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

[MAKG1]

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.

Looks like the same problem as for GM serpentine belts and distributor hold-downs. Those tools are dirt cheap.

 

[flyingron]

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.

Sort of like working on my old Ford distributors.

 

[tehmightypirate]

Since nobody else mentioned it Cessna recommends to turn props backwards in the POH of the 172 SP. Not sure if that advice carries through all Cessna's.

 

[comanchepilot]

what is a vacuum pump again?

 

[gismo]

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.

FWIW, some pumps are unidirectional and the vanes in those are perpendicular to the rotor surface.

 

[peerlesscowboy]

I am in Dan's camp here.

Me too, BTW did these guys ever watch their mechanic do a compression test during annual inspection?

 

[flyingmoose]

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

Yup

I bet I rotate 10 props a day backwards. If you have ever had your timing set/checked you have been rotating it back and forth a bunch, but you all ready knew that being a mech and all.

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 is a place where you can certainly do that. But your claim that rotating backwards is the reason..well maybe in your case, but I think Dan is right. You were going to be changing it soon anyway.