Amateur-built GLASAIR III N54CB accident near Eugene, Oregon

Probable cause

The pilot’s loss of airplane control during the approach to landing due to an asymmetrical flap deployment for reasons that could not be determined based on the available evidence.

NTSB analysis

The pilot departed on a daytime cross-country flight in visual meteorological conditions. Automatic dependent surveillance-broadcast (ADS-B) data indicated a normal flight and a cruising altitude of about 2,500 ft mean sea level. The airplane generally flew a south-southwest heading toward the destination airport. About 1.5 nautical miles north of the destination airport, the airplane entered a descending left-hand spiraling turn, after which the track data ended. During the approach to land, the pilot radioed two times that he was ‘going down.’ The airplane impacted terrain about 1.5 nautical miles north of the airport.

Postaccident examination of the engine revealed no preimpact mechanical anomalies that would have precluded normal operation of the airplane. Examination of the airframe revealed that the flap system is connected via a tubular bar in the center of the wing that extends out into each wing root. At each end of the bar is a welded plate which a flap actuator push-pull assembly is connected to and terminates at the wing flap. The pilot manipulates the flaps from inside the cockpit via a flap switch which electrically actuates the flaps into various positions depending on the phase of flight. The right-wing flap hardware remained connected from the wing root out to the flap. The left-wing flap was disconnected at the flap bar welded plate. The left-wing flap non-threaded steel clevis pin and cotter key, which connected the welded plate to the flap actuator turnbuckle fork at the wing root, were missing and not located within the wreckage.

Review of the aircraft records revealed no evidence of any recent maintenance within the area of the missing clevis pin and cotter key. A condition inspection was completed 45 days prior to the accident. The aircraft records also identified maintenance performed on the landing gear six days prior to the accident.

This situation likely created an asymmetrical “split” flap condition, with only one flap deploying when the pilot extended the flaps during the approach to landing, causing the airplane to depart controlled flight. The pilot would likely not have been aware of a faulty flap until the landing approach.

Text from the NTSB. Open the NTSB docket for WPR21LA204 for the full record.