HP-11 Service Advisory
by Bob Kuykendall

When I went to pre-flight my HP-11 last weekend, I found two sheared rivets in the reinforcing strap at the root end of the wing. The reinforcing strap goes on the outside of the wing skin at the root rib, and is secured by two rows of rivets, the inboard of which also goes through the root rib. The reinforcing strap helps stiffen the wing skin and root rib to transfer lift and inertia loads from ahead of the main wing spar into the main spar and drag spar.

The rivets that had sheared were AN426AD4-7 (now MS20426AD4-7) in the outboard row that go through the strap, the skin, and the spar angle, but not the root rib. Beach marks and wear marks on the one rivet portion recovered suggest a fatigue fracture to me, but I do have the metallurgical background to verify that. It did not appear that any other rivets in the area were affected. The aircraft in question has around 750 hours total time accumulated over 35 calendar years, with several minor but no major repairs.

I believe that these rivets sheared sometime during the Air Sailing contest, which involved some pretty rough flying on the wave days. I didn't notice their failure until after the contest, since that row of rivets was covered with white tape that seals the gap between the wing and the wing canopy. I first saw the sheared rivets when I removed the wing canopy for a prospective purchaser (no doubt that didn't make what you'd call a good first impression).

I plan on replacing these sheared rivets with Cherrymax CR3242-4-5, since the root rib makes them hard to replace with hard rivets in the field.

I encourage all HP-11 operators to inspect the condition of these rivets at the next practical opportunity. Sheared rivets will reveal themselves with either factory heads that have risen above flush on the exterior (as in my case), or in shop heads that show gaps between the shop head and the inner skin or rib surface on the inside. In an extreme case, the condition may also manifest itself with either factory or shop heads (or both) entirely absent.

I suspect that this is an isolated occurrence of sheared rivets in this area. However, it would be good for anyone else who has observed this or a similar condition to report it on this forum so we can get an idea of its prevalence.

Roger Fowler provided the following related comments:

We had one of these rivets shear the first year we had our HP-11A (1000+ hours). It was absent. Further investigation showed that one of the other rivets was also absent and the hole had been filled with filler. I replaced them with Cherry Max rivets as you suggest and have seen no other problems.

A couple of other things, there were numerous working rivets in the spar to skin joint near the root when we bought the glider. I replaced those with Cherry Max rivets also. Our glider had been modified with 1M wingtip extensions, which certainly don't reduce the load on these rivets.