Originally Posted by
eric.leaver
Some of you have asked for an explanation of the failure of the wires. We apologize that we have not responded immediately but we have focused out attention on finding a solution and making certain that parts can be shipped as soon as possible. Also, it is difficult to condense all the work that has been done into a few paragraphs.
We initially suspected that the wires oscillated during certain phases of flight. Therefore, a flight test program was initiated with carefully instrumented wires. It included among other things 2g turns, stalls and dives to VFE and VNE. Tests were repeated with the wires set at different tensions. Tests were carried out on a CC11-100, a CC11-160, a CC11-160 EX that had a CC18 tail and a CC18. The latter was included because there is ample service history with the Piper PA-18 style of wire attachments that is used on the CC18 and it provided a baseline for the tests. The tests were carried out by an FAA DER that specializes in fatigue. A DER that specializes in flutter was also consulted.
It was found that as the stabilizers deflect, they tend to bend the wires, putting stresses on the exposed threads. On the CC18, the design of the attachments of the wires to the stabilizers is different and deflections in the stabilizers are not as likely to produce bending and compression stresses in the wires. The new design that will be retrofitted in the fleet will be similar to the CC18’s- in fact there will be several parts in common.
The phase of flight where the problem was most prevalent was during takeoff, immediately after the aircraft came out of ground effect.
Our aim with the new design is to ensure that the cyclical stresses in the wires are comparable to the stresses experienced by the CC18’s wires.
Finally, it is worth adding that during the investigations, the failed wires, wires taken at random from production aircraft, new wires from the stock room and wires removed from a Piper PA-18 were subject to laboratory analyses. The analyses included examination under a stereomicroscope, scanning electron microscope, energy dispersive spectrometric analysis and hardness testing. Observations of the threaded portions of all wires revealed no significant differences.