Turning Engine

[RVadmirer]

I've recently read that a good part of engine care is to rotate the prop backwards a few times before starting to get the feel of the compression of all cylinders and to be able to note any changes.
I had also read that rotating a vac pump backwards is a good way to shorten their life. Can any engine techs comment on this?
Thank you.

[bharral]

Rotating the prop by hand can serve as a poor man's compression check. I don't do that as a regular (every flight) practice, but do it on occasion. I do that check in the forward direction, and apply the usual cautions.

In the event that you had impulse coupled mags, AND left them hot - then it would probably be safer to rotate in the reverse direction!

Some vacuum pumps have canted vanes, so rotating them backwards means they are not running in their "design direction" (however, they are also not turning as fast as they do at cruise). I do not hesitate to turn the prop backwards for 1/4 revolution (as opposed to 3/4 forward) when I find it convenient to do so.

I don't think there is much 'hard data' on the effects of turning the prop backwards. I expect you will discover that practices in this regard are much more correlated to personalties then to any significant statistical data (BTW, that does not mean that they are wrong!)

[RScott]

As my engine winds down at shutdown, it turns backwards for about 3/4 of a revolution. It's normal. But I don't have a vacuum pump. I think all or at least many engines to that.

Since I hand prop the plane, I get a sense of the compression every time I start it. You can feel a 20 pound difference in compression. When rotating by hand, always do it as if you are hand propping it--with mags off, of course. Pull it thru in a manner that gets your hands out of the way just before the impulse coupling snaps.

[penguin]

Originally Posted by RVadmirer

I've recently read that a good part of engine care is to rotate the prop backwards a few times before starting to get the feel of the compression of all cylinders and to be able to note any changes.
I had also read that rotating a vac pump backwards is a good way to shorten their life. Can any engine techs comment on this?
Thank you.

I'm not sure that is good advice. Lycoming recommends to never turn an engine backwards, it may well shorten the life of a vac pump. If you are interested in 'feeling' the compression then why not rotate forward? I also think that is not a great idea as it rubs any residual oil off the cylinder walls without working up sufficient oil pressure to send any oil around the system.

I don't turn the engine at all before I start up. If its been a while since I've flown I will not prime until I have oil pressure (5 to 8 seconds). I can't see what you gain by pulling then engine through by hand - especially not backwards.

Pete

PS I don't agree with RScott that its normal for engines to run backwards at shutdown - most Lycomings don't.

[asav8tor]

Originally Posted by penguin

Pete

PS I don't agree with RScott that its normal for engines to run backwards at shutdown - most Lycomings don't.

I think what he was describing was on shutdown as the prop slows to the last compression stroke it often doesn't make it past TDC and flips back a little. Not runs backwards.

Aside from that, turning the prop backwards seems like the oil pump would be pumping backwards.

[Andy Hill]

AAIB Report

A Textron Lycoming Service Instruction, No L180B, issued in November 2001, contains advice on engine preservation for active and stored aircraft. In particular, it recommends a procedure to be followed if it is known that the engine is to remain inactive for 30 or more days. It additionally cautions against pulling the engine through by hand prior to start, as this simply wipes the oil film from cylinder walls, cams and followers, thus extending the period of exposure to which these components are subjected before oil is circulated from the pump.

[Bill Dicus]

We were taught on our Pitts S-2A to pull 20 blades through before start. The purpose we were told was to get oil pressure high enough to bring some to the front main bearings. We do get an oil pressure indication part way through the process; whether oil actually reaches that bearing is a question. Our instructor felt that front bearing was so important due to loads on it during gyroscopic manuevers (spins, snaps, lomcevaks, flat spins). He felt a lot of wear could occur in the first seconds of running without this procedure. We've done what we were first taught for many years and can say there have been no negative effects that we're aware of - and maybe we're in better shape! Bill