What happened
On June 5, 1997, a Cessna U206C, registration OH-CWW, was performing a skydiving flight at Turku Airport when the aircraft's elevator began vibrating violently. The flight involved a pilot and four skydivers. After the skydivers exited the aircraft at 10,000 feet, the pilot initiated a descent. Upon reaching an indicated airspeed of approximately 100 MPH, the elevator entered a severe flutter, oscillating two to three times per second. The pilot managed to mitigate the vibration by reducing the airspeed to 80 MPH, allowing for a safe landing.
Upon ground inspection, it was discovered that the trim tab's screw actuator attachment points had torn away from the stabilizer because the stabilizer spars had fractured under the stress of the vibration. There were no injuries during the event.
The investigation
The investigation focused on the structural failure of the elevator and the cause of the aerodynamic imbalance. Technicians at a maintenance facility inspected the elevator and performed a weight check according to the maintenance manual. The results revealed that the trim tab weighed 1400g, which was double the weight of a new 700g component. This significant weight increase made the elevator markedly tail-heavy.
The investigation established that the excessive weight was caused by moisture absorption within the urethane foam filling the trailing edge of the elevator and the interior of the trim tab. Furthermore, the investigation found that this was not an isolated incident; maintenance reports indicated several other cases where foam-filled components had become saturated, leading to similar imbalances.
Findings
- The primary cause of the flutter was a significant aerodynamic imbalance of the elevator and trim tab, resulting from the absorption of moisture into the ureton foam filling.
- The increased weight of the trim tab shifted the center of gravity of the control surface, lowering the critical airspeed at which aeroelastic flutter occurs to within the aircraft's normal operating range.
- While a manufacturer service bulletin (SEB85-7) existed for inspecting the condition of the foam and skin, it did not address the risk of moisture absorption or the resulting weight changes.
- The investigation noted that the manufacturer had transitioned to providing spare parts without foam filling, but older, foam-filled components remained in service.