FLIGHT CONTROL CABLE MAINTENANCE….
Acknowledgements: Thomas P. Turner (Mastery Flight Training, Inc.)
The issue of flight control cable maintenance and corrosion is emerging as a hazard in many makes and models of airplanes. Looking back to see if I’ve ever addressed this in FLYING LESSONS I found this item from 2012. This National Transportation Safety Board report bears repeating today.
“Shortly after take-off, the pilot of a Cessna 172 reported ‘...the aileron cable broke and ...trouble keeping the aircraft in straight and level flight....’ According to numerous eyewitness reports the airplane appeared to be maintaining level flight and subsequently banked to the right, as viewed from behind, descended, and then impacted the water in a right wing down attitude.
One eyewitness, in an aircraft that was following the accident airplane on final, reported that the airplane had ‘made some zigzagging’ prior to the aircraft banking to the right. The airplane fatally impacted the water surface with the right wing-tip. The airplane cartwheeled and disappeared below the water surface.
The airplane was recovered from the water. Inspection of the airplane showed the right aileron cable was separated at the pulley near the top of the left aft doorpost”.
Now, in this case it may not have been possible to detect any problem with the aileron controls during pre-flight inspection, or even in the Before Take-off control checks.
But certainly, if there is any unusual noise or obstruction to movement when you move the flight controls to the stops during your exterior walk-around, or anything at all out of the ordinary when you conduct your ‘Controls FREE AND CORRECT’ checks before taking the runway for departure, cancel the flight and get the controls checked by a maintenance professional.
With the average age of a general aviation airplane exceeding 40 years, we (and our mechanics and inspectors) need to pay special attention to items that didn’t use to be problems but now have the potential to cause catastrophe given the thousands of hours of fatigue exposure on a frequently flown airplane.
What may be even worse is an airplane that is notflown frequently. Airplanes that sit for long periods between flights are subject to something more hazardous than normal fatigue - corrosion. This brings us back to one possible factor in the fatal Cessna crash. Most aircraft control cables are impregnated with corrosion-protection oil when new. Over the years the oil within control cable strands will attract dirt and grime.
Now, it’s natural when seeing a grungy cable to wipe it clean. Doing so, however, can remove the corrosion-proofing lubricant from the cables, making them susceptible to rust and breakage. So if you do clean a control cable, you need to follow up by re-lubricating it properly. We don’t know if this was a factor in the Cessna crash, but it has been found to be a factor in control cable strand breakage on other airplanes; damage that might lead to a tragedy like that described by the NTSB.
Many FLYING LESSONS readers like to participate in the maintenance of their airplane. If you’re a hands-on airplane owner, don’t do anything without discussing it with an experienced mechanic. There’s a good reason all maintenance must be done by or under the direct supervision of a certificated mechanic. Because without knowledge of the hazards, even seemingly simple and obvious things like cleaning the crud off control cables can have devastating long-term effects on airworthiness if done incorrectly.