What happened
During the pre-flight sequence while taxiing for takeoff, the copilot attempted to extend the landing gear. The intended action was to place the landing gear hydraulic handle in the down position to increase pressure within the downline system. However, the copilot believed he had confused the procedure and inadvertently pulled up on the landing gear lock handle instead of pushing the hydraulic handle down.
This incorrect manipulation of the controls released the mechanical locks securing the landing gear. Shortly after this action, the left main landing gear collapsed. At the same time, hydraulic fluid and associated fumes entered the cockpit, indicating a breach in the system integrity.
The investigation
A subsequent examination of the aircraft revealed that a hydraulic line connected to the pressure gage had failed due to overpressure. This failure was likely exacerbated by the improper handling of the gear controls. After the damaged line was repaired, the system was tested and operated normally, confirming that the mechanical components functioned correctly once the specific control error was addressed.
Findings
The primary factor in this incident was the misapplication of the landing gear controls. The confusion between the hydraulic handle and the lock handle led to the unintended release of the gear locks. The failure of the hydraulic line to the pressure gage was a secondary mechanical consequence, but the root cause remained the procedural error during the taxi phase.