Whilst reviewing setup of a MGL Challenger EFIS today I came across this information relating to the recording of hours. Written by the CEO of MGL it suggest that hours logs should be adjusted according to the Revs used as opposed to 100% power. Eg. as an example if you are running a 912S with 100% continuous max power at 5500 RPM then you can call the TBO for that engine 2000 hrs but if you only cruise around at 4000 RPM most of the time then you can increase the TBO by at least a proportion of 5500/4000. The EFIS will do this for you if setup in that manner

"Tach time is calculated relative to your 100% HOBBS Tach RPM as setup in your engine monitoring setup. 

For example, for a Rotax 912 engine you could select 5500 RPM here. This is the RPM Rotax tells us you should be able to run continuous and reach TBO (assuming correctly loaded prop and everything else in order). 

Tach time then credits you for any time spent below 5500 (excluding zero of course) and penalizes you for any time spent above this value. 

The credits and penalties are linear with RPM so in this case assuming you would run at 2750 RPM you would get a 50% credit (so TBO is effectively extended to double). 

There is also a setting where you choose if your maintenance timer is based on Tack or normal Hobbs. 

Hobbs is subjected to a certain minimum RPM before it counts (you can set that too). Normally you don't want idle time to count towards the hobbs (unless you are a flight school and charge for this). "

I have never heard of this before is there any validity in this or is it BS.


  • Re: Recording engine hours

    by » 5 years ago

    Maintenance Manual, Section 05-10-00

    1) 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 misunderstandings 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.

    NOTES: A mechanical hour meter is directly coupled to the engine
    speed, the readings may deviate considerably from those
    given by electronic remitters (e.g. TCU, FlyDat). Maintenance
    and overhaul intervals are always dictated by the
    readings of the electronic hour meter.

    - The planned inspections to be performed at certain intervals are based
    on experience from long test runs and field observations. They are
    intended as precautionary maintenance measures

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

    Thank you said by: john hunt, jimi james

  • Re: Recording engine hours

    by » 5 years ago


    Whilst the initial part of this makes complete sense I am afraid I do not understand the second part.

    If all hours that the engines actually running are to be counted then surely the Maintenance and overhaul periods should be based on the mechanical meter, which will record all the hours that the engine runs as opposed to the electrical hour meters either in a TCU or Flydat which can be switched off but the engine is still running?




  • Re: Recording engine hours

    by » 5 years ago

    It is a historical thing.

    The Mechanical meter is not the little DC clock motor-driven HOBBS meter you might be thinking of.

    The Classic HOBBS Meter falls into the Electric Hour Meter category.


    The mechanical meter referred to dates back to WWII or earlier. Think DC3 era.

    The Tachometer was strictly mechanical and was cable driven similar to a speedometer.

    The "Odometer" Hourmeter was calibrated for each engine but might record an actual hour at 2500rpm.

    It ran slower at lower RPMs and faster at full power. 

    It was an Odometer geared down to register an hour for every 150,000 revolutions. (2500*60)

    The Electrical Hour meter is assumed to be an Electronic Digital Clock actuated by the activation of the engine or any device that operates in the same manner.

    It will record actual hours regardless of engine RPM.

    With the advent of the Computer is is possible for the Electronic Hour meter to operate in a manner simulating either instrument or a hybrid of the two.

    In summary:

    Mechanical Hour Meter = Mechanical Tachometer.

    Electronic Hour Meter = Electric HOBBS Meter.



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

    Thank you said by: john hunt

  • Re: Recording engine hours

    by » 5 years ago

    Thanks. That makes perfect sense. I understand from a post elsewhere that the method proposed by MGL is quite standard practice in the world of Lycontinentisaurus and is very much standard practice in the USA, so presumably this where the idea came from. I guess that if Rotax adopted this practice then the TBO would need to come down as well as the maintenance intervals which would make the engine look less desirable.


    This clarifies for me my approach at inspections where I regularly see engine hours that match the airframe, the owners saying that warm up time doesn't count( although I guess much of the wear actually comes during this cold period).

  • Re: Recording engine hours

    by » 5 years ago

    can you share the picture of mechanical meter for info and purchase ?

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