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PA12 One Flap Up, One Flap Down

Steve Pierce

Steve Pierce

BENEFACTOR
Graham, TX
Customer with a PA12 I inspected back in November just called and his girl friend was flying their 150 hp PA12 and upon landing had one flap pop up while the other stayed down. I am not as intimate with the PA12 as I am with a PA18 or PA20/22 but can not figure out how this could happen unless a cable broke or came off a pulley. Any ideas? Airplane is on it's way here now.
 
That makes sense, I did have to massage one of the brackets on the butt rib at annual that was awfully close to the cable. Will start in the wing roots and post some pictures. Thanks Stewart.
 
Not sure if the system is the same, but in a PA-22, the flaps are held up by springs, and pulled down by the cables. If one of the "return springs" breaks, the corresponding flap will fall down (unless the wind/air load is holding it up). Thus, even with a broken spring the flap will stay up during flight, as it is pulled to the down position by the flap handle and cables. But on the ground, without air load, the flap will hang down. Might be the same in a -12.
 
In my experience with it a tail gust blows the flap down. Since the cable wasn't pulled the swaged end stays in it's normal orientation as the bell crank rotates and the thimble jams in that position (approx 90* from normal) so the flap stays down on that one side. Freeing it up with fingers through the inspection cover was quick and easy. It can't occur in flight so not a big deal.
 
Turns out the flap cable for the left wing jumped the pulley. No sign of a cable guard. Don't have the STC paperwork to see if there is one called out. I can make one similar to the one on the front elevator pulley on a Super Cub.
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Steve Pierce said:
Turns out the flap cable for the left wing jumped the pulley. No sign of a cable guard. Don't have the STC paperwork to see if there is one called out. I can make one similar to the one on the front elevator pulley on a Super Cub.

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Click to expand...

That's how I would handle it Steve...

We have a PA-12 project in our big hanger... Going to start on it next year (2019) sometime, will have to get lots of pictures of the flap system in it... The airplane has just about every major "Alaska" mod done to it... Will look closely at cable guards now...

Brian.
 
Made a cable guard just like what is used on the front elevator pulley on a PA18. Made it a bit longer to guard both pulleys. After installation I figured out what probably happened. I pulled the flap down while watching the slack in the cable and it balled up in front of the cable guard. Without the guard I think it would have popped off again.
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Steve Pierce said:
Made a cable guard just like what is used on the front elevator pulley on a PA18. Made it a bit longer to guard both pulleys. After installation I figured out what probably happened. I pulled the flap down while watching the slack in the cable and it balled up in front of the cable guard. Without the guard I think it would have popped off again.
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1cf2cc121c354c00231afee07d8d58bb.jpg


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Click to expand...
Some Italian pilots from Shepherd Air Force Base picked the owner up when he dropped the airplane off. They wanted to see what we found. They wanted to know who did the previous work. The owner is a retired crop duster. With out missing a beat he told them the airplane came with an Oklahoma Warranty, if you break it you get to keep both parts. [emoji13]

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Sound to me like the cables are too loose in the up position.I keep mine slightly taut, but no so much that the flaps don't reach the up stops of course. With zero slack: 1) That can't happen. 2) You get maximum possible flap extension. I'm surprised how many flap cables I see that are floppy loose when retracted.
 
Seems to have normal tension to me but when you pull the flap down and the handle is still in the "flaps up" position there is quite a bit of cable pushed back into the fuselage against that pulley. Worse on the left side since that is the side the pulleys are on.
 
PerryB said:
Sound to me like the cables are too loose in the up position.I keep mine slightly taut, but no so much that the flaps don't reach the up stops of course. With zero slack: 1) That can't happen. 2) You get maximum possible flap extension. I'm surprised how many flap cables I see that are floppy loose when retracted.
Click to expand...

only thing on a cub like flap installation is to set is full down deflection.... what cables do when in up position is not relevant...
 
Well, the tighter they are the more deflection you get. My point on the cable being taut when up is that there is no slack to let the cable jump off. Obviously if the aircraft in question had proper gaurds/guides, this wouldn't be an issue, but still I like all the flap I can get for landing. Also when there is minimal/no slack, they don't bang around when taxiing on rough strips. That drives me nuts, and it's a short drive...
 
I have trid rigging flaps to get more deflection but end up drooping the flaps when in the full up position. Would need a longer flap ratchet or flap bel lcrank I would think.
 
I have found it is hard to meet the TCDS without getting it almost drooping. Pretty fine line in that system.
 
Steve Pierce said:
I have tried rigging flaps to get more deflection but end up drooping the flaps when in the full up position. Would need a longer flap ratchet or flap bellcrank I would think.
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I found that clocking the crank (where the cable attaches) on the flap handle was critical. Even after messing around with that, it still seems like a suboptimal geometry. I used to have what I thought was a superior cable geometry, but my IA made me redo it in accordance with the STC drawings (Day and Night) during rebuild.
 
PerryB said:
Well, the tighter they are the more deflection you get. My point on the cable being taut when up is that there is no slack to let the cable jump off. Obviously if the aircraft in question had proper gaurds/guides, this wouldn't be an issue, but still I like all the flap I can get for landing. Also when there is minimal/no slack, they don't bang around when taxiing on rough strips. That drives me nuts, and it's a short drive...
Click to expand...

Stewart described above in post #5 a scenario where there IS slack in the system. Obviously cable gaurds are a necessity.
 
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