Turbulence wrinkles Debonair

This April 2010 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.

Aircraft: Beech Debonair. Injuries: None. Location: Burbank, Calif. Aircraft damage: Substantial

What reportedly happened: According to the pilot, the airplane was in a 15° banked turn and descending through 5,000 feet when it encountered turbulence. The tail started to flutter. The pilot said the airspeed was between 155 and 160 mph. He quickly reduced power in accordance with the airplane’s Pilot Information Manual. This stopped the vibration.

The rest of the approach and landing at the airport was uneventful. After the flight the pilot noticed wrinkles in the ruddervator. A mechanic determined that the left ruddervator had a cracked front spar and that the left stabilizer had wrinkled top skin and a cracked rear spar, which was bent up 5 inches.

The maximum structural cruising airspeed for this model variant of the Beech 35 series is 161 mph, and the maneuvering airspeed is 131 mph. By definition, maximum maneuvering speed is that speed at which full and abrupt control surface deflection or any turbulence encounter will result in an aerodynamic stall before the structural limit load will be exceeded and damage will occur. Above this speed it is possible to incur structural damage with maneuvering inputs and/or significant turbulence encounters. The pilot was flying above maneuvering speed but within the green airspeed arc when the airplane encountered the turbulence. The pilot stated that he had no reason to believe that the airplane would encounter turbulence.

Probable cause: The airplane’s encounter with turbulence while at an airspeed above maneuvering airspeed, which exceeded the limit load of the stabilizers and ruddervators.

For more information: NTSB.gov. NTSB Identification: WPR10CA226



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