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We have a 914 in a Eurofox glider tug that has a strange problem that only happens on the first flight of the day. As you increase to full power and the manifold pressure & rpm approach the normal 40"/ 5500rpm values, suddenly the tacho needle kicks instantaneously to over 6000rpm for a fraction of a second and the boost reduces before increasing back to normal.

Having logged TCU data with a laptop (see attached file) I can see that the turbo control unit opens the wastegate in response to it seeing the rpm go over the max of 5800 (which is what you would expect). Once the recorded rpm is below max, it re-closes the wastegate and everything is fine.

The log shows the engine rpm increasing by 800rpm in 1 second which I cannot believe it is capable of, so I suspect an rpm sensor problem (ie: the rpm is not really exceeding 5800), but why only on the first flight of the day ?

The rpm sensor is just a simple coil excited by a magnet on the flywheel so you would think it either works or does not. The cable & connector seem ok.

The wastegate cable is properly adjusted and everything is normal for flights the rest of the day, but the problem is there again in the morning.

Any suggestions ?

Thanks, Mark

10045_1_TCU Data.JPG (You do not have access to download this file.)
  • Re: 914 RPM/ Boost problem – only 1st flight of the day

    by » 10 months ago


    I first need to ask if this is fitted with an overload clutch?  If so given what you gave from the log file it appears that the clutch has may have slipped some.  I am not sure why you do not believe the RPM in the log.  Anything over 5800 will cause the turbo to reduce boost and you should have a flashing red lamp in the system.  Remember that the engine TCU is looking at the information you are looking at in the log file.  It thinks you are over rpm and is trying to save the engine from damage.  

    Cheers


  • Re: 914 RPM/ Boost problem – only 1st flight of the day

    by » 10 months ago


    Just a supplemental comment.  Perhaps you simply have too small a load (pitch) on the propeller.  If this is a fixed prop then for sure it is way too small a load.  if this is constant speed you need to load it more, same if it is inflight adjustable type.  I would look at this before getting into a lot of hunting for strange things in the TCU.  

    Cheers


  • Re: 914 RPM/ Boost problem – only 1st flight of the day

    by » 10 months ago


    Rotax Wizard wrote:

    I first need to ask if this is fitted with an overload clutch?  If so given what you gave from the log file it appears that the clutch has may have slipped some.  I am not sure why you do not believe the RPM in the log.  Anything over 5800 will cause the turbo to reduce boost and you should have a flashing red lamp in the system.  Remember that the engine TCU is looking at the information you are looking at in the log file.  It thinks you are over rpm and is trying to save the engine from damage.  

    Cheers

    Thanks for the reply - I agree the TCU appears to be doing it's job and saving the engine from excessive RPM, but I don't understand why it only does this on the first flight of the day. I am suspicious of the high RPM data as the TCU thinks it increased 800rpm in 1 second and I don't think the inertia of the engine would allow it to do that, but that's just a gut feel.

     

    Cheers, Mark


  • Re: 914 RPM/ Boost problem – only 1st flight of the day

    by » 10 months ago


    Rotax Wizard wrote:

    Just a supplemental comment.  Perhaps you simply have too small a load (pitch) on the propeller.  If this is a fixed prop then for sure it is way too small a load.  if this is constant speed you need to load it more, same if it is inflight adjustable type.  I would look at this before getting into a lot of hunting for strange things in the TCU.  

    Cheers

    Once the engine gets over this glitch, the rpm & everything else is fine for the rest of the day, so I'm pretty sure the prop pitch is OK. It hasn't changed and the engine has done 750 hours. I'm stumped !

    Cheers


  • Re: 914 RPM/ Boost problem – only 1st flight of the day

    by » 10 months ago


    Marc,

    I can follow your concern but do not find the 800 revs jump in the log. 

    The comment on the clutch sounds very plausible to me (sudden lack of load). However, the question remains open why this effect only occurs once a day during the first flight.

    If I understand your description correctly your prop setup seems OK to me (below 5800 revs at full noise).

    Keep us updated.

     


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