For more than a year I have been repainting my 360.  On reinstalling the flap to the flap push-rod bracket, I stripped (too easily) a nut plate inside of the flap.  With the best surgeon-like precision I could muster, I cut a whole in the inboard end of the flap to remove/replace the stripped nut-plate.  None of the bolts extended to the surface of the nut- plates and none was closer than 3 threads from the surface -of course 3 threads should be showing beyond the end of the nut-plate.  I had used the proper AN3-7A bolts called for in the plans (page 10-20).  Further, I believe I had used (15 years ago) the recommended procedures for constructing that bulkhead -meaning the flap forward bulkhead is of proper thickness

 

My conclusion is that many 320 and 360 aircraft are flying with bolts too short to take advantage of the full strength of the nut-plates.  Those bolts are in tension when the flap is retracted and in shear and/or tension when the flap is fully extended.  Always those flaps are in high stress and vibration environment.

 

Recommendation:  Check the length of penetration of these bolts on your next annual.  Use an AN3-8A if the present bolts are too short to extend beyond the embedded nut-plates. 

 

As incentive to remember to check these bolts at your next annual, think of of those bolts  the next time you extend your flaps in flight.    Greg Nelson