Shortly after take-off in challenging weather conditions, a small, general aviation plane crashed. ESi was retained to investigate in litigation arising after the accident. An investigation focus concerned the aircraft’s autopilot pitch trim servo. It was being alleged that the servo gearing experienced binding caused by environmental particulate entering the servo on the accident flight. The experts espousing this theory identified gear teeth contact marks as being unique evidence of this binding and servo malfunction.
Our Approach
Prior to attending an inspection of the subject, an exemplar aircraft and its installed pitch trim servo were examined. The servo has a cylindrical capstan with grooves that engage a wrapped control cable extending from a cockpit control wheel back to a pitch trim bell crank in the tail. The arrangement allows for pilot manual control of the pitch trim or electric servo control by the autopilot. In autopilot mode, the servo motor drives a small diameter pinion gear that is engaged to the capstan larger diameter gear.
The subject pitch trim servo had been extracted from the wreckage that suffered extensive impact and fire damage. Initially, the servo was scanned using X-ray Computed Tomography (CT) to memorialize its internal condition prior to disassembly. After disassembly, microscopic examination was performed on the gears both optically and with a scanning electron microscope. Contact marks were observed on the gear teeth of both the pinion gear and the capstan gear and were alleged to be evidence of the gears binding. Critically however, ESi observed that these contact marks were “fresh” and clear of post-crash fire residues present on the rest of the servo components.
ESi consultants then carefully studied the subject servo CT scan and an exemplar servo CT scan and found, in its post-accident condition, the subject servo’s pinion gear axis was misaligned relative to the that of the capstan gear. Using both the CT scan data and engineering drawings, this misalignment was reverse-engineered using 3D CAD software. The resulting model CAD model was then animated to conclusively demonstrate that the gear teeth contact marks were caused after the accident by rotation of the gears that were misaligned due to crash impact. Their rotation was likely the result of the pitch trim cable being inadvertently pulled during wreckage retrieval from the site. Notably, this rotation was also confirmed visually by a fire residue shadow on the capstan gear face that had been displaced from a circular window originally exposing it during the post-crash fire.
Services Utilized
Expertise Utilized
The Outcome
ESi consultants testified at trial and our client, the autopilot manufacturer, won a defense verdict.
