Prop surge on PA28R-201 Arrow?

Vince R

Vince R

Pre-takeoff checklist
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Mar 14, 2017
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Southport NC
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Vince R
I'm a fairly new Arrow pilot, and on a recent flight I noticed the prop surging about 50-100 RPM at cruise. I had fairly low power set - 23" and 2300 RPM - and the prop would surge up to about 2400 for less than a second, then settle back to 2300 where it would just repeat over and over. Same thing at several other power settings I tried. Only time the surging would stop was with the prop full forward - it was fine during the climb and landing.

Engine seemed to be operating fine otherwise, with ample (and steady) oil pressure, no changes to MP or fuel flow, and no ill flying effects. In fact, I took off my headset and couldn't hear any audible sign of RPM change as the surging was going on. The prop seemed fine on the ground during runup.

My initial thought is to suspect the instrument, but before I get someone to look at it, just want to make sure this isn't just a stupid pilot error or something unique to the Arrow that I might have overlooked. Any ideas?
 
Never experienced that when i owned an Arrow

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I've had this happen. It's easy to inspect the cable and lube it while you're there. After you see the cable is fine pull the tach and take it to the local certified repair station that also happens to be a speedometer repair shop. They will charge about $75 or so to try to fix it. Maybe you get lucky. When the tach fails again just buy the EI electronic tach. In fact that is the best thing to do...
 
Typically when a prop hunts there is a governor problem in my experience.
 
No audible, check the tach cable first. Replaced mine years back when the needle started to drift. Wound up being freyed near drive end. You can pull the cable completely out of shield to inspect. Might also check how the linkage is on prop lever to governor. If no joy, then as others suggested, dive deeper.
 
What a terrific resource this site is...late on a Saturday night and already got five opinions after barely an hour.:)
 
How was your oil pressure during this time?
 
James331 said:
How was your oil pressure during this time?
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No change at all to oil pressure as the prop was surging - it was right in the middle of the green arc, exactly where it always is. I couldn't detect any visible movement of the needle.

Oil was maybe half a quart below full when we left, and it seemed no different when we returned an hour later. After landing, I checked pretty thoroughly for oil leaks or other obvious problems but found nothing. Except for the odd behavior on the tach, it was a completely normal flight.
 
So you're saying no actual/audible prop surge, just seeing it on the tach? Think about your audible perception of rpm change when you do your runup mag check. A 150 drop is noticeable. Guessing the issue is tach related, not prop/gov.
 
charlie said:
A 100 RPM surge will rock you for and aft in the seat! Take off the ANR headset and you would hear it.
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That's what has me thinking it's some sort of instrument error...I did take off my headset, and I couldn't hear any change at all.
 
455 Bravo Uniform said:
So you're saying no actual/audible prop surge, just seeing it on the tach? Think about your audible perception of rpm change when you do your runup mag check. A 150 drop is noticeable. Guessing the issue is tach related, not prop/gov.
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Yes, that's exactly my thinking...even a 50 RPM drop during run-up is noticeable, yet the tach surge I saw wasn't at all audible. But I am concerned that the tach seemed fine at high RPM during takeoff/landing with the prop full forward, and at really low RPM when taxiing on the ground. I'm also new to this aircraft (10 hours), so I still wouldn't rule out some sort of stupid pilot error.
 
Vince R said:
Yes, that's exactly my thinking...even a 50 RPM drop during run-up is noticeable, yet the tach surge I saw wasn't at all audible. But I am concerned that the tach seemed fine at high RPM during takeoff/landing with the prop full forward, and at really low RPM when taxiing on the ground. I'm also new to this aircraft (10 hours), so I still wouldn't rule out some sort of stupid pilot error.
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It took about 50 hours to go from first wobble to failure on my tach. I had plenty of time to read about them. The pundits say that considering the age of the fleet many tacos indicate a little lower rpm than the engine is actually turning. It's just the nature of the mechanism. The electronic tachs are better since the cable is eliminated. They will also indicate a hot mag if ya pay attention to the lights.

Anyway have fun with the troubleshooting and repair. The aircraft doesn't care if it's not telling you how fast it's turning. It'll fly the same either way. Me? I like knowing how fast the engine is turning so didn't mind the expense of the electronic tach installation. I went with the Horizon tach. Customer service has been great but I recommend the EI. Long story.
 
I flew another airplane with the Horizon - not sure I liked the digital-only display...I felt like I was always overshooting or undershooting the setting I wanted unless I really fixated on the tach. The EI seems better, though I haven't flown with one. Or, perhaps it's time to spend a little more and go for something like a JPI 900.

I guess the first step is to have it looked at - maybe it's something minor, though that never seems to be my luck.
 
Vince R said:
I flew another airplane with the Horizon - not sure I liked the digital-only display...I felt like I was always overshooting or undershooting the setting I wanted unless I really fixated on the tach. The EI seems better, though I haven't flown with one. Or, perhaps it's time to spend a little more and go for something like a JPI 900.

I guess the first step is to have it looked at - maybe it's something minor, though that never seems to be my luck.
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The Horizon seemed easy enough after a couple hours. I've found control position to be consistent and just flying one aircraft helps a lot.

A JPI would be nice and it would eliminate other gauge failures. EI has some competitive TSOd options as well. From what I've heard EI is a bit more flexible with setup than JPI so if the engine has been modified they are the way to go. I have a UBG 16 so haven't looked seriously at the full setups.
 
I had this happen with my Mooney on climb out and some on cruise at high power settings. Adjust the RPM down would help but it was still there. Did the annual and sent the governor off. $975 dollars later no more surging.
 
Mechanic took a look and found nothing wrong and I went on a short flight and it seemed fine. I hate intermittent problems.

One thing that turned up is that the aircraft was washed just before I flew it on the day it was acting erratic. The FBO has a couple of kids that wash planes, and they managed to do the Arrow by mistake (there are a few Arrows and I think they just washed the wrong one). They tape over the static ports and plug the engine openings, but I'm wondering if it's possible that they might have got water somewhere it doesn't belong, and when it dried out, the plane was fine. Does this sound plausible to anyone?
 
Vince R said:
Mechanic took a look and found nothing wrong and I went on a short flight and it seemed fine. I hate intermittent problems.

One thing that turned up is that the aircraft was washed just before I flew it on the day it was acting erratic. The FBO has a couple of kids that wash planes, and they managed to do the Arrow by mistake (there are a few Arrows and I think they just washed the wrong one). They tape over the static ports and plug the engine openings, but I'm wondering if it's possible that they might have got water somewhere it doesn't belong, and when it dried out, the plane was fine. Does this sound plausible to anyone?
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Washing shouldn't matter. The tach system isn't bothered by water for the most part. It'd be really tough to get wash water on the instrument itself. If the problem is the governor then no, washing shouldn't affect it. I don't think it's the governor since you reported no audible engine surging.

When my tach failed it took about 50 hours from first wobble to the point it was unusable. Just another thing to look forward to on these aircraft as they age. The good news is that the electronic tachs pick up signal from the mags so the tach drive cable and firewall penetration goes away.

At any rate, keep an eye on it but it's not something big to worry about. Fly and enjoy the plane.
 
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