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TBO question

in 912 / 914 Technical Questions 9 years ago
I apologize for my ignorance but I really don't know the correct answer. When determining tbo, is the time based upon Hobbs time or tach time? My tach time right now differs from Hobbs by approx. 70 hrs. so over the life of the engine it could make a difference when I actually would need to do an overhaul. Thx. for the advice.
Terry

  • Re: TBO question

    by » 9 years ago

    Hi Terry,

    Use the Hobbs time. Usually Hobbs time records when your engine has 15 psi of oil pressure and that means the engine must be running and it records at a steady rate regardless of rpm. Use Hobbs for inspection times too.

    Some Tach times can be 15%-20% off because it varies it's recording time when at idle versus high rpm. Some of the older tachs would register even when just the power was turned on.

    If you have an digital EMS and have the option to pick between Hobbs and Tach like Dynon does then select Hobbs.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: TBO question

    by » 9 years ago

    Thx. again Roger, always helpful as usual and very informative.
      
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  • Re: TBO question

    by » 9 years ago

    Hate to contradict both Rotax and Roger but:
    FAA 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.”
      
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  • Re: TBO question

    by » 9 years ago

    Engine rebuild at TBO is not a mandatory requirement with the FAA or the engine manufacturer; it is only a recommendation. This is true whether the engine is certified or not. Mike Busch, a well known and respected AP/IA, who writes a column for the EAA Sport Aviation magazine is very clear on this. He is very comfortable in going beyond, in some cases well beyond, TBO if there are no indications of a need for engine teardown; just keep doing the recommended preventative maintenance and keep a close eye on oil analysis results, pressures, temps, etc. If you go with his philosophy there is no need to be anal about whether you use hobbs or tach time. I prefer to record both just for my own information.
      
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  • Re: TBO question

    by » 9 years ago

    I have no problem with a responsible owner going on-condition. If it is done as you say with an oil analysis and monitoring of oil consumption, compression and leaks it can be very successful.
    I believe commercial operators cannot do it and correct me if I wrong but LSA aircraft must follow the manufactures maintenance manual requirements?
      
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