Close Call: An Aircraft Maintenance Human Factors Tale

Flying Safety, August, 2001

12. The technicians, on the other hand, viewed their task as lending a hand to the crew chief, who was responsible for the work. All of the work related to the reinstallation of the elevator and stabilizer was completed on the night shift.

13. Everything seemed to be progressing OK at this point. The following night, both lead AMEs were available, so the crew was at full staff. On this shift, the crew chief instructed one of the AMEs to complete an "independent inspection" of the work. After inspecting the work, the AME pointed out to the technicians several items that had not been properly completed, including missing cotter pins and locking clips, a nut that was not fully installed on its bolt, and lockwire that was not of adequate thickness. They then re-did their work and presented it for reinspection.

14. Because of concurrent tasks, the AME did not reinspect the work until the end of the shift, and he did not have any assistance while accomplishing the inspection. Since the details had been completed satisfactorily, he checked the trim for freedom of movement but failed to have someone outside the aircraft to observe what was happening on the tailplane. As a result, he missed the most important failure in the process: the fact that the trim was operating in a reverse direction.

15. At the end of the shift, the lead engineer assisted the crew chief in filling out the aircraft logbook, indicating that the horizontal stabilizer and elevator were reinstalled and the rigging was checked as per the maintenance manual, although no one actually completed a rigging check because the crew chief had asked a technician to follow the rigging procedure as detailed in the maintenance manual, and he had highlighted two of the important tasks: special attention to the cable tension and dimensional check. The technician understood the instruction as a request to check the cable tension and dimension, which he did; however, the rigging was not performed properly.

In conclusion, the maintenance entry was signed as having been completed by the AME who had actually completed the "independent inspection," while the "independent inspection" was signed off by the crew chief who supervised the task. This occurred at the end of the shift when the logbooks from several aircraft were being completed and signed by the two AMEs.

Both AMEs felt confident in the other's work, and they simply signed off the work completed by the crew, regardless of their personal involvement.

There were five people who had a hand in the installation/rigging/inspection of the elevator trim tab control system of this aircraft, and it was still released with the elevator trim control operating in reverse.

The task of hooking up the control cables is, in itself, very basic. There are only two cables, and it does not require training to expert levels to understand the system and to recognize that the consequences of hooking the cables up backwards can be disastrous. This story could fill another page or two, but I think you have the main safety message related to managerial changes, shift changes, minimally trained technicians, inadequate supervision, poorly communicated instructions and log entries. This all added up to a simple but near-fatal mistake, and the whole mess could easily be avoided if manufacturers paid more attention to designing control hookups with different cable ends that could not be applied in reverse, if AMEs paid more attention to clearly tagging cable ends and connection points at the time of removal, and, finally, if those responsible applied some knowledge of aerodynamics with a physical check of the operation of flight controls before releasing the aircraft for flight.