Aircraft Maintenance: Understanding and maintaining flight control cables
Part 1: What you don’t know can hurt you
As many of you know, I tend to think of aircraft as sentient beings. I believe in taking care of them, listening to them and, truth be told, I’ve been known to talk to an aircraft or two when no one is looking. After all, we rely on these machines to take us miles into the air and return us safely with big grins on our faces, none the worse for wear.
So, it didn't come as much of a surprise when my own airplane reached out for attention recently regarding the autopilot trim system. My Bonanza is equipped with a Century III autopilot with electric trim. While the Century is an excellent autopilot, it has always been a breeding ground for gremlins in the form of the unique, blue plastic electrical connectors that tie the system together. These connectors are notorious for intermittent connections that have been the source of premature hair loss to many an avionics technician.
However, as I was wrapping up my work inside the tail, my aluminum friend reached out and yanked on my sleeve (quite literally) for a little more attention. A tiny strand of broken trim cable caught my shirt sleeve and drew my view to the trim servo where the trim cable wraps around the capstan and pulley assembly on its way to the elevator trim tab mechanism. Upon further inspection, it became apparent that the cable was severely damaged with many broken strands that were easily hidden by the tight turns and confines of the servo.
Replacing the cable was easy enough. Sourcing the part was not quite as simple. Although the cable came installed in the aircraft straight from the factory, the autopilot was installed by supplemental type certificate, which is often the case with aircraft options. Therefore, there was no part available from Beechcraft directly. The part had to come from Century Flight Systems which, in this case, meant it had to be made-to-order from the original drawings. I could have sent the old part elsewhere to be duplicated, but there wouldn’t have been much time or money saved by doing that in this case. So, I waited a couple of weeks and the installation went smoothly.
I was fortunate to discover the fraying trim cable before it failed in flight. While the design of the trim system would have made an accident unlikely, it would certainly have caused some trouble, and may have stranded the aircraft away from home for the two weeks it took to get the part. That said, flight control cable failures are very critical safety issues that can and do lead to accidents. So, it’s important to know how your flight control systems work, as well as how to inspect, maintain, and adjust them regularly.
During the next segment, I will get more in-depth with cable maintenance—what to look for and how to keep the system in top shape. Happy flying!
