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  #1  
Old 09-25-2017, 07:42 PM
bob1393 bob1393 is offline
 
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Location: Port Orchard, Wa
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Default Hobbs Meter or Tach for Hours

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?
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Old 09-25-2017, 08:01 PM
PCHunt PCHunt is offline
 
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Quote:
Originally Posted by bob1393 View Post
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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Last edited by PCHunt : 09-25-2017 at 08:03 PM. Reason: Add last sentence
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  #3  
Old 09-25-2017, 08:13 PM
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tomkk tomkk is offline
 
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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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Last edited by tomkk : 09-28-2017 at 07:51 AM.
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Old 09-25-2017, 10:30 PM
bob1393 bob1393 is offline
 
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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.
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  #5  
Old 09-26-2017, 07:26 AM
Robert Anglin Robert Anglin is offline
 
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Location: houston, texas
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Default 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
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Old 09-26-2017, 10:15 AM
PilotBrent PilotBrent is offline
 
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Quote:
Originally Posted by PCHunt View Post

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.
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Old 09-26-2017, 11:15 AM
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Raymo Raymo is offline
 
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I was told during training to use Tach x 1.2 to get an estimated Hobbs time for my pilot logbook.
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  #8  
Old 09-26-2017, 11:29 AM
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tomkk tomkk is offline
 
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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.
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  #9  
Old 09-28-2017, 07:44 AM
Jolly Jolly is offline
 
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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.
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  #10  
Old 09-28-2017, 08:43 AM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by tomkk View Post
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.
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