History of Flight
On August 31, 2012, about 1140 Pacific daylight time, a Beech G35, registration N4454D, encountered clear air turbulence while on descent to Oceano County Airport, Oceano, California. The owner/pilot was operating the airplane under 14 Code of Federal Regulations Part 91 for a cross-country personal flight. The flight had departed Tulare, California, about 1100, with a planned destination of Oceano. Visual meteorological conditions prevailed, and no flight plan had been filed.
The pilot reported that while descending to land at Oceano at 2,500 feet, the airplane experienced a turbulence event that lasted about 2 seconds. The indicated airspeed was about 180 miles per hour. The pilot immediately reduced power and slowed the airplane to about 140 miles per hour, then continued the approach without incident and landed.
After landing, the pilot observed deformities along the fuselage. Maintenance personnel examined the airplane and discovered substantial damage to the tail section of the fuselage. The pilot notified the NTSB on September 6, 2012, after being advised by maintenance of the damage.
Examination Findings
On September 11, 2012, investigators from the FAA and the airplane manufacturer's representative examined and documented the airplane. The examination report documented the following damage:
Rear Fuselage
- Wrinkles were observed on the left and right side fuselage skins, running diagonally down and aft from the top of the fuselage.
- The lower fuselage skin was torn.
- Wrinkles radiated from the left and right bottom corners of the fuselage.
- The left and right I-stringers, which join the bottom sides of the fuselage side skins to the lower fuselage skin, were buckled.
- The bulkhead was cracked in three areas around the inner lightening hole cut-out.
- Interior inspection noted buckling of I-stringers and G-stringers in the area of the bulkhead.
Flight Controls
- The left elevator had a trailing edge heavy moment of 19.87 in.-lbs., and the right elevator had 19.86 in.-lbs. These values exceed the published Shop Manual limits for the trailing edge heavy moment airworthiness specification.
The manufacturer's representative stated that the rear fuselage damage to the fuselage sides and lower skin was consistent with damage seen on other G35 airplanes that had experienced turbulence while in high speed descents—a one-time overload in a tail downward direction. The presence of abundant oil and grime in the rear fuselage, and the lack of oil and grime in the bulkhead cracks, were consistent with the cracks having occurred recently.
Additional Information
A review of the Beechcraft Bonanza G35 Pilot's Operating Handbook and FAA Approved Airplane Flight Manual indicated the following airspeed limits:
- Normal Operating Range: 66 to 176 mph (green arc)
- Caution Operating Range: 176 to 202 mph (yellow arc)
- Maneuvering Speed (Va): 131 mph at maximum gross weight. The manual states not to make full or abrupt control movements above this speed.
- Maximum Structural Cruising (Vc): 176 mph; do not exceed except in smooth air and then only with caution.
The pilot reported that the accident airplane was being operated in the yellow or caution airspeed range, which is greater than Va. At airspeeds greater than Va, the airplane can sustain structural damage during operation. An unairworthy airframe material condition, such as the observed cracks in the fuselage bulkhead and the elevator trailing edge heavy moments, would act to lower Va by an unknown amount.