Don't turn the prop backwards, even just a little!!

comanche pilot

comanche pilot

Pre-takeoff checklist
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Everett, WA
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Kevin
I know better, being a mechanic and all. I park my little Porsche 914 in front of the wing of the Comanche. To get the best clearance I swing the prop vertical so I can back in or pull out with ease. Well, the last time it was just a little off and i pushed it backwards and thought I heard a little "tink".:no: Didn't think much of it until I flew down to Eugene, Or. and noticed the vacuum gauge needle flickering a bit. Upon the return trip no vacuum at all. Needless to say the Comanche has a new vacuum pump on it now... Oh and the one inner lower nut, washer and lockwasher was a real pain in the butt too.:yes:
Dumb-a**
Kevin
 
It's happened to me just like that too. When I posted about it there were plenty of people in opposition to the idea that turning it backwards could have any possibility of breaking a pump vane. However it was fine when I landed, I heard the 'snick' of a breaking vane when I turned the prop to put the tow bar on, next when I started it, no vacuum. Changed pump and all was well.

I do think the vanes can only get canted a bit when they have some age on them.. but still would avoid turning the prop backwards wherever able.
 
I understand it depends on the pump. Airwolf sez it is fine the turn their wet pump backwards and so far mine has never complained.
 
Yeah, it was an old Sigma-Tek dry vacuum pump. I know that durring my PPL training I was told not to rotate the prop backwards as well. I'm a good listener...:rolleyes2:
 
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Clark1961 said:
I understand it depends on the pump. Airwolf sez it is fine the turn their wet pump backwards and so far mine has never complained.
Click to expand...

Beat me to it, I was ready to respond with "get a wet pump, problem solved...along with many others."

:)
 
I did take it apart after removal. The vanes looked rather long still. It was past it's prime though. It had at least 300 hrs of use that I know of. Plus what ever from the previous owner. I'm a strictly VFR pilot. I would seriously consider a wet pump in the future if I was doing any IFR flying.
 
Good to know!
I heard over and over again not to turn the 9XX series Rotax propellers backwards, and thought I heard that it was ok for traditional Lycoming and Continental engines. From an engine point of view I'm sure it is ok, but vacuum pumps aren't cheap either!
-Jim
 
Fer Pete's sake. I've worked in aircraft maintenance for almost 20 years now, turned props backwards every day all that time, never had a vac pump break.

They will break if they're thoroughly worn out. We don't let them get that far. We install pumps with the inspection ports and replace them when the vanes are at limits.

The OP was about to buy a new pump anyway.

So you take your pick: test that pump by turning the prop backward so it breaks on the ground instead of in flight, or never turn the propeller backwards and risk a mag snapping and losing some important part of your anatomy. I think most will agree that vacuum pumps are much more dispensable than an arm or leg or head.

Dan
 
Dan Thomas said:
Fer Pete's sake. I've worked in aircraft maintenance for almost 20 years now, turned props backwards every day all that time, never had a vac pump break.

They will break if they're thoroughly worn out. We don't let them get that far. We install pumps with the inspection ports and replace them when the vanes are at limits.

The OP was about to buy a new pump anyway.

So you take your pick: test that pump by turning the prop backward so it breaks on the ground instead of in flight, or never turn the propeller backwards and risk a mag snapping and losing some important part of your anatomy. I think most will agree that vacuum pumps are much more dispensable than an arm or leg or head.

Dan
Click to expand...

I am in Dan's camp here.
 
comanche pilot said:
I know better, being a mechanic and all. I park my little Porsche 914 in front of the wing of the Comanche. To get the best clearance I swing the prop vertical so I can back in or pull out with ease. Well, the last time it was just a little off and i pushed it backwards and thought I heard a little "tink".:no: Didn't think much of it until I flew down to Eugene, Or. and noticed the vacuum gauge needle flickering a bit. Upon the return trip no vacuum at all. Needless to say the Comanche has a new vacuum pump on it now... Oh and the one inner lower nut, washer and lockwasher was a real pain in the butt too.:yes:
Dumb-a**
Kevin
Click to expand...
Trade in the Commanche for an experimental with an EFIS, or install your own, and problem solved. just kidding.

I just installed a new air pump on our RV-9A and it just happened to be less than 1 hour after annual condition inspection during which the prop was rotated back and forth for the compression check, hmmm.

That inner lower nut is a pain. There is a tool made for the job:
http://www.aircraftspruce.com/pdf/2013Individual/Cat13446.pdf
That is in addition to the wrench made for the nut which is also seen on the link above.
 
Dan Thomas said:
...
The OP was about to buy a new pump anyway.
Click to expand...
I don't see anything about that in his post.

Dan Thomas said:
...So you take your pick: test that pump by turning the prop backward so it breaks on the ground instead of in flight, or never turn the propeller backwards and risk a mag snapping and losing some important part of your anatomy. I think most will agree that vacuum pumps are much more dispensable than an arm or leg or head.

Dan
Click to expand...
So...if you are going to turn it backwards you should listen for the *tink* so you know what is coming.
 
Kiddo's Driver said:
I don't see anything about that in his post.
Click to expand...

It was implied. If turning the prop backwards is going to break a vacuum pump vane, it was going to fail soon anyway.

So...if you are going to turn it backwards you should listen for the *tink* so you know what is coming.
Click to expand...

Not sure what you are trying to say here. But yes, that would be an indication of a failure.
 
Greg Bockelman said:
Not sure what you are trying to say here. But yes, that would be an indication of a failure.
Click to expand...
Pretty much exactly that. If turning the prop backwards is preferred for safety (no accidental hand proping), then you might as well listen and try to catch the tell-tale sound of a vane cracking. If I heard that I would know to consider not launching into IMC and that I might want to order a new/rebuilt pump.

Jim
 
Kiddo's Driver said:
Pretty much exactly that. If turning the prop backwards is preferred for safety (no accidental hand proping), then you might as well listen and try to catch the tell-tale sound of a vane cracking. If I heard that I would know to consider not launching into IMC and that I might want to order a new/rebuilt pump.

Jim
Click to expand...

Don't you check suction during run-up?

And your AI and DG during taxi?

I'd expect those to be high on the list prior to launching into IMC.
 
So what happens at shutdown when the prop comes up against the compression and backs up a bit? Sometimes it even wiggles back and forth a few times before it comes to rest. No way to stop it.
 
MAKG1 said:
Don't you check suction during run-up?

And your AI and DG during taxi?

I'd expect those to be high on the list prior to launching into IMC.
Click to expand...

Sure do. Go back and re-read the first post.

comanche pilot said:
...the last time it was just a little off and i pushed it backwards and thought I heard a little "tink".:no: Didn't think much of it until I flew down to Eugene, Or. and noticed the vacuum gauge needle flickering a bit. Upon the return trip no vacuum at all...Kevin
Click to expand...

He heard the *tink* and managed a flight before it failed. The sound could just be it cracking. The break follows some time later.

FORANE said:
...I just installed a new air pump on our RV-9A and it just happened to be less than 1 hour after annual condition inspection during which the prop was rotated back and forth for the compression check, hmmm...
Click to expand...

In the case of the guy just coming out of an annual it was less than an hour after the annual. He didn't hear the "tink" but there was a lot of prop rocking going on.

Jim
 
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I'm in the "pump was about to fail" camp as well. Perhaps turning the prop backwards was a good thing...it brought on the inevitable while on the ground rather than in the air...
 
The thing is, there you are 300 mi from home, as you might be 50% of the time if you travel with your plane like me & many others.

You could take steps to avoid being aog, such as not turning the prop backwards - or you could go ahead and take that chance.

Yes the chances are miniscule, in this case.... but if you do enough of these things, I think you are increasing your chances to 'no longer miniscule'.
 
The prop turns backwards "just a little" each time you fly. You really have no control over it, since every time you shut down the engine, the prop will recoil backwards slightly as it stops. No fish to fry here, IMO.
 
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Dan Thomas said:
Fer Pete's sake. I've worked in aircraft maintenance for almost 20 years now, turned props backwards every day all that time, never had a vac pump break.

They will break if they're thoroughly worn out. We don't let them get that far. We install pumps with the inspection ports and replace them when the vanes are at limits.

The OP was about to buy a new pump anyway.

So you take your pick: test that pump by turning the prop backward so it breaks on the ground instead of in flight, or never turn the propeller backwards and risk a mag snapping and losing some important part of your anatomy. I think most will agree that vacuum pumps are much more dispensable than an arm or leg or head.

Dan
Click to expand...
+1 .
 
Let'sgoflying! said:
The thing is, there you are 300 mi from home, as you might be 50% of the time if you travel with your plane like me & many others.



You could take steps to avoid being aog, such as not turning the prop backwards - or you could go ahead and take that chance.



Yes the chances are miniscule, in this case.... but if you do enough of these things, I think you are increasing your chances to 'no longer miniscule'.
Click to expand...


Or just replace the pump at shorter intervals.

Personally I'd rather break it before takeoff than have it go after, if IMC.

One could make the silly argument that if you're about to launch into low IMC, go spin that prop backward and hope it breaks, if you're the type to run it to failure. ;)

If you've ever looked at a cutaway of one, the vane is either quite worn or it's too loose in its channel if turning the prop backward breaks it off. It was time for it to die anyway. They're pretty simple devices.

Attending a type class at Cessna Pilot's Assn this has to be one of the things that makes that course well worth the entrance fee. John has cutaways of damn near everything on the aircraft and you can finally see the guys of those things and see how they really work. He also has lots of broken stuff to show ya which parts are problematic on your specific model and how someone else's broke. ;)

Well worth the trip and the money if you own a Cessna. We little guys rarely get the equivalent of a "systems" course.
 
One of the best days of my aircraft owning life was when we removed the old vacuum pump, dropped it in a box, and put a cover over the spot where it used to reside on our Lycoming O-360.
 
So exactly what is the mechanism that supposedly causes a vacuum pump vane to break if it's rotated backwards?

If the engine were running in reverse and the aerodynamic force on the blades was reversed, I could see the potential for blades to be damaged.

But moving the prop backwards a few degrees, by hand? I don't get how that's supposed to stress the vanes. They probably see harsher load when you roll the nosewheel over the lip of the hangar.
 
8061.jpg


This pump turns clockwise from the view we see here. See the angle of the slots that carry the vanes? That angle causes the vanes to jam if the pump is turned backward and if the vanes and slots are worn enough. Shortened vanes in widened slots will jam and we get this:

vacuum_pump_inside.jpg


Dan
 
Jim_R said:
So exactly what is the mechanism that supposedly causes a vacuum pump vane to break if it's rotated backwards?
Click to expand...

There is nothing "supposedly" about it. Friction would be the mechanism. The vanes wear due to friction and form a microscopic feathered edge where it contacts the housing. Turning the pump backwards causes the trailing edge to catch in a worn spot of the housing or the edge of a port and "snap" you just bought yourself a pump. Please note I did say that this was a well-worn pump and probably at its life limit anyhow.

I thought this forum was used to discuss lessons learned. I fly VFR and want everything working in my aircraft as it should be. If I hadn't turned the prop backwards and broke a vane I'm sure it would still be functioning normally at this time. Instead I did something I know I shouldn't have and I'm paying for it. Lesson learned.
 
It looks to me, in your picture Dan, the upper photo turns counter clockwise and the lower turns clockwise? Thanks for pics, reguardless! : )
 
comanche pilot said:
It looks to me, in your picture Dan, the upper photo turns counter clockwise and the lower turns clockwise? Thanks for pics, reguardless! : )
Click to expand...

That's right. Top clockwise, bottom counterclockwise. The idea is that centrifugal force keeps the vanes in contact with the bore, and the angle reduces the pressure on the vane edges.

Get a look at how the vanes in the top pump (a new pump) completely fill the slots. In the lower pump, the vanes are 35 or 40% gone, and when they slide out to contact the bore, there's very little left in the slot to guide them. Turning that pump backwards will jam and break it. It's worn well past limits, in any case.

See this: http://www.avweb.com/news/maint/182905-1.html?redirected=1

Dan
 
What isn't likely visible unless you know how it works is what pushes those vanes against the housing are the rotational forces. Unless you're spinning the engine at speed the vanes are free to slip. If you got something on the case that's going to snag the vane, it's going to be bad news once the engine starts anyhow.

I'm only on my second vacuum pump in thousands of hours and don't give much thought to the direction of rotation.

This is a lot of hogwash that people extrapolate from the fact that some pumps indicate which way they're SUPPOSED to rotate, but that only has any meaning at the operating speeds.
 
comanche pilot said:
There is nothing "supposedly" about it. Friction would be the mechanism. The vanes wear due to friction and form a microscopic feathered edge where it contacts the housing. Turning the pump backwards causes the trailing edge to catch in a worn spot of the housing or the edge of a port and "snap" you just bought yourself a pump. Please note I did say that this was a well-worn pump and probably at its life limit anyhow.

I thought this forum was used to discuss lessons learned. I fly VFR and want everything working in my aircraft as it should be. If I hadn't turned the prop backwards and broke a vane I'm sure it would still be functioning normally at this time. Instead I did something I know I shouldn't have and I'm paying for it. Lesson learned.
Click to expand...

Yes, this forum is for lessons learned, and I wasn't learning anything because I wasn't "getting it". I asked a question because the discussion wasn't making sense to me. The missing piece was that I had never seen the inside of a dry pump before and didn't understand the physical mechanisms involved...my imagination had the innards looking more like a fan or a windmill that could spin freely in either direction.

With the benefit of Dan's photos, I now understand better what's going on. I bet I'm not the only one who learned something after I asked that question. Many thanks to both of you for the explanation.
 
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I understand the reasoning behind not turning the prop backwards, but how do you time the mags without doing so?
 
I turned my A65 backwards 6-8 blades the other day because it got flooded trying to do a hot start. Seemed to be the fix for the problem, as it started on the first blade afterwards.

(Throttle management procedures also involved.)
 
There is a special little wrench they charge you an arm and a leg for that makes getting that one lower nut and washer off and back on a slightly (infinitesimally) lesser PITA to get to. :rolleyes:

10-13911.jpg
 
jnmeade said:
I turned my A65 backwards 6-8 blades the other day because it got flooded trying to do a hot start. Seemed to be the fix for the problem, as it started on the first blade afterwards.

(Throttle management procedures also involved.)
Click to expand...

Do you have a vacuum pump on your 65? ;)
 
jnmeade said:
I turned my A65 backwards 6-8 blades the other day because it got flooded trying to do a hot start. Seemed to be the fix for the problem, as it started on the first blade afterwards.

(Throttle management procedures also involved.)
Click to expand...

Yep, you have no vac pump as mentioned. And for clearing a flooded engine, turning backwards works no different from turning forwards. Both do the same thing. It's just that turning backwards will prevent the mags from firing in the event of a freakly-timed mag switch problem which could cause the engine to start at a high throttle setting while you're clearing it.
 
RoscoeT said:
...And for clearing a flooded engine, turning backwards works no different from turning forwards...
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It does work differently because the intake valve opens at the bottom of the stroke, piston moves up forcing air out through the intake tube and carburetor. The exhaust valve opens at the top of the stroke, piston moves down drawing air in through the exhaust rather than the intake.
 
Silvaire said:
It does work differently because the intake valve opens at the bottom of the stroke, piston moves up forcing air out through the intake tube and carburetor. The exhaust valve opens at the top of the stroke, piston moves down drawing air in through the exhaust rather than the intake.
Click to expand...

Doesn't have an effect on clearing a flooded engine though. I've done both and with the throttle open turning either direction gives the same result. As mentioned, backwards just won't allow the mags to fire.
 
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