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I recently started experiencing vibration at cruise with my 2018 912iS, and all signs point to a gearbox issue. The aircraft has just over 420 TT, and the original gearbox was already replaced just about 60 Hobbs hours ago with the new misting version #893-341.

With the gearbox being so low hour, and the new version, I'm hesitant to just remove it and pay to have it overhauled. My concern is that I might spend a lot of money and just have the same issue again in less than 100 hours. What could cause a new gearbox to begin failing so early? Oil changes are up to date, fuel has been unleaded ethanol free, the motor is kept at 2000 for startup idle, etc. Any help is greatly appreciated for this new owner and pilot.

 

 

  • Re: New Gearbox Failure on 912IS - Causes?

    by » 3 months ago


    I would first confirm that it is the Gearbox and not something else.
    Gearbox vibration is a symptom of improper backlash (Freeplay) in the gears.
    Backlash should be minimal but NEVER Zero (Binding).
    Rotate the prop slowly by hand and feel for any damaged gear teeth.
    At least 1 rev of the prop will be needed to engage all gear teeth at least once, but 2 revs or move revs would be better.
    At all prop positions, the backlash of the gears as measured at the propeller tip should be about 1/16 Inch.
    1/8" or more of backlash is considered a cause for concern. Zero (Binding) is even worse.

    You might also want to test this with a spark plug removed from each cylinder for a lighter feel.

    Also, check your Magnetic plug for metal chips.
    It is there to collect any gear bits that may go astray.
    Magnetic "Hair" on the plug that feels like grease (no grit) between your fingers is normal.

    - - -
    This is something for everyone to consider while "Burping" the engine to check the Oil level.


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


  • Re: New Gearbox Failure on 912IS - Causes?

    by » 3 months ago


    Bill Hertzel wrote:

    I would first confirm that it is the Gearbox and not something else.
    Gearbox vibration is a symptom of improper backlash (Freeplay) in the gears.
    Backlash should be minimal but NEVER Zero (Binding).
    Rotate the prop slowly by hand and feel for any damaged gear teeth.
    At least 1 rev of the prop will be needed to engage all gear teeth at least once, but 2 revs or move revs would be better.
    At all prop positions, the backlash of the gears as measured at the propeller tip should be about 1/16 Inch.
    1/8" or more of backlash is considered a cause for concern. Zero (Binding) is even worse.

    You might also want to test this with a spark plug removed from each cylinder for a lighter feel.

    Also, check your Magnetic plug for metal chips.
    It is there to collect any gear bits that may go astray.
    Magnetic "Hair" on the plug that feels like grease (no grit) between your fingers is normal.

    - - -
    This is something for everyone to consider while "Burping" the engine to check the Oil level.

    The vibration occurs at altitude, after the aircraft has been trimmed out in cruise. It's not instant, but once I settle into cruise it comes on and is definitely noticeable. If I start to pull back on the stick and initiate a climb, it goes away very suddenly. Would anything other than the gearbox be causing an issue like that?

    I can fly the traffic pattern down here at 1200' and not feel anything. But after settling in for a X-Country, it definitely happens.


  • Re: New Gearbox Failure on 912IS - Causes?

    by » 3 months ago


    Sounds more like airframe resonance.


  • Re: New Gearbox Failure on 912IS - Causes?

    by » 3 months ago


    What propellor type/diameter are you using?  CSU?


  • Re: New Gearbox Failure on 912IS - Causes?

    by » 3 months ago


    Sean Griffin wrote:

    Sounds more like airframe resonance.

    Wouldn't I have been experiencing that before though? This problem just started occurring within the last couple flights. Also, wouldn't I be noticing something outside of just cruise flight?


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