Question about engine

DavidWhite

DavidWhite

Final Approach
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I have an O360A4M and an electric tach. When I'm flying sometimes the RPM is jumpy. It'll go up and down by 15RPMs or so and you can feel it when it does. Anyone know what this might be?
 
Oil temp/pressure and EGT is normal. Its just really frustrating. Sometimes it does it and other times it doesnt.
 
and you can feel it when it does
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Can you explain this part David? I don't think I would hear an engine lose 15rpm, or feel it in the controls/seat - or do you mean you have your hand on the instrument, maybe something else?
 
Let'sgoflying! said:
Can you explain this part David? I don't think I would hear an engine lose 15rpm, or feel it in the controls/seat - or do you mean you have your hand on the instrument, maybe something else?
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You cant hear it, but you can feel it. Its just like a bump. The engine is really smooth otherwise and if I keep an eye on the tach I can feel the bump when it drops/climbs
 
Seeeeee what that Instrument Rating did to you? Noticing 15 RPM variances now? ;)
 
It sounds like the magnet/spring set in your tach is getting weak and the needle is bouncing in turbulence to me.
 
Are you talking about how it quits running for a split second? Doubt you'd even see it on the tach. It did the same thing with the old tach and the new one is a brand new electronic. Sometimes it'll do it in flight, other times in the air. I've right seate probably 70 hours in that plane.
 
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jesse said:
Are you talking about how it quits running for a split second? Doubt you'd even see it on the tach. It did the same thing with the old tach and the new one is a brand new electronic. Sometimes it'll do it in flight, other times in the air. I've right seate probably 70 hours in that plane.
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"quits running"?

That would be sensed without a tach indication, I would hope!
 
That'd be the momentary misses that exist. Not uncommon when you're leaned out, especially if your plugs or harnesses are old/weak. Your mag checks will still pass just fine when this happens.
 
You hear these much better at night.

Am I right?
 
I have noticed this syndrome when over vast expanses of water or mountains as well.
 
Mine (IO520) does the same thing, always has - and I'm not worried about it.
 
How does your electronic tach get its rpm data? From the tach drive cable, or is it connected to the mag p-leads?
 
DavidWhite said:
I have an O360A4M and an electric tach. When I'm flying sometimes the RPM is jumpy. It'll go up and down by 15RPMs or so and you can feel it when it does. Anyone know what this might be?
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Over-lean?...Ted's right ....I've seen this too. These planes get old and the mags, harnesses, plugs etc get old...just did a 182 firewall forward. Look in the logs and just see when any of these items were replaced. Also look at the voltage regulators (solid state needed and alternators)...stray voltage can kill all sorts of stuff.
 
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It seems that RPM behavior changes with mixture, according to the video you posted.
That would seem to rule out electrical issues, and point more toward ignition issues.

Other than that, I'm out of my depth on this one.
 
Let'sgoflying! said:
Mine (IO520) does the same thing, always has - and I'm not worried about it.
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Yer the smart one.. the electronic tach is much more sensitive than the mechanical governor. It will pick up the 1 or 2 RPM deviations in load that the Governor won't.
 
Tom-D said:
Yer the smart one.. the electronic tach is much more sensitive than the mechanical governor. It will pick up the 1 or 2 RPM deviations in load that the Governor won't.
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I have the EI electronic tach, which is filtered to a resolution of 10rpm. So you don't see changes of less than that. It makes things easier, and little rpm fluctuations don't capture your attention during your scan (you don't see it out of the corner of your eye constantly changing).
 
There's nothing I'm worried about here. As I said, you might have a weak plug or lead. If you have an engine monitor, do an in-flight mag check. If one of the EGTs drops out quickly when you turn off a mag, you should know which plug is causing any issues.

Given how small of a fluctuation that is, though, you might not even have anything wrong. I wouldn't worry about it, a leaned out in-flight mag check will tell you if there's something wrong.

Frequently, I've talked to A&Ps who don't like engine monitors because once their customers buy them, they discover all kinds of "problems."

"Oh no, my CHTs are 50 degrees apart from highest to lowest!"
"A travesty, my EGTs aren't exactly in line with eachother!"

etc. etc.
 
Henning said:
The FAA allows a tach pickup on the p-leads? Wow, that's stupid.
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I'm in the other camp, the one that sez it's okay. The tach has an indicator for no signal, i.e. broken lead. It uses both leads so pick-up is redundant. There is no singal point of failure until inside the instrument, unlike mechanical or signal generator driven instruments.

All that said, the horizon tach in the frankenkota did fail in flight once. Had to cycle the master switch restore function. Horizon did repair the tach under warrantee. They were quite interested in seeing the instrument when I just called to report the behavior. It had only done it once in a couple hundred hours and I thought it might just have been a voltage spike that gave it temporary insanity.
 
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