Industry Insights

Sheet Music

By Steve Carbone

A good friend of mine plays guitar by ear; he's one of those guys who plays any song just by listening to it once. I hate those guys! I've been playing for 35 years, but still need my sheet music.

I have the same handicap working maintenance; I can't wing it. I need instructions and pictures to make sense of the work task. Instructions, like music, must be specific. Regrettably, aircraft manufacturers do not always walk us through a maintenance task, so we rely on knowledge from experienced mechanics.

With the increasing wave of veteran mechanics retiring, we are in danger of losing the techniques needed for working important items, like control cables.

It takes time and practice to properly bring an aircraft "in rig." There are rig pins, allowable threads, no-go holes, and various ways to safety the turnbuckles. Each task must be done in sequence and done correctly for the rigging to be accurate.

After 19 years working for a major cargo operator, I joined the National Transportation Safety Board (NTSB) as an accident investigator. That work taught me that standard cable-rigging techniques are not common throughout industry. Cable blocks, for instance, are not practiced by some mechanics. Blocking is a simple technique where a clamping device is put on the cable near a pulley or fairlead; tension on the cable is held, preventing it from unwinding down line.

I investigated a maintenance-caused accident several years ago, brought about by incorrect rigging procedures. Two pilots died when their aircraft crashed after take-off due to an un-commanded pitch problem; a preventable accident.

The flight data recorder was deferred, but the voice recorder tape showed the pilots frantically trying to trim nose up, but inadvertently trimming nose down. They tried to correct the pitch angle, but the situation worsened and became unrecoverable. Why? The aircraft's pitch angle was responding in reverse to the normal input. This illogical situation confused the pilots and they dove into the ocean at a 50-degree angle. What caused the accident wasn't immediately known until a repair performed prior to the accident was investigated.

The NTSB found the elevator trim tab cable, which runs from the trim tab actuator to the trim drum and back, was wound backwards around the drum. This cable was replaced just prior to the last flight. While the improper routing of the replacement trim cable was the cause of the accident, to understand what happened you have to ask why the original trim cable was replaced.

The answer: the original trim cable became pinched between the drum and airframe during an operational check because it unwound from the drum during maintenance. Why? Someone used masking tape instead of cable blocks when replacing a trim actuator.

The masking tape adhesive was useless against the cable lubricant and it allowed the cable tension to relax. As the cable tension loosened the cable began to unwind from the trim drum inside the cockpit pedestal. When the trim actuator was tested the trim cable became kinked and needed replacing. The mechanic working on replacing the trim actuator felt masking tape was a means of keeping the cables out of his way while he worked. If he had maintained positive tension on the cable with a cable block, the cable wouldn't have unwound around the trim drum and the cables would not have been damaged. The replacement cables would not have been installed incorrectly.

The NTSB determined that the probable cause of the accident was the reverse threading of the cable around the drum, aggravated by a bad maintenance manual illustration. The masking tape blocking technique was not mentioned as a direct cause, so that fact was missing in the report. Being ignorant of a technique does not make it any less relevant. The manufacturer's manual isn't required to teach techniques, but information on how to block cables is found in Advisory Circular (AC), 65-15A. This AC gives mechanics the methods and techniques needed to perform maintenance tasks, like cable rigging.

Page 70 of AC 65-15A addresses, in detail, working with cables. There is a note in the Aircraft Rigging section that says, "(1) positioning the flight control system in neutral and temporarily locking it there with rig pins or blocks ..." The AC states this is one of the first things to do when rigging. The keyword is: locking.

Cable will be used in aircraft for many years to come. It's lightweight, malleable, strong, and easily adjusted. It must be worked correctly or the result could have unforeseen consequences. We have to know where to find the skills and to use them. The information is out there, we just have to look. That way we play from the same sheet of music.