When I first energized my avionics, my Hobbs showed 0 hours, naturally, and the EMS showed 0.5 engine hours, which I assumed was from the factory break-in testing.  Slowly, the Hobbs time has exceeded the engine time.  It's currently showing 111.7 on the Hobbs, but 111.5 on the engine run time.  So, my question is what are the conditions for which the Hobbs runs vs. engine time?  I assume when the oil pressure exceeds some threshold the engine timer runs, but not sure about the Hobbs.  Any insights so I can better understand my system would be appreciated.


8829_1_EMS.pdf (You do not have access to download this file.)
  • Re: Hobbs Vs. Engine Time

    by » 5 weeks ago

    Edit: Nevermind, you're not talking about Tach time. Looks like I can't delete my reply, so it'll just have to stay here, making me look dumb. :)


    Hobbs is actual engine runtime, Tach is engine revolutions converted to time. There's a setting in the EMS setup to tell the Dynon what your cruise RPM is, and it'll calculate Tach time based on that. If you spend most of your time below that value (warming up, idling, taxiing, runup), or if the value is set higher than your actual cruise RPM, Tach will be lower than Hobbs.

  • Re: Hobbs Vs. Engine Time

    by » 5 weeks ago

    Use Hobbs

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

  • Re: Hobbs Vs. Engine Time

    by » 5 weeks ago

    What EMS brand is this?
    If the "Engine" hours are ECU Hours, that is the Actual Engine running time.
    A Mechanical HOBBS actuated by an Oil Pressure switch is also quite accurate.
    The software HOBBS in the EMSs often is a Tach/Hobbs hybrid.
    If the Software assumes 5200 rpm is full speed and you fly mostly at 5300, the software Hobbs will outrun the ECU hours.

    I have an MGL brand EFIS that stops the HOBBS engine OFF/ Lanes ON,  but runs at 1/4 speed when the EFIS is ON and the LANES are OFF.
    It records 0.1 hrs every 24 minutes.  Crazy !!!

    I would interrogate the ECU to see if it matches what you are calling "Engine" hours.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

  • Re: Hobbs Vs. Engine Time

    by » 5 weeks ago

    Thanks, Bill.  I did, in parallel, run this past Dynon, my avionics manufacturer, and they just confirmed that they do generate the Hobbs time, which runs on sensing positive oil pressure and stops on shut down.  This all the more makes me wonder what ENGINE time means.  If it didn’t display on my EMS I’d be oblivious, but since it does I have to ask.  And it’s obviously coming from the ECU or it wouldn’t have shown 0.5 hours on first energizing the avionics.  Perplexing…

  • Re: Hobbs Vs. Engine Time

    by » 5 weeks ago

    For my Dynon Skyview:



    Whenever oil pressure is above 15 psi or engine is above 200 rpm.



    Engine time normalized for cruise RPM (cruise RPM parameter must be properly set in system setup).



    Hobbs time for current flight. Resets first time engine is started after Skyview is powered on. This allows you to view previous flight’s engine run time any time before you start your engine.



    Runs any time SkyView determines the aircraft is in flight. Resets as SkyView detects that the aircraft has transitioned to “in the air.” This time is also optionally used to calculate the Switch Fuel Tank Reminder.



    Hobbs time that can be reset by pressing ENGINE > CLR TMR.



    Runs any time that SkyView determines that the aircraft is in flight, just like air total. Unlike air total, it can be reset by pressing ENGINE > CLR TMR




    Runs any time that SkyView determines that the aircraft is in flight. Can be reset or adjusted in the Aircraft information menu within SkyView Setup.

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