ENGINE DIAGNOSIS, What's your guess?

[Bill Greenwood]

Ok, all you A&Ps, engine builders and just plain smart guys, here is a real case. What is your diagnosis or at least best guess?
While this happened on a factory built airplane, it could also apply to some, (not all) warbirds and many other planes.
I was flying my 1988 Beechcraft B36 TC Bonanza when a problem occurred. I flew from Colorado to Texas, no problems on that leg, cruising at 11,500 at 28" and 2300 rpm, burning 18 gph. After a fuel and lunch stop in Amarillo the run up was normal. We took off and climbed to 9500. I thought I noticed a very slight vibration. I leveled off at 9500' and reduced to cruise power, 28, 2300 and leaned to about 18gph, just like many times before. I noticed a small, but definite vibration. I could see a slight ripple in the compass fluid, and I could see it on the top of the panel, just a slight wiggle. I looked at all the engine gauges, all seemed well and normal. Even the Gem gauge showed all cylinders egts and head temps as normal. I could not hear any miss in the engine, but there seemed to be just enough vibration that I couldn't ignore it. I even considered that there was very light turbulence, but that did not seem to be it. I returned to landing. Post flight runup was normal, just as it was before. I parked and checked the engine compartment; no leaks or obvious problems. No prop nicks, the spinner was firmly attached.
I had the A&P check the engine our throughly. Good compression, plugs good and clean, just like the annual a few weeks earlier. No metal in the filter, no leaks. So no problem found.
It ran fine on the next flights, but still seemed to be not quite as smooth as before.

So what is your guy's best guess of the cause?

[shrike]

Start with simplest first. Plugs and plug wires would have been my first suggestion, but if they're clean and tight, move to the mags. At that low an altitude, it's probably not mag pressurization, but a bad hose to the fitting could create just enough of a difference between them to cause a little bit of roughness. If you don't have pressurized mags you can forget that. Check mag timing. How many hours on them? Bendix and Slick both like a service every 500hrs.

[AndyG]

Shot engine mount bushes?

(pure guess)

[m charters]

Timing would be my first thought... Maybe dirty injectors, but if power is ok and temps even on cylinders that would likely not be the issue... maybe a partially blocked air reference line?

[The Inspector]

Yep! I'd look @ plugs, wires, contacts, distributor cap fingers, 'P' lead ground connection, mag timing check, and then see what might be doing the backstroke in your fuel samples (take more than one from different places/drains).

[Bill Greenwood]

Nope, plugs were fine, wires were tight. Remember that not only was the runup was fine both before the takeoff and after landing. Also the Gem Insight indicator shows all the cylinders and both egt and head temps were normal in flight. If there was a missfire in one cylinders it would show up.

I am not sure if he looked in the mags or not, but no reason to believe there was any fault there, we wern't at high altitude, only 9500' so the mag missfire is not likely.

Remember, all I felt or really saw was a very slight vibration,not even evident to someone who might not be a pilot or used to that plane.

I will put up the explanation in the morning. It is a simple thing, once you know it.

And since I did just refuel before the takeoff, we went back and drained the tank sumps. No water found, and if there had been any it would have been burned off long before I turned back. The FBO does a lot of business and is well aware of keeping the fuel clean.

Thanks

[Bill Greenwood]

Here is one more hint. This could not happen on a J 3 Cub, nor even on a T6/SNJ. It could happen on another B36 or even A36 and I once had something not exactly the same but related happen on the Spitfire that I flew in Canada. Not likiely on most Spitfires though, or a Mustang. More likely on some, not all, twins and multis.

[AndyG]

Here is one more hint. This could not happen on a J 3 Cub, nor even on a T6/SNJ. It could happen on another B36 or even A36 and I once had something not exactly the same but related happen on the Spitfire that I flew in Canada. Not likiely on most Spitfires though, or a Mustang. More likely on some, not all, twins and multis.

That's very naughty, you are going to keep us up all night now!!!

For a laugh, and I suspect your answer might be along the same lines: My father had a brand new large RV (motor home for all brits and 'small' if you are American) which was his pride and joy. One day it started making an expensive resonant howl from under the engine with vibration at certain engine speeds. He took it back to the dealer many times and they change this and they changed that, it still didn't cure it. Then they changed the expensive turbo thinking that would finally kill it. Still no joy.....
Anyway finally a smart young mechanic removed the non-standard chrome tail pipe trim dad had fitted, low and behold the noise and vibration dissappeared! The trim was regrettably held on by father by drilling a bolt right through, so that a bolt spanned the bore of the pipe, causing a vortex induced vibration to occur. The mechanic handed the pipe to an embarassed dad and walked off, nothing more was said. (They made their money as it was warranty work)

[b29flteng]

Ice in the spinner?

[Bill Greenwood]

B-29, nope no ice that day, perhaps 60 degrees and clear. However you are thinking better.And nothing on the tailpipe, nothing added aftermarket, all factory stock.

Last edited by Bill Greenwood on Sat Feb 05, 2011 3:59 pm, edited 1 time in total.

[The Inspector]

A piece of FOD of some kind in the air scoop inlet?

[Tiger Tim]

Something to do with the propeller's counterweights or possibly a stuck blade?

-Tim

[davidbray]

After noticing the TS, I'm gonna lean towards the turbocharger area.

[EW439]

hanging gear door

[b29flteng]

Something plugging the alcohol line to one of the prop blades, such as a mud dauber?