Partial power loss

Mike Smith

Mike Smith

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May 15, 2011
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Prattville, Alabama
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Fresh Prince of PrattVegas
Today it happened. I was flying a PA28-140 that belongs to a friend. I pulled it out, preflight was fine, runup was fine, everything in the green. The takeoff was fine, making full power and the climb out was typical. At 500' agl I begin my crosswind turn and the engine stumbles for a second but only a second and resumes full power. I continue my turn and at about 600' I lost 500 RPM and the engine began to run rough. It was still making power, just not full power, I continued my turn back to the runway. I announced my intentions and landed without any problems. I turned off the runway and began to attempt to find out what was wrong. Oil pressure and temp were fine. Fuel pump was on. I had pulled carb heat during the event, there was no change. On the ground it idled fine and a ground run up only got a very slight almost not noticeable miss. However, when I would go to full power, I'd only get 2000 RPM and a very noticeable engine roughness. I leaned aggressively to clean the plugs, that's not it. I parked it and looked everything over and can't find anything that jumps out at me. I probably left some details out, but what in the world could it be? I flew it last weekend and it was fine. The other guy flew it Thursday and it was fine.
 
Sticking valve would be my guess. When you lose compression on 1 cylinder the motor will act exactly like you describe - max rpm's of about 2000, very rough running motor that will shake the whole plane.
If you were on the ground and it was still doing it, just manually rotating the prop you would feel the complete lack of compression on the effected cylinder.
 
Mike, do you always lean aggressively during ground ops and in cruise flight? Glad to hear you got on the ground in one piece!
 
Bad magneto? Any loose lead wires to plugs?
 
mscard88 said:
Bad magneto? Any loose lead wires to plugs?
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No loose wires, P-leads etc. When I did a run up after I landed, at 1800 RPM, both mags seem fine. @75 RPM drop each. It *might* be running a little rough @ 1800, if so, it is very slight. Go to full power, only get 2000 and it runs ROUGH.
 
Mike Smith said:
No loose wires, P-leads etc. When I did a run up after I landed, at 1800 RPM, both mags seem fine. @75 RPM drop each. It *might* be running a little rough @ 1800, if so, it is very slight. Go to full power, only get 2000 and it runs ROUGH.
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When you pull carb heat during the runup, do you get the standard drop?
 
Could still very well be fouled plugs that did not get cleaned off during your burn off attempt. I had the SAME thing as you described and the A/P found several pretty cruddy plugs so he pulled them all, cleaned them and was good to go.

That has been my one and only true pucker factor moment to date!

I would at minimum pull the plugs and check 'em.
 
Could still be carb ice if conditions were right for it. If you had a long taxi prior to take off and if there was enough build up turning on the carb heat might not be able to clear it and you could run up at 1700 but when you go full power there is enough restriction to cause a problem.
 
DFH65 said:
Could still be carb ice if conditions were right for it. If you had a long taxi prior to take off and if there was enough build up turning on the carb heat might not be able to clear it and you could run up at 1700 but when you go full power there is enough restriction to cause a problem.
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Sure, but once we were on the ground it would not clear up. Ran it very lean trying to clear plugs if that was it, did several runups, never cleared.
 
I just got off the phone with the Mx, he is going to pull the plugs and check them. Start there and see what happens.
 
My guess is a stuck exhaust valve based on what you describe. You can do a quick compression check by pulling the prop through its cycle.
 
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Somedudeintn said:
My guess is a stick exhaust valve based on what you describe. You can do a quick compression check by pulling the prop through its cycle.
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Honest question (not an attempt to second-guess):
I had always heard they were most likely to stick on startup ("morning sickness") or in a quick descent with power pulled out (cooling too quickly). They sometimes stick in high power setting scenarios I take it?
 
No matter the outcome, nice job getting you and the plane back on the ground safely.
 
Omalley1537 said:
Honest question (not an attempt to second-guess):
I had always heard they were most likely to stick on startup ("morning sickness") or in a quick descent with power pulled out (cooling too quickly). They sometimes stick in high power setting scenarios I take it?
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Mine happened after about 1 hour scud running at 1000-1500 agl. I was full rich mixture but at that altitude my egt's were a couple hundred greater than they typically run at altitude.
 
I went by this morning and pulled the prop through several revolutions, I could not tell any difference. It felt like I got a compression stroke every half turn. I admittedly don't really know what I was trying to feel though. Mx didn't get a chance to look at it today, but the guy who owns the plane talked to him and he reiterated that he believes it to be a bad plug, but will have a better idea tomorrow.

ETA, full disclosure, the flight was to be my first instrument lesson. My CFI-I was in the right seat, I had a ton of help and the fact he was there, I never felt worried.
 
Mike Smith said:
I went by this morning and pulled the prop through several revolutions, I could not tell any difference. It felt like I got a compression stroke every half turn. I admittedly don't really know what I was trying to feel though. Mx didn't get a chance to look at it today, but the guy who owns the plane talked to him and he reiterated that he believes it to be a bad plug, but will have a better idea tomorrow.

ETA, full disclosure, the flight was to be my first instrument lesson. My CFI-I was in the right seat, I had a ton of help and the fact he was there, I never felt worried.
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IME if an engine runs smoothly at low power but rough and weak at full throttle it's almost always fuel system related.
 
Not the most likely thing but I've had similar problems, chased them for weeks, went through everything and found nothing, and then a friend told me to drain the carb bowl. Low and behold I had a little bit of water in there. Apparently water doesn't like to feed up the main jet but will sit in there and disrupt fuel flow at high power settings. One more thing to add to the list if nothing else makes sense. I run some isopropyl every now and then to prevent it from happening again.
 
My gut feeling without running it is a MAG problem. One single plug wouldn't cause that kind of roughness.
Order of my troubleshooting would be:
-Check the tailpipe like others suggested
-Compression checks
-Clean and test plugs
-Check the strainer and tanks for water....again :)
-Could certainly be an intermittent valve issue. Hopefully, it would show on the compression test.
Good luck I am sure you MX will do those things
 
Omalley1537 said:
Honest question (not an attempt to second-guess):
I had always heard they were most likely to stick on startup ("morning sickness") or in a quick descent with power pulled out (cooling too quickly). They sometimes stick in high power setting scenarios I take it?
Click to expand...

This is what a stuck exhaust valve did to me today. Mag check good. Normal flight until I saw EGT on one cylinder go to 1460. Immediately turned back to the airport.

After shutdown, a big puddle of oil underneath the aircraft. Pull the cowling and a bent pushrod and cracked spacer that goes to the rocker arm. Don't know yet how the tappet will look like. Probably have something else internal broken with my luck.
 

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Velocity173 said:
This is what a stuck exhaust valve did to me today. Mag check good. Normal flight until I saw EGT on one cylinder go to 1460. Immediately turned back to the airport.

After shutdown, a big puddle of oil underneath the aircraft. Pull the cowling and a bent pushrod and cracked spacer that goes to the rocker arm. Don't know yet how the tappet will look like. Probably have something else internal broken with my luck.
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Where's the "don't like" option. At least you made it to the ground safely.
 
Well, the news is in and it isn't good. Mx say it was a stuck valve/bent push rod/shattered valve and something I don't really understand. I got this second hand from the owner, apparently there was something that broke into pieces and some pieces went into the case. Mx says he has been unable to get all the pieces out and therefore the engine will have to be removed and the case split. The engine has high time and I suspect there is an overhaul in its future.
 
Mike Smith said:
Well, the news is in and it isn't good. Mx say it was a stuck valve/bent push rod/shattered valve and something I don't really understand. I got this second hand from the owner, apparently there was something that broke into pieces and some pieces went into the case. Mx says he has been unable to get all the pieces out and therefore the engine will have to be removed and the case split. The engine has high time and I suspect there is an overhaul in its future.
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Wow just glad you made it back to the runway. I'm familiar w/ P'ville and there's some rugged terrain depending where you're at in the pattern. You're probably correct that an overhaul might be in order. Might be a great time to make an offer on it? ;)
 
mscard88 said:
Wow just glad you made it back to the runway. I'm familiar w/ P'ville and there's some rugged terrain depending where you're at in the pattern. You're probably correct that an overhaul might be in order. Might be a great time to make an offer on it? ;)
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We were taking of on 9 which has the most options if you have a problem. Like I said, it was still making partial power, so the runway was never in doubt. I had already turned about 90 degrees when it happened and I am very confident that we could have made the runway had it completely quit. I am considering an offer of some sort to become a partner, The guy JUST bought the airplane. I think it has been flown @ 3 hours since he bought it, i was just lucky enough to be flying it when it decided to die. Needless to say he is a little sick, but he and I talked about this very scenario before he bought it and I encouraged him to have a prebuy which he declined. I feel bad that it happened the way it did, but we had talked about this possibility.
 
Lesson learned concerning prebuys, but at least it's not in a crumbled heap off the end of 27. Hope it all works out for y'all.
 
Would a pre-buy have discovered the issue?
 
I once had a total power loss and mistook it for a partial power loss. Fortunately, I was within easy gliding distance of a runway. I found out it was a total power loss after landing, when I got slowed enough for the prop to stop windmilling. :eek2:

Since then, I have come to the realization that the same method can be used to find out if it is a total power loss as is used to confirm which engine is dead on a twin: pull back the throttle and see if the RPM (or manifold pressure) changes.
 
FORANE said:
Would a pre-buy have discovered the issue?
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I suppose that is unknown. Every prebuy is different, some dig deep, some not so deep. Even then, who knows. One thing is certain, nothing is caught on the prebuy you dont have.
 
Obviously.

In my line of work, "making power" is pretty much understood as being synonymous with "making takeoff, or rated power". The only time I've ever heard something similar to "partial power" used is in reference to "part power" which is a Pratt & Whitney rigging term.
 
Ok, you win. Your definition of making power is correct. In this case the airplane was not making power, its was just not not making power, not all the power since it wasn't making that.
 
GlennAB1 said:
Obviously.

In my line of work, "making power" is pretty much understood as being synonymous with "making takeoff, or rated power". The only time I've ever heard something similar to "partial power" used is in reference to "part power" which is a Pratt & Whitney rigging term.
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So how would you prefer us to distinguish between producing less than rated power, and producing zero power? Or is that a taboo subject?
 
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