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...