Accident Sequence
At 0347, the pilot received a standard weather briefing indicating instrument flight rules would prevail along the entire route of flight. The briefing included advisories for mountains to be obscured, an Airmet for occasional moderate turbulence below 15,000 feet msl, icing, occasional moderate rime or mixed icing in clouds and precipitation, and a freezing level to 16,000 feet msl. At 0540, radar contact was established with the flight. At 0635, while at 11,000 feet msl, the pilot requested a higher altitude. By 0639, the pilot reported being at 13,000 feet msl and clear of rime ice. At a time which could not be determined, the pilot advised air traffic control that he had just lost his suction gauge, and shortly thereafter radioed, "Mayday, Mayday, Mayday." Radar data from 0650:52 showed the airplane at 13,100 feet msl on a south-southwesterly heading. By 0650:59, it was at 12,900 feet msl on a southeasterly heading. From 0651:04 to 0651:28, the airplane was between 12,800 and 13,100 feet msl, turning left from south-southeast to east-southeast. Between 0651:28 and 0651:52, it descended on a northeasterly heading to 10,700 feet. The airplane then made a right turn of approximately 60 degrees to a south-southeast heading, and seven seconds later at 0651:59 was observed at 10,500 feet msl, indicating its descent had been arrested. Six seconds later, at 0652:05, the flight had descended to 6,800 feet msl on the same heading. The last radar hit was at 0652:17 when the airplane was on a northwest heading at 6,400 feet msl.
Wreckage Examination
The engine and fuselage were located 1 mile northwest of Warren, Oregon, while both wings, the tail section, and the left stabilizer were located approximately 1 mile northwest of the main wreckage. Separation signatures were consistent with the left stabilizer separating in an upward and aft direction, while both wings had separated in an upward and positive direction.
Vacuum Pump Analysis
Examination of the vacuum pump and its history indicates that it is probable that the pump failed due to vane failure as a result of highly worn vanes being subjected to impact bending loads from the uneven interior surface of the housing, with a possible contribution from contaminating material between the vanes and the housing.