A discussion about how neglecting engine control cables can lead to catastrophic failures, highlighting the critical importance of regular inspections and timely replacements.
By Steve Ells
Fellow ace aviator and Cessna Flyer contributor Tom Machum contacted me to ask about the existence of any printed recommendations from manufacturers about replacement times or periodic maintenance for the engine controls (throttle, prop, and mixture). The incident that prompted Machum to ask me about control cable life was the loss of an airplane due to a failed engine throttle control and subsequent crash.
His question is a good one. Most owners know by now that the active components of engine operation (such as the propeller, propeller governor, and magnetos) need regular maintenance.
But what about the static firewall-forward components? In the past, I’ve written about the need to send exhaust systems and welded steel engine mounts to specialized facilities during engine overhaul, because even the smallest failure of either could lead to serious issues during flight, resulting in a crash with injuries or fatalities.
It’s easy to see how a crack in the exhaust system can lead to a negative outcome. Even a small crack in the wrong spot could admit carbon monoxide (CO)—an odorless and colorless toxic gas—into the cabin, where it will cause unconsciousness and eventually death if levels of CO in the bloodstream exceed certain limits. Fortunately, this threat can be managed because pilots these days can buy aviation-focused CO detectors for around $100.
Time for a survey: How many owners know the total time since new on their airplane’s engine controls? I do, because between 2017 and 2019 I replaced the prop, throttle, and mixture control cables on my airplane. So, I’m good for a while. But for how long? I didn’t know the answer, so I reached out to McFarlane Aviation (which manufactures replacement control cables for Cessnas) and looked at Cessna service manuals.
McFarlane Aviation
McFarlane Aviation offers FAA/PMA-approved throttle, propeller, mixture, cowl flap, carb heat, and cabin environmental controls for many Cessna aircraft. They publish a handy PDF document with a listing of Cessna models and corresponding control parts. It also contains installation tips (see link in Resources).
If McFarlane does not have the model you need in stock, they can produce custom, made-to-order, high-quality engine control cables that are approved for installation under the “owner-built parts” rules. I sent in my ancient throttle and prop governor control as patterns in 2017. The replacement parts from McFarlane fit perfectly.
I asked Austin Beine, the Product Development Manager at McFarlane, about their control cables and expected service life. The following is his reply:
“When compared to other aircraft push-pull controls, McFarlane controls are engineered and manufactured with materials that improve both the function and lifespan of the controls. Due to these improvements in materials and design, our customers never need to lubricate or service these controls throughout the lifespan of the control.
“Regarding your question on service hours, there are no published numbers specific to McFarlane controls. We’ve observed over many years of producing our controls that pilot usage is the most significant factor and leads to different wear patterns over the same amount of logged time. For instance, a pilot frequently flying short routes with multiple landing cycles will experience different wear than a pilot flying longer distances with fewer landings.
“Inspecting those two push-pull control sets at the same amount of time in service will show different wear trends. Inspections of our controls returned from service indicate that we can expect a control to remain in service easily up to 2,500 hours, with some controls remaining safe and functional well past that hour count.
“Rather than using a fixed time limit, we provide our maximum recommended time-in-service for McFarlane controls as equal to the maximum time between engine overhauls as recommended by the engine manufacturer. We also recommend that engine controls be replaced when the engine is overhauled or replaced.
“We do believe it is important that pilots monitor how their controls are performing and watch for changes. Push-pull controls should always move smoothly through their entire range of travel. There should be no rough spots in feel or areas of travel where control forces increase or ‘drags’ through. All of these reductions in control performance indicate that the control is worn, or potentially fouled, and should be replaced at that time.”
Replacement costs
Recent research for this article showed that engine control cable assemblies are widely available. My first move when looking for a new cable(s) would be to type the manufacturer’s part number into your favorite internet search engine. Most often, the vendors that had ready-to-ship assemblies were McFarlane Aviation, Aircraft Spruce & Specialty, and Van Bortel’s aircraft parts subsidiary, Air Power.
Often there’s a used serviceable part available from various salvage yards such as Dodson International Parts, Preferred Airparts, or Wentworth Aircraft. You can also find others listed on eBay. Since it’s difficult to determine the time in service of a part being sold on eBay, before buying I would check to determine if there’s a known time in service for any control assemblies, and verify that the seller offers a money-back guarantee.
My research indicates that $2,000 to $2,500 is a reasonable cost estimate to purchase a complete set of new engine control cable assemblies for a single-engine airplane with a constant-speed propeller; for an airplane with a fixed-pitch propeller, budget $1,500 to $2,000. That’s not too much to pay for the knowledge and confidence that your connections to your airplane’s engine(s) are going to be in an airworthy condition for at least the life of a modern engine (2,000-2,400 hours), if not more.
Conclusion
It’s pretty well known and understood that manufacturers and assembly suppliers err on the side of conservative parts life suggestions. To the best of my knowledge, there are no mandatory replacement times for any engine control cable assemblies. While replacement at every engine overhaul may seem too conservative, where the airplane is based and how it’s flown must be factored into the replacement time decision.
The controls must be checked for freedom of movement during annual inspections, and, since there are no field-serviceable items on the cable assemblies, they must be replaced if there’s binding or restriction of movement. I suggest that any of these control cables that have been in service longer than 15 years or over more than two engine changes are past due for replacement.
Steve Ells is contributing editor and tech rep for Cessna Flyer Association. He has been an A&P/IA for 45-plus years. He is a commercial pilot with instrument and multiengine ratings and loves utility and bush-style airplanes and operations. He’s a former associate editor for AOPA Pilot. He owns Ells Aviation and lives in Templeton, California. Send questions and comments to [email protected].