stuck drain plug

During leak testing on the wings, I installed the AN913-3D NPT plugs (square head) in the tank (see p 18-09 Fig 2) along with the other fittings etc.

After the leak test (ok) I tried to remove the AN913-3D plugs to add some thread sealer. One of the plugs came out easily, but the other one came out about a 1/4 turn and then started getting tighter. I tried taking it forward and back but the tightness just increased until I was afraid of breaking something. I'm not sure how to proceed, drill it out or what but it has really seized.

Wondering if anyone else has experienced anything like this and what cause or the solution might be....

Half of the VA-141 fuel flanges I received had really bad threads on them. Make sure you check the fit and finish of the plug and flanges before installation.

I’m afraid you learned how to weld at the wrong place at the wrong time..NEVER put bare aluminum pipe threads into aluminum threads, or it’ll gall and seize. Im Surprised you got the first one out! Even bare brass could cause galling if the tapped hole is rough to begin with.. you MUST use thread sealer as it lubrications the threads. When it first got tight, you might have had a chance with some tap magic for aluminum, but going back and fourth until it seized is about the worst thing you could have done. Even if you just kept wrenching it out one direction, you may have had a chance to retap 1/8 pipe tap deeper, or even upsize to 1/4 pipe if there’s enough thickness. They make 1/4 drain sumps.

If you can’t get that one out with thread penetrating oil, and retap, you could also drill out the rivets holding the bung, and install a new one by bucking through the access hole. That’s probably your best bet. If it’s already installed on the plane, perhaps countersunk pulled rivets gooped up with proseal could work too.

Center punch the plug and drill a hole starting with a 3/32 and upsize and keep it centered. Continue till you get close to the threads... At this point, the remaining threads of the plug will collapse and you can dig them out with a dental pick.

A tap can also be used at this point. Or finally, you could install a heli-coil.