What happened
On 11 June 2001, a Cessna Citation 550, registration HB-VMX, was performing a passenger flight from Zurich to Oxford Airport. After completing standard pre-landing checks, the crew executed a visual approach to runway 20, performing a short field landing. The aircraft touched down normally, and the crew subsequently applied the brakes, deployed the thrust reversers, and extended the speed brakes.
Shortly after these actions, the landing gear warning horn activated, the landing gear warning light illuminated, and the green light for the nose landing gear extinguished. The nose landing gear then collapsed, causing the aircraft to slide along the runway until it came to a stop. The crew performed an emergency shutdown and evacuated the five people on board, consisting of two crew members and three passengers. No injuries were reported. The Airport Fire Service attended the scene, but no fire occurred. Damage was confined to the nose landing gear and the forward section of the fuselage.
The investigation
The AAIB examined the aircraft's systems and consulted with the manufacturer, Cessna. The investigation focused on why the nose gear retracted despite the landing being otherwise normal. Evidence from the manufacturer indicated that this specific failure had occurred in six other instances. Engineering tests conducted by Cessna revealed that a non-return valve within the hydraulic system's return circuit could permit a pressure spike to travel back through the valve. This spike is capable of unlocking the nose landing gear actuator downlock.
Findings
- The nose landing gear collapsed due to the unintended unlocking of the nose landing gear actuator downlock.
- This unlocking was caused by a pressure spike in the hydraulic return system.
- The pressure spike was triggered by the simultaneous activation of the thrust reversers or speed brakes during the landing roll.
Safety action
Cessna issued a revised Airplane Flight Manual as an interim safety measure, instructing flight crews to avoid operating speed brakes or thrust reversers until the nose wheel has made firm contact with the runway. Additionally, the manufacturer is working with the Federal Aviation Administration and a supplier to certify a redesigned check valve to prevent future occurrences.