What happened
On 7 October 2000, a Cessna U2/06A Super Skywagon, registration G-BGWR, was performing a parachute dropping operation near Whitchurch (Tilstock) Airfield in Shropshire. Following a routine touchdown on Runway 33R, the pilot reduced the throttle to idle and began retracting the flaps while applying brakes. During this process, the pilot noted unusual stiffness in the brake pedals and observed the nose of the aircraft beginning to sink.
Before the pilot could shut down the engine, the propeller blades struck the runway surface. The nose landing gear subsequently jammed as the aircraft settled, causing the plane to skid to a halt to the right of the runway centerline. There were no injuries to the pilot, though the aircraft sustained damage to the engine, propeller, and nose landing gear.
The investigation
The investigation focused on the structural integrity of the nose landing gear. Examination of the aircraft revealed that a 3/16 inch diameter bolt, which secures the nose landing gear drag brace to the fuselage, had failed. While this bolt is designed to operate in double shear, the failure occurred in the plane of the drag brace and fork fitting interface, allowing the gear leg to pivot forward.
A metallurgical analysis of the failed bolt by DERA showed that fatigue cracks had grown through the majority of the bolt's shank. The examination identified multiple crack origins and evidence of corrosion pitting along the shank. The presence of both corroded and uncorroded regions suggested that the cracks had been developing over a significant period and that the bolt may have been reinstalled in a rotated position after being previously removed.
Findings
- The primary cause of the incident was the failure of the nose landing gear drag brace bolt due to fatigue crack growth.
- The fatigue cracks likely originated from corrosion pits on the bolt shank.
- The failure was driven by plain bending loads rather than the intended double shear loading.
- These bending loads may have been caused by clearance gaps within the fitting, potentially exacerbated by wear or high-load events.
- The material strength of the bolt was found to be within the required specifications.
Safety action
- It is recommended that the Federal Aviation Administration and The Cessna Aircraft Company implement an inspection or lifing policy for the nose landing gear drag brace to fuselage fitting bolt on Cessna 206 and 207 aircraft.