The critical hardware that attaches an aircraft control yoke to the control surfaces should be inspected regularly, but if you own a Cessna, you might consider doing an immediate inspection.
That’s because McFarlane Aviation in Baldwin, Kansas, recently issued a service bulletin for its FAA-PMA aftermarket universal joints that attach the control tube to the shaft for controlling the flight control surfaces in Cessna models. These are models ranging from the 120 to the 210.
While rolling out from a landing, a Cessna 170 experienced a lockup of its yoke and subsequent failure of the control tube/shaft U-joint. McFarlane Aviation’s Dave McFarlane told us that his company developed its own FAA-approved U-joints a few years ago after quality problems (hydrogen embrittlement and play) with U-joints sourced from two different aerospace manufacturers. The McFarlane hardware is designed and fabricated using parts certified to MilSpec standards. These parts are made of a hardened stainless steel alloy, with the idea of reducing wear and corrosion, while increasing strength.
But the recent McFarlane service bulletin (SB-9) addresses a potential failure of the company’s part number MC0411257 universal joint. When it fails, the joint can separate. The event in the C-170 was preceded by the report of a late-model Cessna 172 experiencing stiff controls, and upon inspection it was determined that material contamination and lubrication had caused the problem. All affected articles were shipped between July 1, 2015, and September 30, 2016.
After the first report, the company revised the build materials and engineers determined that the like materials used in the part might have suffered from galling—an adhesive phenomenon that takes place at the molecular level, resulting in one part wanting to stick to another—and will generally result in metal transfer between two parts. Galling is generally not a problem with hardened steel parts or in applications that have good lubrication.
The failed universal joints showed evidence of a perfect storm of contamination, lack of lubrication and metal transfer locking free movement of the joint parts. McFarlane nitrides the inner steel parts (similar to the hardened finish in engine cylinders), resulting in a slick, hard component that’s impervious to contamination and has a molecular resistance to galling. There’s a long-lasting, non-petroleum-based dry lubricant added to the petroleum-based lubricant. McFarlane completed the redesign by including a boot that covered the part, similar to the boot that Cessna used on some models in the past. Samples were subjected to rigorous testing, far beyond what could be expected out in the field, including tremendous torque loads, running dry of lubricant and cycling until they were too hot to touch. None experienced failure.
There are over 500 of the original-design universals in the field, and the service bulletin calls for inspecting and lubricating the part every 25 hours. The U-joint must be replaced within 100 flight hours after the initial inspection or one calendar year. U-joints that are yellow or gold in color are not affected and no further inspection is necessary. The company will issue a $50 labor credit (and a new part) for replacing the U-joints from the affected lots.
Contact www.mcfarlaneaviation.com, 800-544-8594.
