A-740 cable broken

[1bigdog]

Good Morning VAF

I was nearing completion of my first annual. Part of my annual maintenance checklist is a simple pull test of the A-740 cable for the alt-air on the airbox. What could go wrong. Someone in the cockpit to pull alt-air knob while I watch the door swing open. Nada.

The cable pulled out in the cockpit with no resistance. I unscrewed the end at the airbox and about 9" of cable pulled out the FWF side.
The cable has no kinks, no wear. There are no cuts. It's just cleanly broken. The cable follows a very gentle curve from the cockpit to the alt-air door.

I have a USB microscope at home. In looking more closely I can see an obvious dark spot within both broken ends that extends out to each of the matching sides.

I'll try to find a place to post this if people are interested (I don't have anywhere that I post images online - not on social media)
but my question would be if others have seen this type of issue. These are 0.050 stainless steel cables. They are incredibly difficult to wrap around the end at the alt-air door so I can't imagine how this could break.

Cheers.

Michael B.

 

[rv6n6r]

It can be difficult to get the geometry just right, such that there isn't undue stress at or near the door attach point that causes early failures. But From your description that's not it, in which case I would suspect a manufacturing defect.

 

[PilotjohnS]

Not just you

The alt air has been my headache since I started flight testing. First the door broke away from the fiberglass at 10 hours. Then at 40 hours, found it hanging loose, not sure where the pivot screw went. Now I am using a bolt in a locking nutplate with locktite and hope that holds together. This is the most rinky-dink thing I have on the plane. Can be a slider valve, but needs springs and stuff to hold it close.

Good luck

 

[1bigdog]

slider is possible

The alt air has been my headache since I started flight testing. First the door broke away from the fiberglass at 10 hours. Then at 40 hours, found it hanging loose, not sure where the pivot screw went. Now I am using a bolt in a locking nutplate with locktite and hope that holds together. This is the most rinky-dink thing I have on the plane. Can be a slider valve, but needs springs and stuff to hold it close.

Good luck

I never had a problem with the OEM door, just this cable.

I do like the slider idea. Z brackets at the side and front would capture the edges and keep it close to the FG body. But would not be changing it for the moment since the FG of the airbox would have to be reworked.
I'm trying to figure out how to make a better swivel mechanism. I know there are other ways published on VAF.

For the cable I'll probably have to go back to ACS. But doesn't seem like many have suffered this failure.

 

[Ulrich]

I would check your engine ground wire.. sounds like that cable was being used as a ground and arced inside.

 

[Ralph Inkster]

The alt air has been my headache since I started flight testing. First the door broke away from the fiberglass at 10 hours. Then at 40 hours, found it hanging loose, not sure where the pivot screw went. Now I am using a bolt in a locking nutplate with locktite and hope that holds together. This is the most rinky-dink thing I have on the plane. Can be a slider valve, but needs springs and stuff to hold it close.

Good luck

Missing pivot screw happened to me too. Since than I have been drilling the screw head & grooving the washer to accept safety wire & tying them down. Problem solved.

 

[SPB W10]

#2 for lack of good current carrying ground.

 

[1bigdog]

Nope

I would check your engine ground wire.. sounds like that cable was being used as a ground and arced inside.

It’s not a bad guess when there is no photo to work from. The ground to the engine is perfect. Big fat ground wire and star washer safetied to the engine block.
This is a clean separation between the 2 ends. No evidence of arcing on the inner wire or outer casing.

Thanks.