Ok, the mags are shut off and you have to push the plane back with the towbar but a propeller blade is in the way. Which way should you move the blade? Turn it the way it normallly rotates or the opposite? Does it matter?
I don't think it will hurt the vacuum pump if its just turned a slight amount in the reverse direction (enough to get the blade out of the way).
I have personally had two occasions of broken P-leads on mags., so turning the prop in the direction of rotation makes me a little uncomfortable. I guess if I personally was the one to do the mag. ground check before shutdown, then I would be a little more ok with it.
My vote would be opposite the direction of rotation if its just part of a turn to move a blade out of the way.
Pratt powered turboprop props are not directly connected to the engine, so you can turn them any way you want at any speed you want. Garretts are different being that they are direct-drive units but I still don't think you'd harm anything by turning it backwards, and it's a very good idea to turn them by hand before and after shutdown in the right direction.
Curious on your input on this one..... I have never (in my modest 600+ hours) had a p-lead problem as you specified. HOWEVER, I HAVE had many occasions where the key can be removed from the ignition switch while the ignition is in either an R or L position.
I am only bringing this up because I am beginning to develop a hypothesis that P-leads arent the weakest link... the C-172 Ignition switches are...
Well, the one case was after mag work was done...mechanics completely forgot to attach one P-lead. The other time, the connecter (at the mag) just wore out and broke.
I've had the switches themselves do funky things too, but only one that I could pull the key out from with it on. The problem I always had was they moved too easily and the "click" was too slight, so students would go from both to off during the runup (BANG....•!!!!!), or hit the key with their knee in flight and unknowingly turn it to the left mag only (gee, why are we not climbing so well after that touch&go?!?!). There is an A.D. on most of those mag switches, but its just for the wiring, not for the switch itself.
Where I work, we turn the prop in the opposite direction of normal rotation. This is safer, since it does not engage the impulse coupling in the left magneto, making it very unlikely that the engine will accidently start. There are almost 100 airplanes here and the prop is moved on almost all of them every time they are fueled. The fuelers here can tell if an airplane needs gas if the prop is not parallel to the ground. After fueling, the prop is turned so everyone knows it is full. Since we have not had any problems with vacuum pumps, I would not give any weight to the 'turning engines backwards is bad' theory. Turning the prop a half turn to get it out of the way should be ok.
The only time I know of that turning a prop in the wrong direction can cause engine damage is when people turn a radial backwards to clear a hydraulic lock instead of removing the sparkplugs on the lower cylinders like they are supposed to. This can destroy an engine.
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I still don't think you'd harm anything by turning it backwards.
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Keep in mind that those turboprops, as well as some piston a/c, use electric anti-ice panels on the prop. The electrical power is transferred to the prop by brushes/metal ring. Turning the prop backwards can damage or break off the brushes that supply this power.
There is a huge difference between turning a prop by hand and installing the wrong type of pump on an engine turning at 2700 rpm. Also as the article notes, the vanes are held into contact with the pump housing by centrifugal force. Since you are turning the engine at about 1 rpm, this is very low to non-existant. You can believe what you want, but I think if we were replacing lots of vacuum pumps on a weekly basis due to turning the props the wrong way, maintenance would not have us do this.
You as the pilot have to make the determination. Do I believe what someone wrote in an article just because it made it into print? Does what they are saying even apply to me? The author of your article is knowledgeable and I respect what he writes, but I do not think it applies to our problem of moving the prop to use a towbar.
The same goes for moving aircraft by pushing on the prop. I have heard alot of people say how it is bad to do this. On some aircraft it should be avoided. If you have electric prop deice, then this is a no-no, you can damage the electric heating elements on the prop. On a small single, it is no big deal. Just push or pull as close to the hub as possible. The loads the prop experiences in flight far exceed anything you can subject it to while moving the aircraft.
I provided the article because I was reacting to the phrasing of your original post (which didn't include the qualifications of your second post), making it sound I am am some kind of airliners.net idiot making up stuff.
The risk of damaging a vacuum pump turning it backwards a small amount at a slow RPM is very low, but it is not zero. The "low" vanes of the rotor are in contact with the housing whether there is centrifugal force or not. The result might not be catastrophic damage, but rather a shortening of the pump life.
That being said, as I stated above, I agree with your overall conclusion that a little backwards turning is probably better the the risk of an unintended engine start.
Turning a prop is kind of scary to me bacuase of the possibility of the engine starting and the prop sredding me to bits. I would not even consider turing the prop unless I personally did a grouding check on the mags before shutting down the engine and then shut it down with the mixture. Then I would turn the prop in the direction that it would have to be turned the least to be out of the way. For example, if it would be out of the way by turning it 6 inches to the left or 12 inches to the right, I'd turn it to the left.
The only problem with that method is that the distance you move the prop has nothing to do with the likelyhood of the engine starting. If you move it in the direction of normal rotation, you increase the risk that it will start. In the left magneto there is an impulse coupling that engages at low rpm. The impulse coupling is spring loaded and 'catches' as the engine rotates. It then springs forward, accelerating the magneto. By doing this it retards the timing and generates a hotter spark than would otherwise be possible at low rpm, increasing the chances of a start. If you turn the engine you will hear the impulse coupling 'click' as it releases. At higher rpm when the engine is running it disengages.
If you turn the engine the wrong way, the impulse coupling will not engage, making an accidental start unlikely.