Incident Details
An airline transport pilot was conducting a mountain sightseeing flight with three passengers under Title 14, CFR Part 135. The pilot reported that while at 11,000 ft MSL and encountering some turbulence, the airplane began to shake violently. He stated that he could not control the airplane and stopped the engine in case it was causing the problem. The shaking continued until he slowed the airplane to 80 mph. He then restarted the engine and landed safely.
Examination Findings
An examination of the airplane revealed that both wings were structurally damaged. Prior to the event, the pilot had been taking a digital video with audio through the window of the airplane. The recording shows the airplane in a wings level attitude in visual flight rules (VFR) conditions. When the camera was dropped to the floor after the initiation of the event, it continued to record. The audio recording revealed a vibration lasting about 3 to 7 seconds in the 8.2 to 8.4 Hz range, which is in the appropriate range for the wing bending/torsional first mode of vibration. Nothing on the visual or audio recording indicated that the airplane was being flown outside its normal operating envelope.
The airplane was examined by aerospace engineers from the Anchorage FAA Aircraft Certification Office. Damage signatures indicated that the rear spars of both wings oscillated up and down with significant amplitude at span station 92.5. The bushing holes in the rear spar attachment fittings were elongated, and the right aileron and rudder were severely under-balanced. The engineers were not able to ascertain if the aileron cable tension was adjusted properly prior to the event.
Historical Maintenance Context
In 1980, prompted by flutter events, the manufacturer produced Service Bulletin 2/29, relating aileron/wing flutter to more than one of the following four conditions: ailerons not balanced; aileron cables in the wing slack; deterioration in the stiffness of the aileron mounting structure in the fuselage; and/or the airplane being flown outside the limits of the flight manual. On February 20, 1980, in response to the manufacturer's service bulletin, the FAA issued Airworthiness Directive AD 80-24-02, which required inspections of the airplane's wings, spars, and aileron cable tension and balance, based on service time and part numbers.