ERA16LA293 · Cessna 150G UNDESIGNAT

The student pilot and flight instructor were en route to the airport, with the airplane cruising at 4,500 ft above mean sea level, when the student noticed a roughness and vibration coming through the control wheel. He told the flight instructor about it and then noticed that the left elevator tip was hanging down 6-10 inches from its normal mounting position. The flight instructor took control of the airplane and noticed that the left elevator was moving up and down uncontrollably. The flight instructor made a 7-mile-long, straight-in approach to the runway, where the airplane touched down firmly without further incident. Examination of the elevator and horizontal stabilizer revealed that the airplane’s elevator was substantially damaged. The left side of the elevator was bent downward, and the left outboard elevator attachment bolt had backed out of the nutplate. In addition, the right elevator attachment bolt would move in the nutplate when the elevator was moved up or down. With the outboard left side of the elevator no longer attached to the horizontal stabilizer, it is likely that the air loads imparted onto the partially supported elevator during the flight bent it and resulted in the reduced pitch controllability. Further examination of the attachment bolts from both the left and right side of the elevator revealed that both displayed signs of thread wear on the end of the bolt that would have been fully engaged with the nutplate. Both nutplates displayed corrosion outside the threaded area on the backside of each nutplate, and the nutplate from the left side would not hold specified torque and displayed evidence of cross-threading, which may have occurred either before or during replacement of the bolts about 6 years earlier. Review of maintenance records indicated that two new bushings and elevator attachment bolts had been installed about 6 years before the accident, but the records did not reflect if the hinge parts were replaced at that time. Review of the airplane manufacturer’s service manuals revealed that supplemental inspection guidance had been issued for inspection of the horizontal stabilizer, elevator, and attachments for signs of damage, fatigue, or deterioration. The airplane had undergone an annual inspection about 6 months before the accident. It is likely that the damage and deterioration of the elevator’s left bolt and nutplate should have been evident during the most recent annual inspection, as well as previous inspections. Review of the Federal Aviation Administration (FAA) Service Difficulty Reporting program database revealed that since 1979, seven other reports of the elevator hinge attachment bolt backing out had been submitted. According to the FAA, the main issues identified included excessively worn attachment hardware and the use of incorrect attachment hardware, with corrosion possibly a contributing factor. As a result, in May 2017, the FAA issued a special airworthiness information bulletin to alert owners, operators, maintenance technicians, and inspectors of the potential for loose bolts at the elevator attachment points on certain models of the airplane type and recommended performing the supplemental inspections detailed in the airplane manufacturer’s service manuals.

Verbatim NTSB analysis and probable cause from the NTSB dataset