WPR10CA226

The pilot reported that, during descent and at about 5,000 feet, the airplane was flying at 155-160 mph when it hit slight turbulence while in a 15 degree left turn and the tail started to flutter. The pilot quickly reduced power and raised the nose in accordance with the airplane's Pilot Information Manual. This stopped the vibration, which lasted a total of about 3 to 5 seconds. The rest of the approach and landing at the airport was uneventful. During the post-flight inspection, 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 five inches. Examination of the ruddervators disclosed that they were within the balance requirements of the Beechcraft maintenance manual. The pilot stated that he was between 155 and 160 mph at the time of the turbulence encounter. The maximum structural cruising airspeed for this model variant of the Beech 35 series (Vno or Vc, and the beginning of the yellow caution arc) is 161 mph, and the maneuvering airspeed (Va) is 131 mph. By definition, maximum maneuvering speed (Va) 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 Va it is possible to incur structural damage with maneuvering inputs and/or significant turbulence encounters. The pilot was flying above Va but within the green airspeed arc when the airplane encountered the turbulence; however, he was not flying in the yellow arc above Vc/Vno and had no reason to believe that the airplane would encounter turbulence.

In the pilot's written statement to the Safety Board, he reported that he was traveling from Modesto, CA, to Burbank, CA. The pilot reported that during descent to Burbank at about 5,000 feet, the airplane was flying at 155-160 miles per hour when it hit slight turbulence while in a 15 degree left turn, and the tail started to flutter. The pilot quickly reduced power and smoothly raised the nose in accordance with the airplane's Pilot Information Manual. This stopped the vibration, which lasted about 3 to 5 seconds. The rest of the approach and landing at the airport was uneventful. During the post-flight inspection, the pilot noticed wrinkles in the left ruddervator. The airplane was inspected by a mechanic, who found that the left ruddervator had a cracked front spar, and the left stabilizer had wrinkled top skin and a cracked rear spar, which was bent up five inches. The pilot did not state if there were any mechanical problems with the airplane prior to the accident. The airplane's maintenance records were reviewed. In January 2001, the airplane's two rear bulkheads were straightened and redoubled, short cracks were dressed, and new relief flanges were fabricated and installed. Also in January 2001, the empennage skins were redoubled. The airplane was painted as required, the flight controls and systems were reinstalled, and the static control balance was performed. The airplane was inspected for compliance with the pertinent Raytheon (Beechcraft) Service Bulletin. All work was done in accordance with the Service Bulletin, the appropriate Advisory Circular, and the Beechcraft 35 Shop Manual. In April 2001, the airplane's ruddervators were rebuilt and painted. After the painting, the right ruddervator's static balance was 17.33 inch-pounds (in-lb) moment balance. The left ruddervator's static balance was 17.59 in-lb moment balance. Both static balances were within the Beechcraft Shop Manual's allowable balance parameters of 16.8 to 19.8 in-lb. In February 2009, the airplane's fuselage and wings were repainted by the owner, then repainted by a certified mechanic shop. The control surfaces were not painted. After the accident, a mechanic removed and replaced the spars, re-skinned the airplane, used doublers between the spar and the new skin, added a doubler to the center forward of the aft rib, and checked the ruddervator balance. The mechanic stated that the control surface balancing was "perfect." The pilot reported that he was flying between 155-160 miles per hour (MPH) at the time of the turbulence encounter. The maximum structural cruising airspeed (Vno or Vc) is 161 MPH, and the maneuvering airspeed (Va) is 131 MPH. The pilot was flying above Va when the airplane encountered the turbulence; however, he was not flying above Vno and had no reason to believe that the airplane would encounter turbulence.