Overstressing The Airframe

The National Transportation Safety Board (NTSB) recently completed its investigation into the in-flight breakups of two Beech Bonanzas. One was a V-tail S35 flown by an instrument-rated pilot with 3,200 hours. The other was a 36TC turbocharged Bonanza with a conventional tail, flown by an instrument-rated pilot who had 1,320 hours. The initial problems encountered were different, but the outcomes were the same.

Beech Bonanza S35
On November 25, 2003, at approximately 6:53 a.m., a Beech S35 was destroyed after it impacted terrain following an in-flight breakup and uncontrolled descent from cruise flight near Warren, Ore. The instrument-rated commercial pilot and his three passengers received fatal injuries. Visual meteorological conditions prevailed at the accident site, but instrument meteorological conditions were reported over the pilot's route of flight. An IFR flight plan had been filed for the personal cross-country flight being conducted under Part 91. The flight originated from the Arlington Municipal Airport (AWO), Arlington, Wash., at approximately 5:35, and was en route to Rogue Valley International-Medford Airport (MFR), Medford, Ore.

Approximately two hours before the flight, the pilot had received a standard weather briefing from the Seattle Automated Flight Service Station (AFSS). Later, the pilot contacted the AFSS stating that he was on the ground at AWO, had filed an IFR flight plan to MFR, and that he was about 10 minutes late. The specialist told the pilot to hold while he proceeded to coordinate the clearance with the Seattle Air Route Traffic Control Center (ARTCC). At 5:27, the specialist gave the pilot initial clearance to the WATON NDB, climb and maintain 4,000.

At 5:40, radar contact was established with the aircraft at an altitude of 1,400 feet MSL. At 6:35, after having climbed to 11,000 feet, the pilot requested a higher altitude. ARTCC cleared the pilot to 13,000 feet. Radar data indicated that at 6:39, the aircraft had reached 13,000 feet, and the pilot subsequently reported that he was at 13,000 feet and "clear of rime ice." The pilot radioed to ARTCC, "Ah, we just lost our suction gauge."

The controller responded, "Climb and maintain 15,000? You requesting a higher altitude? Is that what you said?"

The pilot then replied, "Mayday, mayday, mayday." There were no further transmissions from the airplane.

Radar data revealed that at 6:50:52, the flight was at 13,100 feet MSL, beginning a right turn from a southerly heading to a south-southwesterly heading and seven seconds later at 6:50:59 was at 12,900 feet MSL, beginning a right turn to a southeasterly heading. At 6:51:04, the aircraft was at 12,800 feet, starting a right turn from the southeast to south-southeast. Seven seconds later, it made a left turn back to a heading of southeast. At 6:51:16, the aircraft had climbed to 12,900 feet and was making a turn farther left to an east-southeasterly heading, reaching 13,100 feet at 6:51:28. Radar data indicated that at 6:51:28, the airplane's heading changed from east-southeast to a heading of northeast, having descended to an altitude of 10,700 feet by 6:51:52. The airplane then turned back to a heading of east-southeast and descended to an altitude of 10,500 feet at 6:51:59. The aircraft continued on a heading of east until 6:52:05, when its altitude was recorded as being 6,800 feet. It then proceeded on a northwesterly heading until 6:52:17, when the last radar hit recorded the aircraft at an altitude of 6,400 feet.

The fuselage and engine were located approximately 3 nm north-northeast of the Scappoose Industrial Airpark in Scappoose, Ore., and approximately 1 nm northeast of Warren, Ore. It impacted a low-lying marsh area. Both wings, the tail section and the left stabilizer of the airplane were located approximately 1 nm northeast of where the fuselage and engine came to rest.

Records indicated the vacuum pump had been installed 222.5 flight hours before the accident. The vacuum pump was subsequently sent to the NTSB Materials Laboratory in Washington, D.C., for examination. The pump's rotor and rotor vanes had shattered and the drive coupling shaft was sheared. A second examination of the pump was conducted later, under the supervision of an FAA airworthiness inspector. During that inspection, it appeared as if sometime in the recent past, the pump's rotor, vanes and coupling were replaced by an unknown party with parts from an unknown source. Although investigators noted that the pilot held an aircraft airframe and powerplant mechanic certificate, the Safety Board did not suggest that the pilot had worked on the vacuum pump.

The NTSB determined that the probable cause of this accident was vacuum pump failure and the pilot's failure to maintain control of the airplane. Factors contributing to the accident included the instrument meteorological weather conditions and the pilot's improper recovery from the descent.

Beech Bonanza 36TC
On July 18, 2004, at 5:08 p.m., a Beech BE-36TC, operated by a private pilot, broke up in flight at approximately 10,000 feet MSL during a cruise flight near Sylvester, Ga. The flight was operated under Part 91. Instrument meteorological conditions prevailed and an IFR flight plan was filed. The pilot and three passengers received fatal injuries, and the airplane sustained substantial damage. The flight originated from Sarasota, Fla.

The pilot had received a weather briefing from an FAA Automated Flight Service Station at about 1:05. The briefer advised of convective activity from Sarasota, extending up to Georgia, moving east-northeast at 30 knots with tops to FL 450. The briefer also stated thunderstorms were east, west and south of Columbus, Ga., moving east at 25 to 35 knots. The pilot filed an IFR flight plan from Sarasota to Lawrenceville, Ill.

ATC broadcast several messages regarding SIGMETs for convective activity. The pilot had reported on frequency when the messages were broadcast. At about 4:49, the pilot advised the Jacksonville Center that he was level at 10,000 feet. At about 4:56, the controller asked the pilot if he had a Stormscope or weather radar on board, to which the pilot replied, "I have a Stormscope."

The controller advised the pilot that there was a significant cell 10 miles in front of him extending 30 miles. The pilot advised that he was looking and asked, "What does the weather look like if we turned about heading 336 now?" The controller stated that the significant weather was solid, 20 miles wide with level 4 and 5 thunderstorms. The pilot asked if the controller could suggest a heading. The controller advised that there were no openings.

The pilot then asked how far the cell was, and the controller replied five miles and suggested, "If you wanted to go eastbound that's the clearest route---due east about 45 miles then northbound---that's the clearest route."

The pilot replied, "We'll try that." There were no further transmissions from the pilot. At about 5:04, the controller attempted to contact the pilot, but there was no response. At 5:08, radar contact was lost. At 5:14, a local sheriff's department received the first 911 call from a local resident reporting a plane crash. The airplane's wreckage was located at 5:21.

The nearest weather reporting facility was Southwest Georgia Regional Airport, in Albany, Ga., located approximately 13 miles west of the accident site at a ground elevation of 197 feet. There were localized IFR conditions in thunderstorms and moderate rain at the time of the accident. The thunderstorms began at 5:05 and continued through 5:12, with a second round of thunderstorms moving back into the area between 5:24 and 5:46.

Examination of the accident site found the wreckage path to be about 5,900 feet in length on a heading of 360 degrees. A search of the area at the beginning of the wreckage path found cabin interior components, followed by heavier items consisting of cabin doors, an emergency escape side window and some airframe skin. The right wing, left wing and empennage were next, followed by the cockpit and cabin seats. The engine and instrument panel were located at the end of the wreckage path. All of the fractures examined found no indications of fatigue. Examination of the right wing front and rear spars found bending in the upward or positive direction with wrinkling of the upper skin at the fracture sites.

The NTSB determined that the probable cause of this accident was the pilot's inadequate in-flight planning and decision making, which resulted in flight into thunderstorms and an in-flight breakup of the airframe.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.