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I fly a touring motorglider (Phoenix U15) with a 912ULS engine. Sometimes I'll soar with the engine off; other times, I'll soar with the engine idling for 2 or 3 hours at a time. I'm wondering how this prolonged idling might affect the engine.

Specifics:

The engine is two years old, with 230 hours on it. Idle on the ground is 1800, idle in the air is 2300, oil temperature and CHT are kept around 150 deg F using the cowl flap. It starts easily, runs smoothly, and the mechanic that maintains it says there is no evidence of any problems. Most of the time it's using 91 AKI ethanol-free auto gas.
  • Re: How does prolonged in-flight idling affect the 912

    by » 9 years ago


    I haven't ever been ask a question like this and I hope Rob chimes in.

    I would suspect that it would be better to shut down for those long idle times. If you are using 100LL lead build up would be worse. Carbon build up and vibration would be worse. Since you are comfortable shutting down the engine I would think you would be better off.

    Let's see what a few others have to say.

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


  • Re: How does prolonged in-flight idling affect the 912

    by » 9 years ago


    Why would you not just shut it down like every other motorglider?
    At 150F you are building up a lot of moisture and acids in the oil. That's the only concern I can think of. The engine will not wear out, it has no load, but I have not heard of anyone idling for 3 hours before so who knows?

  • Re: How does prolonged in-flight idling affect the 912

    by » 9 years ago


    Why would you not just shut it down like every other motorglider?
    At 150F you are building up a lot of moisture and acids in the oil. That's the only concern I can think of. The engine will not wear out, it has no load, but I have not heard of anyone idling for 3 hours before so who knows?


    Idling is common when soaring touring motorgliders (TMG), for several reasons:

    - Their modest glide ratio of about 30 makes it difficult to stay in reach of an airport

    - Even though the Rotax starts very reliably, there are no guarantees, especially if it's been off for a few hours at altitude, where the air can be 25 to 60 deg F colder - cold engine, cold battery, nervous pilot attempting restart ... not a good recipe

    - Most them can not land safely on a soft field with the gear down

    - Most owners don't have a trailer for a retrieve from a field, and the weight and size of many TMG means it's going to be struggle involving several people to derig it and get it into the trailer from a farmers field

    The easy way around all this in less than ideal conditions is to leave the engine running.

    Maybe it would be worthwhile to have the oil analyzed for moisture and acids, with one sample taken before the flight and another after the soaring flight. The mechanic says the oil looks the same as the oil from similar Rotax-powered airplanes that don't ever idle in flight, and generally, the idling periods are about 25% of the engine's total time, or less, so it does spend a lot of time cruising at 190-200F.

  • Re: How does prolonged in-flight idling affect the 912

    by » 9 years ago


    I guess in that situation it is well worth letting it idle.
    If the oil is not milky then it must be getting hot enough, for long enough, to boil out the moisture.

  • Re: How does prolonged in-flight idling affect the 912

    by » 9 years ago


    How do the plugs look? I would think that with that much idling they would be very sooty and black. I would also think that engine restart would be more reliable with normal (non-sooty) plugs.

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