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HOBBS or TAC Time

MVPILOT

Well Known Member
(1) I am selling my RV-12 SLSA and it has 330 hours of HOBBS time. When listing my RV-12, do I show HOBBS or TAC? (2) I read where the Rotax ULS TBO is 1200 hours but can be increased to 1500h when the instructions in SB-912-041UL are followed. Is SB-912-041UL based on hobbs or TAC time? So far I haven't learned what this SB includes.

By the way I've enjoyed my RV-12 so much I'm taking delivery of a new RV-12 iS in two weeks so I'm quite happy with the RV-12.
 
Isn't it tach, as in tachometer, time?

I count flight time based on Hobbs time and I change oil and do maintenance based on tach time.
 
Isn't it tach, as in tachometer, time?

I count flight time based on Hobbs time and I change oil and do maintenance based on tach time.

Thank you. I guess I had very old information. It seems the TBO on Rotax ULS engines is 2000 hours TACH time.
 
According to ROTAX factory guidance, "HOBBs" time is to be used for engine-related maintenance timekeeping. See ROTAX 912ULS Line Maintenance Manual -- Page 2 of 05-10-00 as below:

"All of the maintenance intervals, such as the 100 hr. inspection and the engine TBO, relate to the number of operating hours of the engine. The operating hours are defined as follows in order to prevent misunderstanding and to ensure safety:

• 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."

I suspect that your 912 ULS engine being a newer model already features a 2,000-hour TBO.
 
Last edited:
From the Rotax Line Maintenance Manual:

"Definition of terms
1.1) Operating hours
Definition All of the maintenance intervals, such as the 100 hr. inspection and the
engine TBO, relate to the number of operating hours of the engine.
The operating hours are defined as follows in order to prevent misunder-
standings and to ensure safety:
- 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."

I think that means all maintenance times, including TBO, are based on Hobbs time not Tach time.
 
I was originally managing the engine by Tach time as that is what I did with my Lycomings, but Hobbs for engine maintenance intuitively makes sense when you consider the relatively high idle RPM of the Rotax and that the reduction gearbox is working pretty much all the time.
 
I have the D180. I have never seen a tach and Hobbs number, just one run time. Reminds me of the old joke: Give a man a watch and he knows what the time is. Give him two and he’s never quite sure.😁
 
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