What happened
An airline transport pilot was conducting a mountain sightseeing flight with three passengers under Title 14, CFR Part 135. While flying at 11,000 feet MSL and encountering turbulence, the pilot reported that the Cessna (type not specified in source) began to shake violently. The pilot stated he lost control of the aircraft and subsequently shut down the engine to determine if the engine was causing the vibration. The shaking persisted until the airspeed was reduced to 80 mph. After restarting the engine, the pilot performed a safe landing. There were no fatalities or injuries reported.
A digital video recording taken by the pilot during the flight showed the aircraft in a wings-level attitude under VFR conditions. The audio from the recording captured a vibration lasting between 3 and 7 seconds in the 8.2 to 8.4 Hz range, which corresponds to the first mode of wing bending or torsional vibration. The recording did not show the aircraft being operated outside its normal operating envelope.
The investigation
Aerospace engineers from the Anchorage FAA Aircraft Certification Office examined the aircraft and found that both wings had sustained structural damage. Damage signatures indicated that the rear spars of both wings oscillated with significant amplitude at span station 92.5. Additionally, the bushing holes in the rear spar attachment fittings were elongated. The investigation also noted that the right aileron and rudder were severely under-balanced. Engineers could not determine if the aileron cable tension had been properly adjusted before the event.
Findings
- The rear spars of both wings experienced significant oscillation.
- The right aileron and rudder were found to be severely under-balanced.
- Bushing holes in the rear spar attachment fittings were elongated.