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We have a 914 S/N 4418263 placed in service in 2005. It now has 1275 hours on it. 

This weekend we performed a differential leak down test with the engine cold and the results were between 60/80 and 65/80.  When repeated on a warm engine they were essentially the same.  We could hear air in the oil tank and a little in the intake manifold on cylinders 1 & 3.

We also did a direct compression test on the cold engine and the results were pretty uniform around 90 psi.

Borescope inspection of the valves and cylinder walls look normal.

All of these results are within Rotax limits, but barely. We have looked at our records and notice that the differential compression started to decline from roughly 79/80 about 200 hours ago (5/2020). 

The  tests were performed along with the crankshaft twist test and crankshaft runout test because of a short duration overspeed.  Both of those tests showed no problems.  We wouldn't have known about low compression except for the overspeed and would have been flying as usual until our annual inspection next Spring. 

We think although we are flying with a worn engine, there are no safety of flight questions and it is safe to continue. Certainly in the near future we will need to address the wear, but for now, it is not a problem, just that we have to live with reduced performance.

We are looking to see if others agree with our analysis or have other ideas.

Thanks

Jim & Heather

 

  • Re: 914 Wear

    by » 4 months ago


    Just a quick question...  Does compression feel any different now when you do hand-burping as compared to earlier when differential pressures were higher?


  • Re: 914 Wear

    by » 4 months ago


    You are the second person to comment "We could hear air in the oil tank...."

    No direct experience however I would suggest that this is normal and would be the same for a brand new engine.

    Why, because the Rotax oil return to tank system, relies on crank case pressure/blow by. So when you apply air pressure to the combustion chamber (with exhaust/inlet valves closed) some of the air pressure will always escape into the crank case and force oil into the tank, just as if you are "burping" the engine. Burping is just air from the crank case entering the oil tank.


  • Re: 914 Wear

    by » 4 months ago


    We quit burping a few years ago since it often was unsuccessful and took a long time.  We check oil level soon after landing before the oil has a chance to return to the sump.  But no, turning the prop by hand feels about the same as long ago.

    yes, we understand that there will be air going to the tank since that is how oil is returned in operation.

    thinking about the situation last evening, we’re starting to consider repair because we don’t clear the trees at the end of the runway as well and it takes a pitch down movement to reach 60 kt gear up speed.  Previously we were well clear of the trees and climb airspeed with gear down was around 65 kts.  

    Jim & Heather


  • Re: 914 Wear

    by » 4 months ago


    "Burping", if for no other reason, is good relative indicator of engine health.  Four individual compression strokes can be ascertained and, if you turn prop slowly, when piston goes over TDC you can get some idea of "play" in the overload clutch drive dogs.  Just saying....


  • Re: 914 Wear

    by » 4 months ago


    After 1200 hours your valve train is leaking and probably at least needs a lap job. Talk to the mechanics at LEAF, they did a short block on my 914 and I’d recommend them.


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