WPR12LA396

The pilot’s exceedance of the design maneuvering speed during descent in turbulence, which resulted in structural damage to the aft fuselage.

The pilot reported that, while the airplane was descending to land at an indicated airspeed of about 180 mph, it encountered turbulence for about 2 seconds. The pilot immediately reduced power and slowed the airplane to about 140 mph. The pilot continued the approach and landed the airplane without further incident. After landing, the pilot observed deformities along the fuselage. A postaccident examination of the airplane revealed structural twisting damage to the aft fuselage, consistent with an overload failure in a tail downward direction. According to the airplane manufacturer, the damage was also consistent with damage seen on other airplanes of the same make and model that had encountered turbulence while descending at speeds higher than the airplane's design maneuvering speed. A review of the Pilot's Operating Handbook showed that the airplane's reported airspeed of 180 mph at the time it encountered the turbulence was within the caution operating range of 176 to 202 mph and was higher than the design maneuvering speed. It is likely that the pilot's exceedance of the design maneuvering speed while the airplane was descending in turbulence resulted in the structural damage to the aft fuselage.

HISTORY OF FLIGHT On August 31, 2012, about 1140 Pacific daylight time (PDT), a Beech G35, N4454D, encountered clear air turbulence while on descent to Oceano County Airport, Oceano, California. The owner/pilot was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot and two passengers were not injured; the airplane sustained substantial damage to the fuselage structure. The cross-country personal flight 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 in descent to land at Oceano at 2,500 feet he encountered a turbulence event that lasted about 2 seconds. He stated that his indicated airspeed was about 180 miles per hour; he immediately reduced power, and slowed the airplane to about 140 miles per hour. He continued the approach without incident and landed. After landing, the pilot observed deformities along the fuselage, and determined that further examination of the airplane was warranted. 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 he was advised by the maintenance personnel of the damage. TESTS AND RESEARCH The airplane was examined and documented on September 11, 2012, by investigators from the FAA and the airplane manufacturer's representative. A complete copy of the examination report is attached to the accident docket. The examination report documented the following damage to the airplane: REAR FUSELAGE Wrinkles were observed to the left and right side fuselage skins that ran from the top of the fuselage diagonally down and aft. The lower fuselage skin was torn. Wrinkles were present in the lower fuselage that radiated from the left bottom corner of the fuselage and from the right bottom corner of the fuselage. The left and right side I-stringers, which join the two bottom sides of the fuselage side skins to the lower fuselage skin, were buckled. The bulkhead was cracked in three areas around the bulkheads inner lightening hole cut-out. An interior inspection noted that the I-stringers and G-stringers were buckled in the area of the bulkhead. FLIGHT CONTROLS The left elevator was determined to have a 19.87 in.-lbs. trailing edge heavy moment, and the right elevator was determined to have a 19.86 in.-lbs. trailing edge heavy moment. 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 observed 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 the abundance of 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, P/N 35-590072-9A4, Section II Limitations, page 2-3 airspeed limitation values and page 2-4 airspeed indicator markings, states airspeeds and conditions associated with those airspeeds as follows: Normal Operating Range Green arc on the airspeed indicator face 66 to 176 mph Caution Operating Range Yellow arc on the airspeed indicator face 176 to 202 mph Maneuvering speed Va 131 mph (at maximum gross weight) Do not make full or abrupt control movements above this speed Maximum structural cruising Vc speed is 176 mph Do not exceed this speed except in smooth air and then only with caution. A review of the FAA Pilot's Handbook of Aeronautical knowledge revealed the following: Design maneuvering speed (Va)—the maximum speed at which the structural design's limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage. It is important to consider weight when referencing this speed. For example, Va may be 100 knots when an airplane is heavily loaded, but only 90 knots when the load is light. The pilot reported that the accident airplane was being operated in the Yellow or Caution airspeed range, which is greater than the Va airspeed. 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 FS272 fuselage bulkhead and the elevator trailing edge heavy moments, would act to lower Va by an unknown amount.