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912 engine cough in humid conditions

in 912 / 914 Technical Questions 11 years ago

  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Hi Steve

    This sounds like classic carb icing. Do you have any form of carb heat fitted?

    Mark
      
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  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Hi Mark: No carb heat. Icing was my initial thought, but it never lasts more than a second. I've just never had a problem like this with the engine when it was on my gt 500. Since I need to get my rpms higher; and I need to synch my carbs, I thought I would do that first. Why would it start icing now, on this plane, different airflow to the carbs? Curious, why do you think its "classic" icing? Another poster thought it unlikely since it happens at cruise. Any thoughts would be appreciated. Steve
      
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  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Hi Steve

    I think the fact it only happens in humid conditions is a bit of a give away.

    I would say you can get carb icing at almost any power setting if the conditions are right, but it happens mostly on lower throttle settings, but not idle.

    Next time it happens, reduce your power to around 3000-3500rpm. If it gets worse or even stops then its carb icing.

    It maybe that the air your engine is drawing in is not being affected as much by the under cowl temp as it was on your other aircraft.

    I think most engine setups with a cowl can get away most of the time without any carb heat as the air being drawn into the intake is pre heated, and this acts as a form of carb heat. Any engine open to the elements will have some form of carb heat fitted.

    Even engines that are cowled get carb icing from time to time if the conditions are right.

    If some people only fly where the air is fairly dry, they have probably never experienced carb icing and don't really see it as a problem.

    I have had 2 complete engine failures on different aircraft in the past due to carb icing. One was flying in low cloud and drizzle which you would expect, the other was on a clear day, but below the inversion which was quite humid. Both times I was in the cruise. The best part about it is you can re-start your engine after around 10-20 seconds as the ice will melt almost immediately the engine stops.

    For what its worth, fit the Rotax coolant heated carb heat and I am sure your problem will go away.

    If it happens when the conditions are not conducive to carb icing, I would suggest you may have some debris in the fuel inlet of the carb which is before the needle valve. This tends to show up as the odd loss of power as the debris blocks the fuel to one carb for a couple of seconds.

    These are my views which I have come across through personal experience.

    Mark
      
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  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Mark: First, I want to say how much I appreciate your time and assistance on this issue. I think icing is the most logical explanation. My primary reluctance has always been the fact that flying this engine for years on the GT-500, I never had these symptoms. Both the Gt-500 and the Aventura are uncowled pushers with very similar installations. Of course, not identical installations. The second is how quick the rpm drop occurs and then recovers. This did not seem to be a "classic" icing symptom, which would last longer, at least in terms of conventional (non-rotax) aircraft engines (as I understand it). Maybe what I'm experiencing is the beginning of an ice built up that doesn't get past a momentary effect. I just don't know enough to judge. As I mentioned in my last post, however, I took the plane up yesterday above the inversion layer (7800 ft) and flew it for about 20 minutes. The rpms were rock solid. At 2500 ft I was experiencing rpm drops. If not carb ice, then what could it be?

    In your view, without a cowling, the 912 would be susceptible to icing? And therefore as a general proposition carb heat should be installed?

    Steve
      
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  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Hi Steve,

    Fix the other things first. They are important and can be playing a part here. After you take care of those couple of items get back to us after a test flight and let us know how it went.

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

      
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