Incident Overview
During the landing roll, the left main landing gear retracted into its wheel well, causing the airplane to settle on its left side. The pilot had received normal gear indications prior to landing.
Investigation Findings
Examination revealed that the landing gear down limit switches were stuck and appeared corroded and dirty. These switches, responsible for signaling the landing gear position, had illuminated the landing gear down light despite the gear not being fully extended and locked. The stuck switches affected the sequence of landing gear operation: once the nose of the airplane was lowered, the sequencing valve transferred hydraulic pressure from the main gear actuators to the landing gear door actuators before the main landing gear had fully extended to the locked position. This premature transfer allowed the left main landing gear to retract.
Aircraft and Operational Context
The airplane had been tied down outside on the airport ramp. Its annual inspection had been completed on February 16, 1996, and the aircraft had flown only 10 hours since that inspection.