Over rev

by in 912 / 914 Technical Questions 12 years ago

Hi. I took off with my IFA prop accidently pitched too fine. The 912 UL briefly got to 6800! which is 1000 rpm more than normal max. After adjusting the pitch I continued flying (circuits) for an hour and the engine ran normally. I have since found out that in such cases Rotax advises replacing the crankshaft. That seems a bit extreme, and I can't afford to do it. I am thinking of checking the straightness of the push rods, and perhaps doing a crankshaft out of round check. Any informed opinions would be greatfully accepted.
Allan

Re: Over rev

by Roger Lee » 12 years ago

Roger Lee

126 5.13k 4.53k

Hi Allen,

Could something have been damaged, yes. Could you have gotten lucky and no damage, yes.
You need to do the proper inspection and check everything. An over speed check is in the manual under special inspections. Do a good job and it may have been your lucky day. Inspect it all and make sure nothing is bent or twisted. This isn't something you want to play with because it could be an in flight failure I'd you pass something over. That was quite an over speed. When you give you engine full throttle and you see it hit 5800 and it doesn't seem to stop be ready and pull back on the throttle.

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

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Re: Over rev

by itemprop='name' » 12 years ago

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Thanks for your input - trouble is I'm not sure what is involved in the "proper inspection". I have looked at two Rotax manuals and they seem to give conflicting advice. One (1998) says "Complete check and/or repair of engine is necessary". The other has a lot more requirements;

"1. The whole engine must be inspected, repaired or overhauled in accordance with the BRP-Powertrain instructions for continued airworthiness.
2. Check that the pushrods are straight
3. Replace the crankshaft
4. Inspect all systems for correct functioning
5. Detailed inspection of affected engine components."

Would checking the pushrods and maybe the crankshaft for out of roundness be acceptable or are you saying the engine must be stripped down (& maybe the crankshaft replaced?)
Allan

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Re: Over rev

by Mike Brown » 12 years ago

Mike Brown

0 39 11

Putting aside the mechanical issues for a moment, where are you located (country), what type of airworthiness certificate does your aircraft have and do you carry passengers?

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Re: Over rev

by itemprop='name' » 12 years ago

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Hi. I'm located in New Zealand. The aircraft operates under a Permit to Fly, it's classed as a microlight in NZ - Ultralight to you? (Titan Tornado) I'm not able to carry passengers yet but hope to be able to soon. Allan

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Re: Over rev

by Mike Brown » 12 years ago

Mike Brown

0 39 11

Hi. I'm located in New Zealand. The aircraft operates under a Permit to Fly, it's classed as a microlight in NZ - Ultralight to you? (Titan Tornado) I'm not able to carry passengers yet but hope to be able to soon. Allan

Not an ultralight. Experimental or light sport aircraft here. I'd guess you're not compelled by your governing body to do an engine teardown for this overspeed. So the question is, what's prudent?

6,800 rpm is a very significant overspeed. I'd not assume that because your engine runs fine now that all is OK.

The 912 has a composite (multi-component) crankshaft with captive connecting rods. This composite crank can be distorted by an overspeed and eventually fail as a result. There's a simple test you can do yourself that will give you more information before you decide whether to do a teardown. Crankshaft Distortion Inspection. You'll need a subscription to Rotax-Owner.com to watch the complete video.

I'd start there and if the crank is out of spec. you have your answer. If the crank is in spec. for "twist" then you can decide whether you want to inspect the pushrods which to the best of my knowledge requires removing the cylinders.

If I were you and were planning to carry passengers I'd go by the book and do what Rotax recommends.

Rotax912Crankshaft.jpg (You do not have access to download this file.)

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Over rev

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