Boeing A75L3 N53466 accident near Yacolt, Washington

Probable cause

The pilot’s exceedance of the airplane’s critical angle of attack while maneuvering at a low altitude in mountainous terrain in moderately high density altitude conditions, resulting in an aerodynamic stall and subsequent impact with terrain.

NTSB analysis

The accident airplane was one of three airplanes traveling together on a cross-country flight. The pilots of the accident airplane and one of the other two airplanes chose to divert from the group’s typical route of flight to fly an alternate route through rising, mountainous terrain.

As the two airplanes approached an area of higher terrain and a ridgeline that required a climb, the pilot of the second airplane flying the alternate route initiated a circling climb to gain sufficient altitude needed to cross the approaching ridgeline. He then continued on toward the intended destination, unaware that the accident airplane was not behind his airplane.

When approaching the same ridgeline, the pilot of the accident airplane maintained an altitude that was lower than the elevation of the ridgeline and did not initiate a climb. The airplane maintained an altitude of about 2,900 ft mean sea level (msl) until it entered a left turn, followed by a rapid descent above the accident location. The airplane’s ground speed had slowed to 40 kts at the onset of the rapid descent and the left turn began about .63 statute miles from terrain that rose to 3,550 ft msl, which was the lowest available terrain ahead of the airplane’s position. The pilot likely initiated a left turn away from rising terrain and exceeded the airplane’s critical angle of attack. The investigation was unable to determine why the pilot did not initiate a climb earlier in the flight.

The pilot of the second airplane flying the alternate route reported that “it wasn’t too smooth out,” which was consistent with terrain-induced wind and/or turbulence. He reported that at the initiation of his climb he added engine power, saw his ground speed decrease and then added additional engine power. He subsequently chose to circle climb to clear the ridgeline. The calculated density altitude near the accident site was 4,626 ft.

Postaccident examination of the engine and airframe revealed no evidence of preaccident malfunction or failure.

Text from the NTSB. Open the NTSB docket for WPR24FA298 for the full record.