Knocking Sound Reported by Ground

M

Mister Derp

Guest
Just wanted to gauge an opinion on what y'all woulda done. This is a '69 Piper Cherokee 140. Great CAVU day. 16 deg with 2 deg dewpoint (Celsius). 30.31.

Oil level was good on preflight. Engine started as expected. Oil pressure looking good. Leaned for ground. Temperature coming up as normal. On the way to run up area, ground reported that they received a phone call from ground crew. They said my plane was making a knocking sound. During runup, oil pressure remained okay. No sound noted. Tried with carb heat hot and cold. Tried with both mags. Tried with mixture lean and rich. Still no sound. Oil pressure and now temperature looking great. On takeoff, oil pressure was pegged at the top of green, but no other abnormalities noted. On parking, nothing was of note under the cowling.

Would y'all have stopped the flight on report of what ground said? I could not sense a knocking or anything not unlike the aircraft's normal operation. My only suspicion was that because I was running lean for ground ops, the plane could have been knocking.
 
I probably would have grabbed a ramp guy and had him listen to get a second opinion before flying
 
Engines make funny noises when running really lean on the ground. If you burble on short final after pulling throttle to idle, it's because you're too rich (but rich in the pattern is good, in case you need to go around).
 
Unless I heard some unusual noise or saw some unusual gauge reading, I would have asked questions as to who heard what, what did it sound like, was it at all RPMs and did anyone else hear the noise. If no one could answer my questions, I would have continued.

I once had a tower guy tell me I had smoke coming from the right engine as I was taking off. It was the exhaust from the Janitrol heater which exits the right side of the fuselage. The temperature was -28F and it was leaving a slight con-trail. But I thanked him for being observant and kept on flying.
 
At OSH each year, we watch dozens, perhaps hundreds, of departures.

I am always amazed at the difference in sound coming from similar aircraft. We have heard engines that sounded like a can of washers being shaken. We have heard planes that sounded like a Harley at rough idle. We have heard planes that made clanking noises.

Only once, in 34 years, have we seen (and heard, on the radio) a guy who couldn't climb. He departed Rwy 27, barely cleared Hwy 41, and just cleared the strip mall beyond. He was afraid to turn at all, for fear of stalling. We figured he was running on 3 cylinders, but couldn't really tell. He apparently made it to wherever he was going, cuz we never read anything about a crash.

Otherwise, it seems, airplanes can sound pretty awful from the ground, and be just fine.
 
Mister Derp said:
...Would y'all have stopped the flight on report of what ground said? I could not sense a knocking or anything not unlike the aircraft's normal operation. My only suspicion was that because I was running lean for ground ops, the plane could have been knocking.
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yup. I would've headed back to the ramp and had someone take a listen while you duplicated your run up. why take a chance?
 
SixPapaCharlie said:
I probably would have grabbed a ramp guy and had him listen to get a second opinion before flying
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I too second the listen-in advice. But by no means am I an expert or mechanic.
And since this is PoA and the OP posted as anonymous, I am surprised I haven't seen any totally negative replies eluding to possible killing of children in schools. This is unlike PoA! :)

Btw, Mr. 6PC, that is quite unusual advice from you. I expected to read "I'd just pull the red handle". *shrug*
:D
 
You assumed they meant an engine knock? Maybe a seatbelt was rapping on the side of the plane (for example)... If someone took the time to notify me of something that seemed out of place, I'd take the time to check it out...
 
Probably heard the mag impulse coupler clicking at low idle. Some of them can be quite loud but it is normal. Don
 
Thanks for the unusually positive replies.

After a considerably thorough runup at all RPMs both with and without my headset on, I decided to fly. Instead of headed out southeast to go over the water, I went northeast to remain over land just in case. Upon landing, I peered around the engine to make sure I didn't miss anything like a cracked manifold or something. The plane had a cracked exhaust manifold in the past which led to some unusual sounds.

Anyway, this particular Piper always has a type of sound that sounds like knocking. I also lean for ground ops, and I'm positive nobody else does this. I think that's what led to the sound.

Ultimately, hindsight is 20/20 and I should have asked more questions. However, I felt comfortable flying since everything looked fine engine-wise, so I went. I had more issues avoiding birds than I did with the engine.
 
KNOCKING and DETONATION are two different things, despite the common nomenclature. A knock is part of your engine banging against another, and may be an early sign of failure. A knock loud enough to be heard over a run up requires a second opinion from a mechanic and not someone random. It is NOT caused by running lean, but rather by your engine parts not fitting.

Detonation that loud requires you to back off the leaning, and certainly don't run up that way. You'll eventually make the airplane use oil rapidly, requiring overhaul. Even severe detonation isn't likely to kill the engine, but it can get expensive.

Other noises have other appropriate responses. But if it is unidentified, it's not wise to fly it.
 
MAKG1 said:
KNOCKING and DETONATION are two different things, despite the common nomenclature. A knock is part of your engine banging against another, and may be an early sign of failure. A knock loud enough to be heard over a run up requires a second opinion from a mechanic and not someone random. It is NOT caused by running lean, but rather by your engine parts not fitting.

Detonation that loud requires you to back off the leaning, and certainly don't run up that way. You'll eventually make the airplane use oil rapidly, requiring overhaul. Even severe detonation isn't likely to kill the engine, but it can get expensive.

Other noises have other appropriate responses. But if it is unidentified, it's not wise to fly it.
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Serious questing: did you actually read his original post before posting?
 
Fearless Tower said:
Serious questing: did you actually read his original post before posting?
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Yup. If someone tells me my airplane is making banging noises, I'm taking it VERY seriously even if I don't hear it myself. There needs to be positive indications there are no problems under that circumstance.
 
Here, let's play FAA jargon bingo.

Which hazardous attitudes are being displayed by someone taking off after being told their engine is making an odd noise? ;)
 
Fearless Tower said:
But you didn't read it. No one said anything about hearing a loud sound over the engine during runup as you implied.

They thought they heard something on the ramp while he left to go to the runup area.
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And again if the engine was below 1200 rpm or so the line guy could have heard the impulse coupler clicking. If the airplane had dual impulse couplers they can be very loud. it goes away once the engine gets above that rpm. Another thing, there is no way you can get detonation taxiing on the ground. If you lean it too much in taxi the engine will just quit. Don
 
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