Citabria strut fork problem.?

Question

Airworthiness inspection came up with four cracked strut clevis forks. They are cracked on the threaded end. This is not the first time the AME's have found this problem. What is causing these cracks?

Answer 1

How hard are you landing?

Are you landing on unimproved strips with potholes or rocks on it?

Are the cracked parts part of the same "batch" of parts? (it could be that the metal was overtempered and brittle in a particular batch... not likely, but possible.)

I'd also ask around other Citabria owners and see if this is a problem elsewhere. (It could be that it's a problem like a 172 shimmy damper... everyone just knows that they stink..)

Oh, and it might be that your AME is a little bit TOO particular about this part... at my club we used to rotate annuals through different inspectors to be sure things weren't being missed or mis-diagnosed.

Hope this helps.

Answer 2

Are you sure the forks are actually cracked? The more common problems are corrosion and fretting between the fittings, causing erosion of the threads on the attachment forks. This is fairly common so they should be looked at as part of the annual inspection. If they are indeed cracked this is a serious problem and the manufacturer of the parts should probably be notified.  Causes could be from improper installation, faulty manufacturing (or "fake" parts made overseas), hard landings, or excessive stress from aerobatic maneuvers.

Answer 3

Piper also had this problem with the pa- series aircraft. It became an AD to inspect/replace the rod end forks with a heavy duty end that had the threads manufactured in a different manner. You might research to see if there is an approved heavy duty fork end that could be installed.