Introduction
On 21 April 2001, a Cessna 441 Conquest, registration G-FCAL, sustained damage to its nose landing gear, forward fuselage, both engines and propellers during landing at Teesside International Airport, County Durham. The aircraft was conducting an aerial work flight with three crew and no passengers on board; none were injured.
Flight Sequence
After take-off from Sumburgh Airport, the landing gear did not retract correctly, with the 'Unlocked' light and hydraulic pressure 'On' light illuminated. Recycling the gear produced no change, and selecting gear down resulted in three green 'Down and Locked' lights. A flypast of Sumburgh Control Tower revealed that the nose landing gear did not appear extended correctly. The commander decided to return to base at Teesside with the gear extended. A visual inspection confirmed the nose gear was not locked down; the emergency blow-down system had no effect. Fuel was burned down per the checklist for unsafe nose gear indication.
During landing, the nose landing gear collapsed at approximately 70 kt. The aircraft came to rest on its nose, damaging both propellers, engines and the forward fuselage.
Investigation Findings
Examination of the nose landing gear system revealed that the bolt connecting the leg assembly to the drag brace link was missing. The downlock switch location provided a 'gear safe' indication even though the nose leg was no longer geometrically constrained. The loss of geometric locking allowed the leg to pivot backwards during landing. The piston rod of the hydraulic actuator had buckled under high end load from the unrestrained leg.
The head of the missing bolt was recovered at Sumburgh, suggesting it failed earlier and fell out after the previous landing. The remainder with nut and split pin fell out during taxiing before the accident flight.
Laboratory examination found the bolt fractured perpendicular to its axis about halfway along the shank, with fatigue cracking initiating from a single point. No defect was found at the origin, but a corrosion pit was considered likely as the initiation site. The bolt was not plated and showed surface corrosion. Comparison with a bolt from another similar aircraft and an unused bolt showed similar hardness values. Clearance between lugs and the drag brace link would cause bending moments in the bolt, creating fatigue stresses under landing or towing loads.