Metal Fatigue in the Zenith 601HDS Tail Cone

Access hole to tail cone: 14 in. tall, 17 in. wide
During the past 20 years, our Zenith 601HDS, N314LB, has flown 1,725 hrs, all with the original Rotax 912 engine. 

 During the most recent annual, we checked the tail cone as usual, using a bright light and scanning everything. It all seemed okay, until we suddenly noticed tiny cracks in the L-shaped stiffeners of the bottom plate that were hard to see from the cockpit.
Crack in stiffener
Almost every stiffener had cracked at least once! Quite a shock, but in hindsight not so surprising. The outside of the bottom plate experiences positive pressure during climbs and negative pressure in cruise. Thus, the skin is flexing time and again. Of course, a better design would have used a curved bottom part, which avoids the flexing problem altogether.

 The easiest fix seemed riveting new L-shaped stiffeners to the old ones so that they together formed a U channel. Our friend Jack not only gave us the right material, .032 in. Al 7075-T6 aluminum, but also helped us make L-shaped pieces of appropriate length with his shear and brake.

 The entire tail cone is covered with .016 in. 6061-T6 aluminum, which is very delicate. As a result, the cone isn't strong enough to support anybody crawling into it. So we built a support platform that was covered with towels as cushion material and then wedged under the cone.
Support of tail cone
Installation was no fun. The cockpit has two separate openings to the tail cone, each 14 in. high and 17 in. wide; see above photo. That's just enough to crawl through. We used towels to cover the bottom plate so we wouldn't damage anything and had protection against the sharp edges of the existing stiffeners. 

For installation of the new stiffeners we marked the position of holes in uneven distances so that the cracks were closely bracketed. Next we drilled 1/8 in. holes on the bench, went back into the cone, and drilled the corresponding holes in the existing stiffeners using clecos.

 We wanted more than just rivets holding the new and old stiffeners together, so we coated the contact surface with Loctite Metal epoxy, which according to a best epoxy strength test shown on YouTube is quite strong.

 Finally, we installed the new stiffeners with clecos, enlarged the holes to 5/32 in., and riveted them to the old ones with certified pop rivets.
New stiffeners riveted to existing ones.
The epoxy sets within 24 hours. So when we had installed one stiffener, we waited 24 hrs before entering the tail cone for the next one. Tedious and time consuming, but what isn't in airplane maintenance.

 The result: A rigid bottom, even stronger than 20 years ago. No change or paint damage on the outside of the cone, and negligible weight change.

 We also learned yet another lesson: Metal fatigue becomes a real concern as a plane ages.

Comments

  1. AnonymousAugust 14, 2022 at 8:35 PM

    Why were the original stiffeners fluted? My plans show plain, L stiffeners.

    ReplyDelete
    Replies
    1. Klaus TruemperAugust 18, 2022 at 9:59 PM

      You are right. The original plans call for straight L stiffeners. The tail cone calls for 0.016 material, and the L stiffeners are not much thicker.
      After a few years, we had bad oil canning where the bottom sheet cracked at the edge in three places. Besides patching the cracks, we decided to flute the L stiffeners just a bit to prevent the oil canning. After three or four years the stiffeners cracked as shown in the photos. Shortly after we looked at a 601 HDS of vintage similar to ours. The L stiffeners were as originally installed, but had cracked in a number of places just like ours. The fact is: The skin was not thick enough, and the stiffeners were not strong enough.
      During the most recent inspection, in 2022 after 28 years of flying, the bottom skin had two cracks across the fuselage, an alarming fact. We added two strips of 0.025 material externally to cover the cracks. Bottom line: If one designers flat surfaces on an aircraft, the skin itself and the stiffeners must be of sufficient strength. A better approach uses bulkheads. They suppress oil canning completely without adding much weight.
      A friend has suggested that we replace the entire bottom skin. A complex process where we first would have to construct a jig that assures that the geometry of the fuselage is not changed when we remove the bottom skin. We hope that the current fixes suffice and we won’t be forced into such a major repair.

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