Turning props backwards....
 

I operate a piston engined aircraft fitted with Lycoming 290 hp IO-540 engines. The aircraft has a "Wet" Vac pump system.

As such I turn the props backwards to dress them just in case of live mags etc (which I usually check on shutdown anyway).

Somebody the other day was suggesting that this procedure is not correct, and may harm the engine in some way. What would come from this?

So far no-one (engineer or otherwise) has been able to give me a straight answer - for this aircraft type with the above engine/vac pumps.

Cheers
CS

I'm sure you've checked this already, but.... what does the manual say? And if the manual doesn't say anything, then surely no action is required???

FFF
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I'm not an expert on the various types of vacuum pump, but remember reading an article with regard to this. Many vacuum pumps have vanes which are extended due to centrifugal force and are designed to rotate in one direction only. The suggestion was that by turning the engine backwards you risk turning the vacuum pump the wrong way and catching the vanes on the casing and damaging them. I don't know whether this applies to your particular vacuum pump.

If it does, better a crooked-looking prop on shutdown than a new vacuum pump!;)

AFAIK, that applies to dry vac. pumps. Don't think wet pumps are affected.

Some engines used to be turned backwards during the priming procedure for hand swinging.

Stallie, I suppose you're turning the prop backwards to avoid an inadvertent firing?

I was advised once - by the UK CAA - that turning the prop backwards has absolutely no effect on the possibility of engine start - it can still fire up if all else is in place (just as easily as turning it forwards).

The second point they made was about the vac pumps - but, as said, was a bigger drama to the dry type.

The third point was that if - like most - you turn the prop by cupping your hand around the leading edge to rotate backwards or trailing edge to rotate forwards....what will happen if it does start.

Rotating forward - the prop will move away from your cupped hand (may still rap your knucles with the next blade if you don't move quick enough). Rotating backwards - by cupping the leading edge - the engine will fire into your hand (and that will hurt!!).

All three reasons said "if you need to turn the prop - do it forwards".

Agree with the foregoing as reason for turning it forwards. An additional reason is that there are various brushes on shafts and gearboxes. These expect and are designed for the engine to rotate only one way. Rotate the prop in the opposite direction and you can very easily damage the brushes.

Where an engine is designed for hand-swinging, there is little problem. An IO-540 is not so designed.

Gonewest:
I don't think that a magneto would fire if it was rotated backwards .I think the impulse coupling only works in one direction.

Engineer I worked for in KNX said that it's best to pull the prop through in it's normal direction for dry vacs but it wasn't as critical for wet vacs. You'll get a lot longer TBO with a wet vac. I never had any hassle with them. Now dry vacs used to spit it all the time. Even rotating the prop the correct way. Just too dusty/gritty.

IHL - don't you just love living in a World where everybody has the right to their own opinion??

I had never even thought of the question - until the UK CAA discussed it with me over a beer.

FFF...yes I have checked the manuals and nothing in either regarding it.

There is another reason we do it occasionally in the wet season. When parked if there is very heavy rain (tropical) with a tailwind, it is possible for water to be blown up the exhaust into the engine with expensive consequences on the subsequent engine start.

Engine covers are SOP, but occasionally for whatever reason they are not placed on the a/c. As such, engineers have advised to turn the props backwards several turns during the preflight/daily to allow water to drain out the valves.

CS

Thanks for the above replies!

A wet vacuum pump is not nearly as delicate as a dry one and should suffer no ills by being turned backwards.

That said I still would turn the engine in the proper direction.

Reasons for turning in the normal direction of rotation include being kind to the generator brushes and pumping oil in the proper direction. I have always felt that priming a small engine by pulling it through helps to float the crankshaft and give longevity to the main bearings.

Anytime you touch the prop you should treat it as it will fire. Kind of like handling a loaded gun.

Cupping the prop without wrapping you fingers around it is most certainly the correct method.Also use a one foot forward and the other aft stance to minimize the danger of losing balance and falling into the propeller arc.

I have seen many people clear out small engines on Cubs and the like by opening the throttle and turning them backwards. I suppose this works but the same result can be achieved by turning it forward with a qualified operator at the controls.

Some time ago while operating DC-4s with R-2000 radial engines I knew one captain who said to never move the prop after shutdown because it would break the seal of the piston rings and let oil flow into the bottom cylinders.I do not know if this was a valid theory. He was the only one who subscribed to it but it may be worth looking into.

All that said this is really esoterica and the bottom line is if you move the prop by hand be careful and use the proper tecnique.

Blue Skies, Stearperson

might be different on Lycomings, but ROTAX states, that ´engines wines which have had the prop spun for more than 1 turn in reverse direction´ to be inspected for damaged hydraulic valve tappets, which happens if air is ingested into the oil system.
The Lyco´s have also hydraulic valve tappets (except fore some few models) and might also have problems, if air is ingested into the oil circuit.

Stearperson...

I would personally recommend that any piston engine prop not be moved after shutdown, as the rings will scrape off whatever oil is left on the (steel) cylinder walls, enhancing internal corrosion.
Not such a big problem with chrome cylinders however.