We keep getting damage on the Dogs (see Photo’s) on our 912iS Gearbox, this happens at relatively low hours (~150 to 250 hours). We notice high vibration levels which cause other damage to the exhaust, as well as poor starting. We are conscious of keeping the rpm up during idle. Anyone seen this and got any explanation.

9874_1_20230605_130621.jpeg (You do not have access to download this file.)
  • Re: Gearbox Problems 912iS

    by » one year ago

    What is the S/N of your gearbox?  Starting at S/N 74000 Rotax added an oil spray nozzle to the 912iS gearbox. I believe this was to address the early wear problems some folks were having with the 0-degree dog hubs the iS engine uses. I had a similar failure at about 135 hours, and I replaced the gearbox with the upgraded unit at that time and have not had any more problems in the 200 hours since. Also, after the upgraded gearbox was installed, there has been nearly zero metal on the magnetic plug when I change the oil. 

  • Re: Gearbox Problems 912iS

    by » one year ago

    Thanks Jeff, unfortunately our gearbox has the spray bar. 


  • Re: Gearbox Problems 912iS

    by » one year ago

    Below is information from the Rotax installation and Heavy Maintenance Manual (Links at the bottom): 

    Pitting in the gearbox can lead to high-frequency vibrations, causing various issues as the vibration is transferred through the engine to connected parts. These problems include:

    • Wear on the gearbox (gear profile and contact faces)
    • Wear on external alternative accessories
    • Wear on the exhaust system
    • Leaking of the sealing surface of the crankcase

    Note: Dynamic engine analysis units, commonly used for propeller adjustments, can detect this vibration. The normal vibration level for the engine ranges approximately between 1.27 cm and 2.45 cm per second. Alternatively, refer to the values specified by the aircraft manufacturer for the respective installation.

    Please ensure to verify the moment of inertia for your propeller, adhering to the specified system limits:

    • Minimum moment of inertia on propeller: 1500 kg cm2 (3.559 lb ft2)
    • Maximum moment of inertia on propeller: 6000 kg cm2 (14.238 lb ft2)

    It is necessary to conduct a dynamic balancing of the propeller as prescribed by the propeller manufacturer.

    Visually inspect the dog hub for any visible pitting on the gear-tooth system or in the engagement faces of the dogs, referring to section 72-10-00 for wear limits (GB08). 

    Note:  The cam peaks of the gear should never rest in the trough of the dog hub.

    Measure the gap between the cam peak and the cam trough, referring to section 72-10-00 for wear limits (GB07).

    Note:  Slight to moderate traces of wear and pitting on the dogs are permissible.



  • Re: Gearbox Problems 912iS

    by » one year ago

    interresting but to be able to analyze the problem we need more details how long to make those flight hours? What oil was used? What is the inertia of thr propeller fitting on  ? On wich kind of aircraft?and to see all the parts of the GB that can help to diagnose. And at the end there is a last thing to do but before, to be sure of the final diagnosis all the system must be analysed.

    sorry I think a part out of  his system is like a word out of his sentence 

  • Re: Gearbox Problems 912iS

    by » one year ago

    Of course you’ll want to perform a dynamic prop balance, but if you’ve got a ground-adjustable prop, you should verify each blade is pitched to the same angle too.

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