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Hobbs Meter or Tach for Hours

bob1393

Active Member
Is there any consensus in the RV12 community as to whether we should use the hours shown on the Hobbs meter vs. the hours shown on the tach for oil changes and Rotax 100 inspections? Checked mine today and we had 288 hours tach time and 389 hours Hobbs time. We've been doing our oil changes and 100 hours inspections based on Hobbs. Maybe we're spending too much money by not using the tach time?
 
Is there any consensus in the RV12 community as to whether we should use the hours shown on the Hobbs meter vs. the hours shown on the tach for oil changes and Rotax 100 inspections? Checked mine today and we had 288 hours tach time and 389 hours Hobbs time. We've been doing our oil changes and 100 hours inspections based on Hobbs. Maybe we're spending too much money by not using the tach time?

The FAR's require some inspections and some maintenance to be performed based on Time In Service (TIS), which is further defined as the time from takeoff to landing.
And then there are Calendar requirements, not related to TIS, for example, an annual or condition inspection.
OTOH, Pilot flying time is defined as from engine start to engine shutdown.

Tach time roughly approximates TIS, and Hobbs time roughly approximates Flight Time.

A manufacturer (say, for example, Rotax) could define the oil change interval, and that would be the ruling definition.

I don't know what Rotax specifies.

I'm a bit surprised by the large difference in your two times. Do you spend a lot of time idling on the ground? What causes your Hobbs meter to run, oil pressure switch, avionics master, or master switch?
 
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The Rotax maintenance schedule uses Hobbs time ("All time during which the engine is running is counted towards the total number of operating hours. The time is counted irrespective of the load factor of the engine, such as idling or take-off power.")

There is a Skyview system setting for specifying the Cruise RPM (SETUP MENU > EMS SETUP > ENGINE INFORMATION). If the cruise RPM is incorrect the Tach time will also be incorrect. That might explain the large difference between Hobbs and Tach time.
 
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Thank you guys for he reply. Very Helpful. We will continue maintenance based on Hobbs, and will check the cruise RPM suggested by tomkk.
 
Translation

The old rule of thumb is with privet aircraft and pilots you service engines and log time by the oil switch that turns the hobbs on at start up and shuts it off at shut-down. It is when you get into freight or ATP operations that you get some need to be paid or log time from wheels up to wheels down.
Just kid of the way that is looked at down through the years. The O.E.M. sets the rules for your equipment for the most part. Yours, R.E.A. III #80888
 
I'm a bit surprised by the large difference in your two times. Do you spend a lot of time idling on the ground? What causes your Hobbs meter to run, oil pressure switch, avionics master, or master switch?

His TACH number is 74% of his Hobbs. My Hobbs/TACH numbers are 403.7/333.6 i.e. the TACH is 82% of the Hobbs. Probably pretty common ratio with the Skyview/Rotax setup.
 
I was told during training to use Tach x 1.2 to get an estimated Hobbs time for my pilot logbook.
 
From the Skyview installation manual:
"Hobbs time is a simple timer that runs whenever the oil pressure is above 15 PSI or the engine is above 0 RPM. " "RPM is used when calculating tach time." "Tach time is defined as TIME x (CURRENT RPM / CRUISE RPM)."

The cruise RPM that's entered on the engine setup screen determines the ratio between Hobbs and Tach times.

Rules of thumb are sometimes useful but in this case, it's pretty much totally under our control through the Cruise RPM setting.
 
Rotax wants Hobbs. Hobbs usually records when the oil pressure is 15 psi or higher. The tach depending on how it is set up can be influenced by rpm. Low rpms the tach may turn slower and high rpms it may turn faster. The tach can be 15% - 18% different from a Hobbs. Rotax likes maintenance based on engine run hours no matter at what rpm.
 
From the Skyview installation manual:
"Hobbs time is a simple timer that runs whenever the oil pressure is above 15 PSI or the engine is above 0 RPM. " "RPM is used when calculating tach time." "Tach time is defined as TIME x (CURRENT RPM / CRUISE RPM)."

The cruise RPM that's entered on the engine setup screen determines the ratio between Hobbs and Tach times.

Rules of thumb are sometimes useful but in this case, it's pretty much totally under our control through the Cruise RPM setting.

Only partially true.

Extended taxi time, or sitting at the runway end waiting for lots of landing traffic will both change that Hobbs/Tach time ratio, regardless of what number is set into the Skyview.

If you check the wiring diagram of an older certified mall plane you will probably find that the Hobbs is not wired through the Master Switch. This takes away the temptation for renters to get cheaper flights by switching off the Master in flight. :)
 
Only partially true.

Extended taxi time, or sitting at the runway end waiting for lots of landing traffic will both change that Hobbs/Tach time ratio, regardless of what number is set into the Skyview.

If you check the wiring diagram of an older certified mall plane you will probably find that the Hobbs is not wired through the Master Switch. This takes away the temptation for renters to get cheaper flights by switching off the Master in flight. :)

All true, but I was mostly responding to the rules-of-thumb folks were referring to. Just reminding folks that was adjustable in our Skyview system
 
OTOH, Pilot flying time is defined as from engine start to engine shutdown.

Actually, FAR Part 1, Definitions and Abbreviations, defines Flight Time as "Pilot time that commences when an aircraft moves under it's own power for the purpose of flight and ends when the aircraft comes to rest after landing".

So if you start your engine and sit there for 15 minutes to get your oil warmed up, that time wouldn't count. The time taxiing to-from the runway would be included.
I don't know how accurate Foreflight times the flight but I believe the time starts when the airplane starts in motion.
 
Actually, FAR Part 1, Definitions and Abbreviations, defines Flight Time as "Pilot time that commences when an aircraft moves under it's own power for the purpose of flight and ends when the aircraft comes to rest after landing".

So if you start your engine and sit there for 15 minutes to get your oil warmed up, that time wouldn't count. The time taxiing to-from the runway would be included.
I don't know how accurate Foreflight times the flight but I believe the time starts when the airplane starts in motion.

Thanks for the clarification. I guess if you want to be totally accurate, you should have a stopwatch and start it when you begin taxi, and stop it when you end taxi.
 
Overcooked

Guys, just take your actual flight time and add your pre and post flight taxi time. Keeping in mind that most folks track time in 10th?s of an hour, (which is 6 minutes), I just round to the nearest six minutes. In the last year, I?ve made 280+ flights and I find that my taxi out, run up and then taxi in average out to about .2 hours. So, if you can?t remember exactly, .2 is a pretty good number to use. The point being, that you only need to be accurate to the nearest 6 minutes...stop watch not really required.
 
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