What happened
During the approach to the airport, the flight crew confirmed that the landing gear was in the down and locked position. The first officer performed the landing on runway 35L during a slight left crosswind. Upon touchdown, the landing gear warning horn activated.
Immediately following the contact with the runway, the nose landing gear and the right main landing gear retracted. This caused the aircraft to slide across the runway, eventually coming to a halt on the right side of the runway. The crew was not injured.
The investigation
Data from the flight data recorder indicated that the landing gear was extended and showed an indication of being down and locked prior to the event. However, the nose landing gear transitioned to an "unlocked" or unsafe state approximately 2 seconds after the radio altitude reached 0 feet. During this sequence, hydraulic pressure experienced a momentary drop to approximately 2,700 psi before returning to a normal level of about 3,000 psi.
Investigators performed several tests on various aircraft components, including the proximity switch electronics unit (PSEU), the landing gear control assembly, the landing gear selector valve, and the hydraulic pressure sensing valve. The right main landing gear harness and its associated proximity switches were also examined, with no anomalies found.
Additionally, hydraulic fluid samples were collected from the filter pack, the return line for the landing gear selector valve, and the right main landing gear down lock assist actuator. Analysis of these samples revealed no significant anomalies. Partial systemic wiring tests were also conducted and yielded unremarkable results.