6 Dec 2024
Inspection of Trim Tab Operating Cable Attachment to Prevent In-Flight Failures and replacement with Improved Design of Cable Attachment
In 2016, the LAA issued an Alert, LAA/AWA/16/08, regarding reported problems with the thin stranded steel elevator trim cable on Eurofox aircraft fraying at the attachment point to the trim tab operating horn, where the design forced the cable to be bent around the sharp edges of the clevis. The 2016 Alert also provided a link to, and served to promulgate, the Eurofox UK Service Bulletin on the subject, SB 04/2016.
Recently we’ve been advised of a potentially serious incident of elevator flutter on a Eurofox while towing a glider, which was caused by the trim cable failing altogether in flight where it passes around the trim tab operating horn clevis. This resulted in violent shaking of the elevator with serious potential for structural damage or loss of control.
Since the cable had evidently frayed considerably before failing altogether, this occurrence highlights the importance inspecting the trim cable for signs of fraying at this point at each pre-flight and maintenance check, and, as the service bulletin explains, of keeping the clevis free-moving about its attachment to the tab so the clevis remains in line with the cable as the tab moves. This minimises the relative movement between the two which if occurring, would accelerate the wear on the cable.
Eurofox UK advises that since the issue of the 2016 Service Bulletin, they have revised the design of the cable attachment clevis so that the cable now passes around a circular bush rather than being forced into two sharp bends, which reduces the wear on the cable and tendency to fray and eventually break.
MTD-07-2024, issued today, requires Owners/Operators to check whether the old or new design of clevises are fitted and for any signs of trim cable fraying before further flight. Owners/Operators must obtain and use a copy of the up-issued AEROPRO Checklist - B Service/Maintenance plan (Update Nov 2024 LAA aircraft) before the next 50-hour check. Finally, the trim cable system must be replaced immediately if any trim cable fraying is found, as detailed in the MTD. Furthermore, all Eurofoxes with the old-style of clevis must have the trim cable system replaced, as described in the MTD, by 1st June 2025.