Hobbs time definition, a moral question ?

I have a new Flight Data Systems tachometer in my 3B/0320. I have read the discussion re: tach vs. hobbs. This instrument has two options, time starts at start-up ( ie. Tach Time ) or ? what is your normal cruise RPM ?. The latter calculates ?time? relative to actual Revs / ?chosen? revs at cruise RPM.

I cruise at 2300 because I have no reason to go faster. 65% power is another number controlled by prop selection & map. Some guys cruise at 10 K /2700. So the question is, if you were to buy my airplane, what would you consider a meaningful Hobbs number ?

Actual time starting at start up is Hobbs, not tach. The time ratioed by the actual rpm/nominal rpm is tach time.
As to re-sale expectations, I would think either is okay, if the buyer understands the ?time? being quoted.

If I understand your explanation, you have the option of setting it so that the hobbs slows down at less than whatever you set as max rpm?

If I've got that right, then it's no different that how most certified airplanes have been setup forever; Log time and charge airplane rental by Hobbs time that is basically a timer that starts ticking when you turn the master on, but do inspections and count airframe/engine hours by a tach that slows down whenever you pull the power back.

Fine point: Hobbs meter is usually enabled by an oil pressure switch, isn’t it? Master on doesn’t by itself make the Hobbs accumulate time.

Most production singles with a mechanical tach record engine hours as related to engine speed. So for example, an O-360 with a 2700 rpm red line would accumulate 1.0 hrs TACH time if it were to run at 2700 rpm for one hour. Power settings less than that would accumulate tach time at a slower rate. Most flight schools and charter operators will use a hobbs meter that runs on either oil pressure or master switch power for billing purposes, and conduct maintenance using the tach time. Generally speaking it is about 1.2 to 1.
I don't feel that there is anything illegal or unethical about this practice. As far as logging pilot time, once the engine is running you're on the hook with the Fed's for "operating" the aircraft, whether in flight or on the ground, and I consider this legitimate logable time.
My AFS 5600 system displays both "hobbs" and tach time, I strictly use tach time for all of my aircraft record keeping purposes.

DrillBit said:

Fine point: Hobbs meter is usually enabled by an oil pressure switch, isn?t it? Master on doesn?t by itself make the Hobbs accumulate time.

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Usually yes, but it can be either way. There are other ways to do it as well. I seem to remember flying some twins that had the hobbs wired to the weight on wheels switch so it didn't start ticking until you broke ground, but I couldn't tell you what airplane that was.

Thanks to all, that helps.

I will load 2,700 RPM for 'Tach time" base assuming TBO is based on the potential to run 2700 all the time.

Desert Rat said:

Usually yes, but it can be either way. There are other ways to do it as well. I seem to remember flying some twins that had the hobbs wired to the weight on wheels switch so it didn't start ticking until you broke ground, but I couldn't tell you what airplane that was.

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And that twin would be counting the hours in the exact way the FAA defines.

From the FAA definitions page -

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 down at the next point of landing.

However, that method is hard to implement...

Time

part 135 operators almost exclusively use a Hobbs switch either on the gear strut or an air switch. This allows optimizing TBO limits on engines and other components.
It is not correct that a recording tach in an airplane with 2700 rpm red line will record slow at less than 2700. There are various approved tachs available that will record at slower speeds.
Block time is not recorded when the engine(s) are started but when the aircraft starts to taxi with the INTENT to fly. Ends at the end of taxi.

jrs14855 said:

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Block time is not recorded when the engine(s) are started but when the aircraft starts to taxi with the INTENT to fly. Ends at the end of taxi.

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I think that is mixing up flying time (the intent to fly bit) with maintenance time. Taxi time to the fuel pump doesn't count as you say. :^)

See my FAA definition for Maintenance Time Records earlier.

When I purchased my RV7A the difference between Hobbs and Tach time was over 200 hours. Looking at the log books showed both ?tach and Hobbs hours? were being used for tracking purposes (depending on who did the work and log book entry) but often not on the same entry. Therefore, there were many entries where over a hundred hour differences in recorded maintenance entry time bounced back and forth. Not a good thing since often they were not identified as either Hobbs or Tach time.

Further investigation made me realize the Hobbs meter was wired to the battery switch so anytime the battery was turned on the Hobbs meter was running. Tach time was based on GRT EIS/WS data when ever engine RPM exceeded 600 RPM. A much more useful measurement.

Fortunately (or unfortunately depending on your perception) the RV had its engine completely torn down, inspected and rebuilt ten hours prior to my purchase by a major well known engine repair facility due to a prop strike by the original owner while landing at AirVenture. The builder/owner was too elderly to be flying as PIC (my opinion) and his age compounded several build quality issues that contributed to the prop strike (again- my opinion). The build quality issues have since been addressed.

The rebuild gave me a lot of information about actual total hours and hours since rebuild. This information corresponded with TACH time and also illustrated the significant growing difference between tach and Hobbs time.

After purchase I decided to bring the two tracking methods into sink and installed an oil pressure switch and purchased a new Hobbs meter. Before installing the new meter I ran it on a 12 volt source so that indicated Hobbs hours matched TACH time and then installed the new meter. Of course an appropriate log book entry was entered to record this change out.

It?s been almost three years and over 200 hours usage since I did this. The difference between the two tracking methods now is just over one hour. My log book entries show TACH, Hobbs and Prop time (A new prop was installed at time of rebuild). I use the Hobbs meter primarily for pilot log book time since its readily visible after engine shutdown.

I like having both methods. And quite frankly I trust Hobbs meter time a lot more than EIS/EFIS tach time because it?s a lot harder to manipulate a Hobbs meter than adjusting TACH time. In today?s modern electronics, tach time can be changed by a simple data entry change. Besides - who wants to track how long the battery switch was turned on?

FWIW: I use "Tach" time for all the entries into the airplane's logbook. I use "Hobbs" time for all the entries into my logbook. I keep them consistent to avoid confusion. YMMV

jrs14855 said:

part 135 operators almost exclusively use a Hobbs switch either on the gear strut or an air switch. This allows optimizing TBO limits on engines and other components.
It is not correct that a recording tach in an airplane with 2700 rpm red line will record slow at less than 2700. There are various approved tachs available that will record at slower speeds.
Block time is not recorded when the engine(s) are started but when the aircraft starts to taxi with the INTENT to fly. Ends at the end of taxi.

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Confirmed. I'm a maintenance director for a flying club (actual non-profit, not a "club" in only name). Our 172's tachometers all run 1 tech hour when run at 2500 rpm for 1 hour of hobbs time, while the Piper Archer runs 1 tach hour when run at 2300 rpm for 1 hobbs hour. This is the way they came from the factory.