BEECH B35 near Burbank, CA — 2010-04-19

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.

NTSB narrative

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.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Pilot could have reduced speed before encountering turbulence.

NTSB coding

• Turbulence encounter · Approach-VFR pattern base

Evidence available

• 1 docket document

Docket documents1

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form