Incident
During a full-stop landing, after touchdown and during the landing roll, all three of the aircraft's landing gear collapsed. The aircraft had been configured for landing with the gear extended.
Investigation Findings
Subsequent examination determined that the hydraulic pump drive motor had failed during the gear extension process. Although all three green gear-down indicator lights illuminated when the gear was selected down, the pump's failure prevented it from continuing to operate throughout the entire extension sequence. As a result, the landing gear did not reach the full down position, and the gear down-locks were not activated.
Mechanism
The lack of full extension and engagement of the down-locks meant the gear was not securely locked in place. The weight of the aircraft during the landing roll caused the unlocked gear to collapse. The green indicator lights provided a false indication that the gear was down and locked, when in fact the gear was not fully extended and the locks were not engaged.