Throttle progressing on its own

Nathan N

Nathan N

Filing Flight Plan
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Nov 21, 2018
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Nathan
Hello everyone,


Does anyone happen to know why my throttle will progress inward on its own unless the friction lock is set abnormally tight?

I have to make the friction lock tight to the point where it takes some effort to slide the throttle in and out.

It’s a Cessna 172. I’ve flown several others and didn’t have to set the friction so snug.

Thanks,
Nathan
 
If the throttle linkage fails, it is "supposed" to fail to full throttle. It happened to me off the Columbia River in a 185 floatplane that just had an engine changed out at Troutdale. The throttle went to full and I left it there all the way to Renton and pulled the mixture to land.
 
kgruber said:
If the throttle linkage fails, it is "supposed" to fail to full throttle. It happened to me off the Columbia River in a 185 floatplane that just had an engine changed out at Troutdale. The throttle went to full and I left it there all the way to Renton and pulled the mixture to land.
Click to expand...

Do you have a recording of your phone call to Troutdale;)
 
Some carbs had a very light spring on the throttle shaft. It shouldn't have the power to pull the throttle control in. It was intended to open the throttle if the control came undone at the carb. On the bench it won't even open the throttle against the friction of the accelerator pump; it would need some vibration to get the throttle open.

I would inspect the throttle cable routing to see that it isn't being yanked by the engine as it moves. Make sure someone hasn't added an unapproved spring to the system. Then I would get a dynamic balance done. Excessive vibration can create all sorts of headaches. Even if it isn't causing the throttle creep, it will save wear and tear on the instruments and radios.

The heater control in the 172 also has a spring on the airbox lever, and vibration will creep the heat on. You can flip the spring off the lever to stop that.
 
I've seen this before on a 172 where the cable sheath was not well secured and the vibration caused the cable to increase tension. Adding a few clamps fixed it.
 
IK04 said:
I've seen this before on a 172 where the cable sheath was not well secured and the vibration caused the cable to increase tension. Adding a few clamps fixed it.
Click to expand...
One can find some crazy stuff in old airplanes. The 172 in question might have its cable housing (sheath) clamped to the engine mount instead of the proper bracket that bolts to the sump flange. Engine vibration can cause all sorts of grief with that, including a failed cable. The housing MUST move with the engine.

I once found the drain plug on an O-300 lockwired to the engine mount crosstube. The wire was busy cutting into the tube. That can get really expensive. I have found, on a restart 172, the wiring bundles to the engine clamps to the engine mount within a couple of inches of the bundle clamp on the engine. That's just asking for a failed bunch of wiring. Right from the factory like that. The Dynafocal engines move a lot when starting and stopping, and torque-rotate considerably under high power, against prop drag. Wiring and cables need distance and slack to avoid breaking them.
 
Maybe you have autothrottle?

/sarcasm
 
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