N4606DBeech G35 · 1956

On April 20, 2008, about 1330 eastern daylight time, a Beech G35, N4606D, was substantially damaged following un-commanded pitch and roll oscillations in cruise flight near New Smyrna Beach, Florida. The certificated commercial pilot and the pilot-rated passenger were not injured. Visual meteorological conditions prevailed for the flight that originated at Henry County Airport (PHT), Paris, Tennessee, and destined for Melbourne International Airport (MLB), Melbourne, Florida. No flight plan was filed for the personal flight which was conducted under the provisions of 14 Code of Federal Regulations Part 91.

In a written statement, the pilot described an uneventful flight from departure to the point in the flight where he requested his arrival descent from air traffic control. According to the pilot, the airplane was level at 9,000 feet; at about 125 knots indicated airspeed, and the global position system indicator showing 147 knots true airspeed. Prior to initiating the descent, the pilot "heard and felt 5-6 sharp vibrations with about a 2-3 hertz frequency. The aircraft pitched up and down violently a few degrees and rolled a few degrees to the left." The pilot-rated passenger reported hearing "popping and tearing" from the area of the baggage door.

The pilot disengaged the autopilot, slowed the airplane, and the vibrations stopped. He then performed a slow descent and completed a safe landing at Melbourne Regional Airport. After landing, the pilot noted that the g meter had recorded loads of +3.9 g's to -1 g. Typical loads recorded during moderate turbulence encounters with the airplane were "+0.5 g's to +2.0 g's."

Interpolation of radar data revealed level cruise airspeed above design maneuvering speed (VA-114-kts), and airspeeds above design cruising speed (VC-153-kts), during descent. There was no evidence of airspeeds above the airplane's never-exceed speed (VNE-176-kts).

A hand held Global Positions System (GPS) receiver was on board and in use during the accident flight. The GPS unit was requested from the pilot multiple times, but was not submitted for examination.

The pilot held an airline transport pilot certificate with ratings for airplane single engine land and sea, airplane multi-engine land, and instrument airplane. The pilot also held a flight instructor certificate with ratings for airplane single engine, airplane multi-engine, and instrument airplane. His most recent Federal Aviation Administration (FAA) second class medical certificate was issued on December 15, 2007. The pilot reported 8,200 total hours of flight experience, 3,500 hours of which were in single engine airplanes, and 120 hours of which were in the Beech G35.

According to FAA and maintenance records, the airplane was manufactured in 1956, and had accrued 4,964 total aircraft hours. The most recent annual inspection was completed on May 4, 2007, at 4,937 total aircraft hours. The airplane was in compliance with Beech Service Bulletin 2188 (Tail Reinforcing Kit 35-4016-3S). The kit was installed on October 14, 1987.

At 1347, the weather reported at New Smyrna Municipal Airport (EVB) included scattered clouds at 10,000 feet and winds from 030 degrees at 6 knots. The visibility was 10 miles. The temperature was 19 degrees Celsius and the dew point was 5 degrees Celsius. There were no weather advisories current over central Florida surrounding the period of the accident.

A weather study from a National Transportation Safety Board Senior Meteorologist revealed cumulus clouds developing in the area on the satellite and Weather Surveillance Radar 88 Doppler (WSR-88D) imagery, and therefore the potential for convectively induced turbulence from towering cumulus clouds. The soundings also noted an inversion near 9,500 feet with rapidly drying air above. The vertical wind shear was 5 knots per 1,000 feet at that layer, which corresponded to moderate or greater turbulence. There were no weather advisories current over central Florida surrounding the period of the accident.

Examination of the airplane, a monoplane with a "V" tail, by representatives of Hawker-Beechcraft revealed rear fuselage skin and stringer buckling between flight station 233 and flight station 256 bulkheads. There was downward bending of the rear fuselage with diagonal buckling of the skin on each side, and tearing of the skin on the bottom.

The two elevators were found properly attached, and properly balanced. Control rigging could not be confirmed due to aircraft damage, but the cables were found "taught" using hand force. The propeller and the engine's dynamic shock mounts were in "good" condition. An FAA aviation safety inspector supervised the examination, and concurred with the manufacturer's field notes.