Power rolled back smoothly on me uncommanded

SixPapaCharlie

SixPapaCharlie

May the force be with you
Joined
Aug 8, 2013
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First Officer 6PC
Normally aspirated O-540

Hadn't flown in awhile so took my time, ran through everything a few times. Checklist, sumped. etc .. Runup was fine.

Got on the runway, full power and after about 5 seconds, the engine very smoothly powered back. No cough. No stumbling. It was as if I gently pulled the throttle back.

I pulled off the runway and did several more run-ups and held in full power with no anomalies noted.

Did mag checks. No issue. Idle. Full. And everything in between. Could not recreate.

I'd think fuel contamination would result in stumbling. This was a very smooth loss of power.

Any thoughts?
 
What happened after? Did you takeoff or taxi back to ramp? Strange
Don’t believe 0-540 prone to carb ice?
Did you sump afterwards?
 
Any monitor data*?
Smooth rollback sounds like fuel to me; could a plugged fuel tank vent do this? *and if data, I'd expect a lean signature if it's fuel starvation.
 
Let'sgoflying! said:
Any monitor data*?
Smooth rollback sounds like fuel to me; could a plugged fuel tank vent do this? *and if data, I'd expect a lean signature if it's fuel starvation.
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I have actually had this happen once before with a plug fuel vent but I have since put screens over them to prevent mud daubers from building nests inside.
 
Smooth and still running ? I assume you closed the throttle before power came back? How long was the event?

A big slug of water would end combustion until it passed through - but the engine would take a moment to coast down.

Ain't no A&P, but if it were my airplane I would do a real close look, grab, wiggle, shake of the throttle cable and all the linkage under the cowl just in case something is "slipping" and grabbing. Something like where the throttle cable housing is anchored...

Also, saw an accident report just a day or so ago - loss of power due to baffles in the muffler coming unloose inside.
 
Capt. Geoffrey Thorpe said:
Also, saw an accident report just a day or so ago - loss of power due to baffles in the muffler coming unloose inside.
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Loud pipes save lives?

I had that happen in a camaro years ago. It would occasionally lose power and overheat; turned out it was baffle in muffler obstructing the outflow of exhaust.
 
Capt. Geoffrey Thorpe said:
Also, saw an accident report just a day or so ago - loss of power due to baffles in the muffler coming unloose inside.
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Worth checking the induction side as well; a guy around here ended up in the trees when a bee nest in the throat of the carb dislodged shortly after takeoff.
 
Stupid question... But were you guarding throttle to make sure it didn't roll back?
 
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Salty said:
What are you using to determine "power rolled back"? What does "power rolled back" mean exactly?
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Went from full power to idle.

Edit: well, ~ halfway. Once it was obvious, I pulled the throttle to avoid a mishap
 
Does the Comanche have a RUN/STOP key, like a C64 or a 787?
 
Do you lean during taxi or runup? If so, any chance you forgot to set mixture back to rich for takeoff?
 
Last edited:
eman1200 said:
I was gonna ask if the cufflinks on his epaulette pilot shirt got caught and brought the throttle back lol
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My first thought was giardiniera contamination :dunno:

Fuel, air, spark.
Could be a plugged fuel filter, but that seems less likely. How's the fuel pressure at wot? Boost pump on?
Plugged air filter... Try alternate air?
Intake tubing collapse
Carb ice? How humid was it? First flight in a while... Longer than normal at idle?
Exhaust flame arrestor collapse
Most of those should manifest with a full power run up.
Mags both working after probably rules them out, although I had one that would cut out when it got hot. Took about an hour to get that hot though.
Maybe a governor issue? Again, seems less likely, especially if it was working later.

Id probably check the intake and mufflers, then go do a high speed taxi/aborted takeoff practice to see if I could repeat it, watching MP, RPM, fuel pressure.

Did you ever get your engine monitor put in?
 
Could the throttle friction lock been loose?
 
Jim K said:
My first thought was giardiniera contamination :dunno:

Fuel, air, spark.
Could be a plugged fuel filter, but that seems less likely. How's the fuel pressure at wot? Boost pump on?
Plugged air filter... Try alternate air?
Intake tubing collapse
Carb ice? How humid was it? First flight in a while... Longer than normal at idle?
Exhaust flame arrestor collapse
Most of those should manifest with a full power run up.
Mags both working after probably rules them out, although I had one that would cut out when it got hot. Took about an hour to get that hot though.
Maybe a governor issue? Again, seems less likely, especially if it was working later.

Id probably check the intake and mufflers, then go do a high speed taxi/aborted takeoff practice to see if I could repeat it, watching MP, RPM, fuel pressure.

Did you ever get your engine monitor put in?
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WAY longer than normal at idle.
Like an hour. I was troubleshooting an iPad grimlen before I taxied to the Runup area. Could that lead to something? I did lean on the ground during that period.
 
did you hammer the throttle in quickly? isn't there some kind of issue with throttling up way too fast....forgot what it's called....
 
SixPapaCharlie said:
WAY longer than normal at idle.
Like an hour. I was troubleshooting an iPad grimlen before I taxied to the Runup area. Could that lead to something? I did lean on the ground during that period.
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Like 1000 rpm idle, or idle idle?
 
SixPapaCharlie said:
Went from full power to idle.

Edit: well, ~ halfway. Once it was obvious, I pulled the throttle to avoid a mishap
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I'm still curious what you mean by "power". Few of us have a "power" gauge. How did you gauge a half loss of power?

I still doubt we are understanding the symptom you experienced. I asked about prop governor for this reason. Was your prop going to full pitch slowing RPM's? What exactly happened? "Loss of power" is an interpretation of something. What is that something?
 
Salty said:
I'm still curious what you mean by "power". Few of us have a "power" gauge. How did you gauge a half loss of power?

I still doubt we are understanding the symptom you experienced. I asked about prop governor for this reason. Was your prop going to full pitch slowing RPM's? What exactly happened? "Loss of power" is an interpretation of something. What is that something?
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he told his wife to make dinner. she said make it yourself.

power lost.
 
SixPapaCharlie said:
WAY longer than normal at idle.
Like an hour. I was troubleshooting an iPad grimlen before I taxied to the Runup area. Could that lead to something? I did lean on the ground during that period.
Click to expand...

Did you have the boost pump on? Comanche can be prone to vapor lock on long ground runs with the boost pump on because of the way the fuel lines run near the muffler, but that generally doesn't get your attention until you switch them off.
 
SixPapaCharlie said:
WAY longer than normal at idle.
Like an hour. I was troubleshooting an iPad grimlen before I taxied to the Runup area. Could that lead to something? I did lean on the ground during that period.
Click to expand...
I was thinking of carb ice. Melted when you went full power and bogged the engine down. It would explain why you couldn't reproduce it.
 
SixPapaCharlie said:
WAY longer than normal at idle.
Like an hour. I was troubleshooting an iPad grimlen before I taxied to the Runup area. Could that lead to something? I did lean on the ground during that period.
Click to expand...
Thinking vapor lock somewhere in the fuel system. That’s a long time to idle in the Texas heat.
 
I’m also thinking lead deposits in the top end of the engine. There is a chemical, Ethylene Dibromide, which is a lead scavenger and helps remove lead from the combustion chamber. This chemical only works above a certain temperature, and depending on what RPM you idled at, this chemical may not have been active. This article from Shell recommends running the engine briefly at 1800RPM just before shutdown to be sure the lead is scavenged from the engine before shutdown.

Lead Fouling | Preventing Lead Fouling in Aircraft | Shell Global

Learn about lead fouling in engines using Avgas 100LL, its causes, and practical tips to prevent build-up, ensuring optimal engine performance and longevity.
www.shell.com www.shell.com
 
Bill said:
I’m also thinking lead deposits in the top end of the engine. There is a chemical, Ethylene Dibromide, which is a lead scavenger and helps remove lead from the combustion chamber. This chemical only works above a certain temperature, and depending on what RPM you idled at, this chemical may not have been active. This article from Shell recommends running the engine briefly at 1800RPM just before shutdown to be sure the lead is scavenged from the engine before shutdown.
Click to expand...
:yeahthat: That is why I asked about idle rpm.
 
Bill said:
I’m also thinking lead deposits in the top end of the engine. There is a chemical, Ethylene Dibromide, which is a lead scavenger and helps remove lead from the combustion chamber. This chemical only works above a certain temperature, and depending on what RPM you idled at, this chemical may not have been active. This article from Shell recommends running the engine briefly at 1800RPM just before shutdown to be sure the lead is scavenged from the engine before shutdown.

Lead Fouling | Preventing Lead Fouling in Aircraft | Shell Global

Learn about lead fouling in engines using Avgas 100LL, its causes, and practical tips to prevent build-up, ensuring optimal engine performance and longevity.
www.shell.com www.shell.com
Click to expand...
That was worth the read.
 
If you had this happen before, same symptoms to you, from a clogged fuel vent I would bet a non-Chicago hotdog that this is a vent problem, too. I'm not positive, but on a PA-28 I think each tank usually has three vents - one below on purpose, one in the cap on purpose, and usually a gasket on the fuel cap that's bad enough for rain to get in but that also functions as an extra vent. If you've only got one vent per tank, I'm wondering if something that was lodged in there before got stuck in there like clots sometimes do to people, and you had a mini-stroke. No idea how to troubleshoot or fix.

Not an A&P, and clearly not a cardiologist.
 
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