-10 pulling the prop

[jbell2355]

I'm looking to make sense of the idea that we need to turn the prop post-flight on our -10s. Doing this to remove heat from the engine makes no sense...the prop just turned hundreds of times while it spun down, a few more turns isn't going to make a difference. Preventing shaft bow seems like a good rationale. However, the shaft will make a full rotation with just a few inches of movement of a prop blade so why are we pulling 10+ blades?

If we're doing this to prevent shaft bow, we would only need to pull the prop through when the engine is going to fully cool down. We wouldn't need to do it on a quick turn, right? How long after shutdown should we pull the prop through? Doing it immediately after shutdown doesn't make sense to me, but waiting too long would be pointless. Where is the sweet spot?

Have any of you witnessed a seized -10 with your own eyes or is this an OWT?

[donv]

There are two things going on here.

1) Shaft bow. This happens when the engine is tight, just after overhaul or hot section. You will feel a bit of resistance when you move the prop-- the blade tip only needs to move an inch or two for you to feel it. Sort of a scraping sensation. If this happens, move the blade tip back about an inch and let it rest. This is only an issue when the engine is warm.

I have never personally felt shaft bow on a 331, although I have felt it on a 731.

2) Engine cooling - fuel nozzle coking. This is the reason to spin the engine after shutdown. We started doing it in the 1980s after hearing from the airline Metro operators that over time, it reduced their hot section costs. They flew so many hours and so many cycles that it would make sense that they would know.

Does it work? I don't know. Barry Lane claims it is superstition. I do it anyway.

What is really important in this area (and where Barry and I agree) is the 3 minute cool down after landing. When the RPM comes down to low, you want to keep the engines running for a minimum of 3 minutes. I start a timer when I turn off the runway, and I don't shut down until 3 minutes have elapsed, even if I have to sit there on the ramp for a bit. This is in the AFM and definitely has an effect.

[jbell2355]

Thanks, Don. I really appreciate your reply!

[donv]

In regard to shaft bow, the 980 AFM states the following:

"If the engine is to be restarted within approximately 10 to 45 minutes after engine shutdown, engine rotating group should be rotated 180 degrees, approximately 10 minutes before restart, to prevent shaft bow due to uneven cooling. To rotate, observe engine compressor and slowly move propellor in the normal direction of rotation, until the compressor rotates one half rotation. Propellor tip will move approximately six inches. Failure to accomplish this procedure may cause a hot start and possible engine damage."

[donv]

If you don't have a copy, the "331 Pilot Notes" from Honeywell is a great document. It says this about shaft bow:

ROTATIONAL RESISTANCE DUE TO SHAFT BOW IS UNUSUAL EXCEPT FOR THE INITIAL FEW HOURS OF OPERATION FOLLOWING REPLACEMENT OF THE INTER- STAGE AIR SEALS.

I just checked, and the 3 minute cool down is in the 331 Pilot Notes, not (that I could find) directly in the AFM.

[Bruce Byerly]

Don is right. For some more history; Turning the props after shutdown is legitimate excercise that has been scientifically documented in terms of reduced peak soak back temperatures.  A lower residual temp was found to reduce coking on the nozzles significantly which in turn encouraged clean nozzles and a better spray pattern. If fuel dribbles out from a plugged nozzle, it builds up carbon, which is what causes erosion of the parts you don’t want eroded. The genesis of all the research, as I recall, was premature wear and reduced power output on high cycle quick turning commuter airliners.  Detailed charts of internal temp rise with and without the excercise and some of the other research was presented years ago at some Garrett class and maybe Helmuth too but sadly I don’t see where that info is these days. Someone ought to track Helmuth down for a copy of his materials.

So, believe the engine engineers or don’t, but it can’t hurt anything to give it some fresh air.  I’ve always disagreed with Barry on this out of an abundance of caution rather than my own science because, how are you going to really know?  I try not to bang the gearbox around when turning props. Not sure if that matters either but it makes me feel better.

Shaft bow is totally separate but should be watched like a hawk on new or recently serviced engines. Not an issue if you aren’t turning within about an hour but you can do chin ups on a prop on a new engine and, unless you turn it 1000 times, you’re not preventing shaft bow with a few turns of the prop. As it cools, there will be a period where you can get 180 degrees on the crank and stop and it seem to help the shaft straighten out quicker.  But if you’re not careful, you will machine the clearances down prematurely which is not what you want to do imho. There is a crazy amount of torque available pulling the end of a prop given the reduction gearbox ratio.

[donv]

Here is a copy of the 331 Pilot Notes. If you don't have it, it is a great resource.

[appleseed]

"There is a crazy amount of torque available pulling the end of a prop given the reduction gearbox ratio."

It is actually the opposite, but I agree with everything else you said.

The one thing I don't think has been specifically mentioned yet is to give the props a gentle spin anytime there is a possibility of shaft bow. All it takes is a quarter turn when you walk up to the plane to make sure shaft bow is not causing a rub. Say any time between ten minutes and two hours after engine shutdown. Really, no reason not to be in the hroutine of pulling props before every start.  I'm in the habit of a "last chance" 30 second walk around as pax are boarding. Chocks, gust locks, inlet plugs, ramp furniture in the blind pullaway zone (thankfully this does not exist on Twin-Commanders), props spin free, and fuel caps. I haven't flown any Commanders with the only door underneath the wing, but on Shrikes and Turbos, I standup in the door sill to look at the wing before climbing in.

[donv]

I just pull each prop one blade before start.

[Bruce Byerly]

"There is a crazy amount of torque available pulling the end of a prop given the reduction gearbox ratio."

It is actually the opposite, but I agree with everything else you said.

The one thing I don't think has been specifically mentioned yet is to give the props a gentle spin anytime there is a possibility of shaft bow. All it takes is a quarter turn when you walk up to the plane to make sure shaft bow is not causing a rub. Say any time between ten minutes and two hours after engine shutdown. Really, no reason not to be in the hroutine of pulling props before every start.  I'm in the habit of a "last chance" 30 second walk around as pax are boarding. Chocks, gust locks, inlet plugs, ramp furniture in the blind pullaway zone (thankfully this does not exist on Twin-Commanders), props spin free, and fuel caps. I haven't flown any Commanders with the only door underneath the wing, but on Shrikes and Turbos, I standup in the door sill to look at the wing before climbing in.

You’re right on that gear ratio - I must be thinking about radials and hydrolocking lol

On the other hand, try to hold the impeller while you pull the prop …

[jbell2355]

Don, thanks for the reminder about the Pilot Notes - that document is very helpful indeed. Here are the complete notes related to shaft bow for anyone interested:

 Shaft Bow is known to occur rarely and only between 10 and 45 minutes after a ground
engine shutdown upon completion of flight or a high power run-up.
- Following engine shutdown (no forward airspeed), hot-air eddy currents are generated
within the static engine.
- With no airflow through the engine, heated internal air rises, leading to a thermal gradient
vertically through the engine.
- Cooling starts from the bottom upwards, which causes the main rotating group to be slightly
hotter in the upper half, resulting in a slightly bowed shaft.
- In this situation, when the propeller is turned by hand, contact may be noticed between the
interstage turbine seals and the stationary abradable seal surfaces.
- If shaft bow is suspected during the pre-flight inspection:
PROPELLER – ROTATE BY HAND
- Rotate in normal direction of rotation in order to avoid damage to carbon brushes in the
starter/generator.
- Stop rotation of the propeller at the point of highest resistance, which relates to 180 degrees
displacement of the main rotating group (hotter half of the shaft is now at the bottom).
- This position allows the thermal gradient to neutralize as cooling continues.
- Allow approximately three minutes, depending upon ambient variables.
- After this additional cooling, check again for rotational freedom.
- If complete freedom of rotation is not obtained, repeat process until complete freedom of
rotation is obtained.