condition".) Also, I do know that as an "experimental" there is no requirement to record times or even keep logs, except for recording the condition inspection, but good luck with that on the resale market.

 

[AZ Cactus]

There is no specific mention of "engine", but there is reference to "aircraft". My interpretation of that CFR definition that you mentioned is that it refers to the maintenance logs of the aircraft, not the engine specifically

It is a definition of the term "time in service", no more, and no less. How time in service is applied to the engine, airframe, propeller, or any installed appliances would be as defined in any applicable regulation.

Maybe a read of AC 43-9C would make it clearer. It includes -

(1) Section 91.417(a)(2)(i). Requires a record of total time in service to be kept for the airframe, each engine, and each propeller. Part 1, § 1.1, Definitions, defines time in service, with respect to maintenance time records, as that time from the moment an aircraft leaves the surface of the earth until it touches down at the next point of landing.

Please understand that I'm not telling anyone how their engine, airframe, or propeller time should be logged. I'm only saying that FAA has defined what they expect to be recorded.