IO540 Power fluctuation

[jfrank71]

I had an unusual occurrence on a three leg trip today. First leg, 140nm, all was normal. 169kts TAS cruise on 14gph at 5500; full MP, 2400 rpm. This is very normal. I usually see 169-170kts TAS at cruise with varying fuel burn based on altitude. Second leg, 150nm, 5500feet same settings but only 160kts TAS. Third leg, 190nm, same settings at 8500 feet. Here is the strange part, I was only cruising around 155kts TAS. Then it would drop down as low as 147kts TAS and go up to max of 163kts TAS. It bounced between those min/max speeds about 3 or 4 times. Engine was running smooth, no fluctuations on CHT or EGT, RPM or MP. I have talked with my close RV friends and we are thinking maybe clogged injectors or air induction leak. Any thoughts or ideas out there? Similar things happened to you? Would love some feedback.

[aturner]

If there was no change in engine parameters, then I would be looking at the pitot static system, and not the engine.

[Kahuna]

Ive seen speed fluctuations as high as 20kts, in cruise, smooth air, in thermal waves with the autopilot handling to the point of me not even noticing.

[Sig600]

Originally Posted by Kahuna

Ive seen speed fluctuations as high as 20kts, in cruise, smooth air, in thermal waves with the autopilot handling to the point of me not even noticing.

This. What were the winds aloft? You perfectly describe mountain wave, all be it, gentle.

[rvmills]

With Kahuna on this. On legs 1 and 2, the TAS delta might be due to OAT differences...did you by any chance note them? If you were heavier on leg 2, that could have an impact too, heavier would cost speed. Could be a combo.

On leg 3, with no change in the engine parameters, it could have been updrafts and downdrafts, perhaps similar to mountain waves. Where were you flying?

Cheers,
Bob

[jfrank71]

I was flying in South Central Texas. COT to AQO to CLL to COT. Third leg, I took on 140lbs fuel and 475lbs cargo and passengers. Close to gross on departure third leg. On third leg, OAT between 59-61 deg F. Temps were likely lower with 11am departure for leg 1 and 1:45 departure for leg 2. and 3:15 departure for leg 3. Very low ALT setting of 29.53in. on last leg. Definitely an unusual weather day. Very clear, very windy.

[Kahuna]

Originally Posted by jfrank71

I was flying in South Central Texas. COT to AQO to CLL to COT. Third leg, I took on 140lbs fuel and 475lbs cargo and passengers. Close to gross on departure third leg. On third leg, OAT between 59-61 deg F. Temps were likely lower with 11am departure for leg 1 and 1:45 departure for leg 2. and 3:15 departure for leg 3. Very low ALT setting of 29.53in. on last leg. Definitely an unusual weather day. Very clear, very windy.

And there you have it. Weight, Temps, Wind. A combined set of circumstances. Mostly the latter if you were getting fluctuations, and mostly the former to get the speed reductions.

[Pat Stewart]

Originally Posted by Kahuna

And there you have it. Weight, Temps, Wind. A combined set of circumstances. Mostly the latter if you were getting fluctuations, and mostly the former to get the speed reductions.

Hey Frank, from a guy who knows nothing about your airplane I would agree with Kahuna but yank the cowlings off and check everything just to be safe.

[jfrank71]

Thanks for all of your input guys. Tonight, the cowl comes off, put eyes on everything and quick oil change while I'm down there.

[Paul K]

Frank,

You need to analyze the engine performance if you suspect the engine in any way. CHTs, EGTs, fuel flows, etc. if none of this data changed, then it's not the engine!