When the vacuum system fails, proficiency prevails

Accidents resulting from vacuum pump failures are rare. Unfortunately, vacuum failures can be hard to detect, which can lead to spatial disorientation, unusual attitudes, and death.

On November 25, 2003, the pilot of a Beechcraft Bonanza and his three passengers were killed when the Bonanza broke up in flight near Warren, Oregon, after a vacuum failure and subsequent loss of control. The IFR cross-country flight had departed Arlington, Washington, with a destination of Medford, Oregon.

The pilot got a weather briefing two hours before the flight, which included advisories for mountain obscuration, occasional moderate turbulence and moderate rime or mixed icing in the clouds. Ceilings in the area were forecast to be broken at 2,000 feet and overcast at 4,000 feet with tops at 20,000 feet.

The flight departed Arlington at about 5:35 a.m. Pacific time. At 6:35 a.m., level at 11,000 feet, the pilot contacted Seattle Center and asked for a higher altitude. He was cleared to 13,000 feet. At 6:39 a.m., the flight was level at 13,000 and the pilot reported that he was clear of rime ice. Some time later, the pilot radioed, "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 replied, "Mayday, mayday, mayday." There were no further transmissions from the pilot.

Between 6:50 and 6:52 a.m., the Bonanza made numerous turns to the right and left. At 6:51 a.m., the plane descended from 13,100 feet to 10,700 feet in 24 seconds -- a descent rate of 6,000 fpm. The descent steepened to more than 18,000 fpm, and thirteen seconds later, the Bonanza was at 6,800 feet. The last radar return showed the plane at 6,400 feet.

Wreckage of the Bonanza was located on the east side of Scappoose Bay, 3 nautical miles northeast of the Scappoose Industrial Airpark.

Aircraft records show that a vacuum pump, overhauled on April 22, 1992, was installed on November 21, 2001. The Bonanza had been flown 222.5 hours since the pump's installation. Scoring consistent with an overstress fracture at the coupling's designed shear point was found during disassembly, and representatives from the company that overhauled the unit noted that "it appeared that sometime in the recent past the pump's rotor, vanes, and coupling were replaced by an unknown party with parts from an unknown source."

The pilot was an instrument-rated commercial pilot in both single and multiengine land airplanes. He also held an aircraft airframe and powerplant certificate. He had accumulated 3,263 hours of flight time, with 80 hours in the Bonanza.

The NTSB determined the probable cause of this accident was the failure of the vacuum pump, and the pilot's subsequent failure to maintain control of the airplane.

Early recognition of a vacuum pump failure is complicated because the first warning signs can be subtle. Vacuum- or pressure-powered flight instruments will slowly begin to give conflicting and inaccurate information. Staying proficient with partial panel operation will help mitigate the problems experienced when your vacuum pump fails.

For more information about how vacuum systems operate and how to cope with an in-flight failure, take the Air Safety Foundation's Pneumatic Systems Online Minicourse and read the Pneumatics System Safety Brief.

Accident reports can be found in ASF's accident database.