Duchess RPM fluctuation

[Fearless Tower]

I was recently making a return x-county in a BE-76 and started noticing a very slight rpm fluctuation in the left engine. First thing I noticed was a slight drop in groundspeed (maybe 5 kts) on the gps and while looking at the rpm gauges I saw that the left engine was showing about 50-75 rpm less. Slowly it would return to normal and the groundspeed would also return to what it had been before. This happened maybe 4 times over the course of the 2.5 hour flight. Didn't notice any real change in yaw/roll or prop sync sound. All gauges appeared normal. Only diff between engines was the left engine oil pressure runs a bit higher than the right, but well within limits.

Any ideas, or is this normal? I have about 30-40 hrs in this particular plane and can't say that I have noticed this before. Could this be an early indication of a governor problem?

 

[charlie]

It's a tach error. Anything more than 20 rpm out of synch is audible. 50+ rpm would also cause a slight yaw. Charlie Melot Zephyr Engines

 

[Ted]

Agreed, tach error. On both the Aztec and the 310 the tachs don't read with the needles perfectly lined up. You would notice the engines being out of sync.

 

[Fearless Tower]

That is kind of what I thinking, but wan't sure - 50 rpm off should be audible and I wasn't hearing anything. It sounds like the speed fluctuation may have been just gusts in the headwinds that just happened to happen about the same time as the fluctuation. I guess it was just one of those moments in long quiet cruise flight when you start over-analyzing things.

Thanks,
Andrew

 

[gprellwitz]

That is kind of what I thinking, but wan't sure - 50 rpm off should be audible and I wasn't hearing anything. It sounds like the speed fluctuation may have been just gusts in the headwinds that just happened to happen about the same time as the fluctuation. I guess it was just one of those moments in long quiet cruise flight when you start over-analyzing things.

Thanks,
Andrew

Just be glad you weren't over water!

 

[Fearless Tower]

Just be glad you weren't over water!

Yup....something about hostile environment below that makes pilots extra suspicious!

Probably should add that as another reason not to fly SE over water at night - I'd drive myself nuts looking at the engine instruments.

 

[Ted]

Yup....something about hostile environment below that makes pilots extra suspicious!

Probably should add that as another reason not to fly SE over water at night - I'd drive myself nuts looking at the engine instruments.

I find it's something that you get used to. I'm actually more comfortable flying over water than over land in a number of cases. But then again, the land I frequently fly over doesn't have a lot of landing spots, and iFlyTwins.

 

[GeronimoDriver]

I find it's something that you get used to. I'm actually more comfortable flying over water than over land in a number of cases. But then again, the land I frequently fly over doesn't have a lot of landing spots, and iFlyTwins.

Amen, Southeast Alaska here and I have basically the same plane you do. If all else fails stay over the blue stuff on the GPS, at least you know how high the water is...

BTW, my tachs don't exactly match up either

 

[Ted]

Amen, Southeast Alaska here and I have basically the same plane you do. If all else fails stay over the blue stuff on the GPS, at least you know how high the water is...

BTW, my tachs don't exactly match up either

I'll have to come visit, I'd really like to Alaska.