What happened
On 21 April 2001, a Cessna 441 Conquest, registration G-FCAL, was performing an aerial work flight when the crew encountered issues with the landing gear following departure from Sumburgh Airport. During the initial climb, the pilot noticed that the hydraulic pressure 'On' light and the 'Unlocked' light remained illuminated. Although cycling the gear and re-selecting it to the down position caused the cockpit indications to show the gear was locked, ground observers at Sumburgh noted that the nose gear did not appear fully extended.
Deciding to return to the aircraft's base at Teesside International Airport, the commander performed a visual inspection which confirmed the nose gear was not locked. Despite attempting to use the emergency blow-down system, the condition persisted. Following established procedures for unsafe gear indications, the crew reduced the fuel load to a minimum before attempting a landing. Upon touchdown at a speed of approximately 70 kt, the nose landing gear collapsed. The impact caused significant damage to the forward fuselage, both engines, and the propellers. There were no injuries to the three crew members on board.
The investigation
Investigators examined the nose landing gear assembly and discovered that the bolt connecting the leg assembly to the drag brace link was missing. Analysis of the cockpit indications revealed that the design of the downlock switch on the actuator body allowed a 'gear safe' indication to be displayed even when the nose leg was no longer geometrically secured due to the missing bolt.
Laboratory testing of the recovered bolt head, which was found near the runway at Sumburgh, revealed a fatigue fracture. The crack had initiated from a single point and propagated until the remaining metal could no longer support the load. While no manufacturing defect was found, the investigation focused on the stresses applied to the component during service.
Findings
- The primary cause of the collapse was the fatigue failure of the bolt connecting the leg assembly to the drag brace link.
- It is believed that a corrosion pit likely served as the initiation point for the fatigue crack.
- The loss of the bolt meant the nose leg was no longer geometrically constrained, allowing it to pivot backward under landing loads.
- This movement caused the hydraulic actuator piston rod to buckle under extreme compressive force.
- The bolt head had likely detached during a previous landing the evening before the accident, while the remainder of the bolt fell out during taxiing on the day of the occurrence.