Engine revs and wont respond to throttle or mixture

tehmightypirate

tehmightypirate

Line Up and Wait
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TehMightyPirate
During takeoff, Cessna 172SP (fuel injected Lycoming) revved well past redline, would not respond to throttle OR mixture. Pilot had to kill the mags to land it. Supposedly the club mechanic tore it apart for a month and had components sent out to be bench-tested without finding a cause. Plane runs fine now.

Note, I was not the pilot flying and heard this all through multiple parties, may not have 100% of the details.

What do you think could cause this?
 
Redline is usually restricted in level or climbing flight by prop pitch. In a fixed pitch engine you'd have to be making more than rated power to turn the prop past redline unless you're diving. At least that's how it's supposed to work.
 
The way I heard it was that it happened during climb and level flight, no dives, and the observed the tach was at 2600 RPM. Prop isn't a climb prop or anything fancy so I agree that it should have limited the RPM.
 
tehmightypirate said:
The way I heard it was that it happened during climb and level flight, no dives, and the observed the tach was at 2600 RPM. Prop isn't a climb prop or anything fancy so I agree that it should have limited the RPM.
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Redline is 2700. 2600 is no big deal in level flight.

The usual cause of engine control failure is failing to lock the attachment hardware. It shakes loose and falls out, and the cables then have no control. This is mechanical simplicity, nothing complicated, and it should have been obvious. Someone isn't relaying the full story, I suspect.
 
G1000 Tach. Hmmm, you're right 2700 RPM is redline, I was mistakenly thinking of normal climb full power RPM which is around 2400-2500 RPM for this plane. Either way, it went full throttle and perhaps then some in a climb. The bigger issue seems to be the lack of both throttle and mixture control and that's what I'm most curious about.

Supposedly the cables/controls for throttle and mixture were found to be fine. You're entirely right that I'm not getting the full story and am likely missing or misstating some key info.
 
Maybe the fuel servo went tits up? Don't really know what the failure mode of that unit would be. I would imagine the engine would just run full rich if the servo failed. Hard to tell from info presented... never heard of loss of these controls that wasn't a cable issue.
 
I'm fairly sure that's what they said they sent out to be bench tested and it came back good.
 
zodiacflyguy said:
Maybe the fuel servo went tits up? Don't really know what the failure mode of that unit would be. I would imagine the engine would just run full rich if the servo failed. Hard to tell from info presented... never heard of loss of these controls that wasn't a cable issue.
Click to expand...

The throttle control opens and closes the throttle plate that controls air into the engine, and also works the fuel control valve. The fuel control system could act up all it wants but it could not take the engine to full power if the throttle control was properly connected and the pilot pulled the throttle back, closing off the air to the engine. More fuel makes no more power if the engine can't get oxygen to burn it.

SilverHawk_PosterV2.JPG


The gold lever on the right side is the throttle. Pulling the throttle control back pulls that lever back and closes that round silver throttle plate you can see in the top of the servo bore. The linkage below the throttle lever works the fuel control, the smaller lever at the bottom right. The mixture control lever is out of sight on the front left side, in line with the fuel control shaft on this side.
 
A normally aspirated engine with a fixed pitch propeller that overspeeds in a climb?
 
Dan Thomas said:
The throttle control opens and closes the throttle plate that controls air into the engine, and also works the fuel control valve. The fuel control system could act up all it wants but it could not take the engine to full power if the throttle control was properly connected and the pilot pulled the throttle back, closing off the air to the engine. More fuel makes no more power if the engine can't get oxygen to burn it.

SilverHawk_PosterV2.JPG


The gold lever on the right side is the throttle. Pulling the throttle control back pulls that lever back and closes that round silver throttle plate you can see in the top of the servo bore. The linkage below the throttle lever works the fuel control, the smaller lever at the bottom right. The mixture control lever is out of sight on the front left side, in line with the fuel control shaft on this side.
Click to expand...
I know how the servo works. I just have never seen one break that wasnt related to misrigging. Even then a few twists of a wrench and a cotter pin later.... Back in business. I was wondering out loud if there was a failure point in the fuel servo that had even a remote possibility of that set of symptoms. Can't think of one myself.

Sent from my LG-LS997 using Tapatalk
 
Yeah, I could see the throttle losing response, or the mixture losing response, but not both. About the only thing that could seem possible is literally the entire fuel servo jamming allowing for not adjustment of any component in it. Is this even possible?
 
Any more to the story?

What did it sound like? If you could hear the thing vary speed, check tach. Was the mixture really pulled all the way out and still didn't kill it?
 
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