History of the Flight
On 21 January 2007 at 1206 UTC, a Cessna 401 (registration N401JN) was on a private local flight. The aircraft returned to Blackpool Airport for an ILS approach and landing on Runway 28. The wind was from 250° at 24 knots. Prior to landing, the pilot completed the before-landing checklist, verifying three green landing gear down-and-locked lights. The co-pilot cross-checked and confirmed the lights were illuminated. The landing was described as normal. During the rollout, approximately 100 metres short of taxiway Charlie, the right main landing gear collapsed. The right wing, flaps, and right engine propeller sustained damage. After engine shutdown, the pilot and co-pilot exited normally; both were uninjured.
Aircraft Description and Examination
The Cessna 401 features an electrically operated landing gear extension and retraction system. An electrical actuator in the centre of the aircraft operates a bellcrank, which drives push-pull tubes to the left and right main landing gear torque tubes via landing gear door bellcranks. The rod end of the drive push-pull tube fits into an arm assembly of a torque tube, secured by a nut and bolt through arm attachment holes. The torque tube drives a push-pull tube attached to the main landing gear bellcrank, enabling gear retraction and extension. A positive downlock is achieved when the side brace is rigged over centre.
After the accident, the aircraft was recovered and examined. The right main landing gear torque attachment holes were found to have failed, disconnecting the pushrod that provides drive from the electrical actuator. The torque tube was removed and subjected to detailed metallurgic examination. This revealed a slow progressive fatigue failure, followed by a rapid overload. Fatigue crack striations indicated up to 250 cycles before final overload, with each cycle likely corresponding to a landing gear extension/retraction cycle.
Maintenance History and Supplemental Inspection Document
The aircraft, built in 1966, had accumulated 3,450 flight hours at the time of the accident. Log book review showed that 250 landings prior to the accident flight corresponded to approximately May 2001. The aircraft had previously been on the UK register as G-ROAR. The last disturbance of the right main landing gear occurred in 2003, but the torque tube was not removed or disturbed. The last maintenance inspection, a 100-hour inspection on 6 October 2006, did not require inspection of the right main landing gear torque tube.
In August 2004, Cessna issued Supplemental Inspection Document (SID) 32-10-05 for the main landing gear torque tube assembly. It requires removal and non-destructive inspection (NDI) of the arm attachment holes. The initial inspection is due at 10,000 landings or 20 years, with repeats every 2,000 landings or 4 years. For aircraft exceeding the initial threshold, the SID specifies inspection within 400 landings or by 4 August 2005. This SID is mandatory for European commercial air transport aircraft and will become mandatory for all aircraft from September 2008 under European Commission Regulation EC 2042/2003. However, N401JN was US-registered and the SID was not mandatory for it. The inspection had not been carried out, and visual inspection was difficult due to the torque tube's location.
Analysis
The landing gear collapsed due to failure of the arm attachment holes of the right main landing gear torque tube. The failure resulted from an initial fatigue crack followed by rapid overload. The crack had been present since at least May 2001, based on fatigue striations, and had grown over approximately 250 cycles. The fracture likely occurred as the gear was extended for landing. During the rollout, a side load on the right gear caused it to collapse. The initiation cause of the crack could not be determined. The Cessna SID, if applied, would have required removal and NDI, which could have detected the crack. One safety recommendation was made to the FAA to mandate SID 32-10-05 for Cessna 401/402 main landing gear torque tubes.