What happened
On 4 November 2004, a Cessna 208B Caravan, registration G-BZAH, was being taxied from its dispersal to a holding point at Netheravon Airfield, Wiltshire. During this movement, the pilot heard two loud bangs originating from the nose wheel area. Upon contacting air traffic control, the pilot was informed that a panel appeared loose, prompting an immediate return to the dispersal and engine shutdown. There were no injuries to the two occupants on board.
The investigation
An investigation by the AAIB established that the rear support of the nose landing gear spring had detached from its fuselage mounting point. This separation dislodged a composite fairing located beneath the spring. Engineering examinations revealed that one of the four attachment bolts had failed, and the remaining three bolts had been pulled from their self-longing anchor nuts.
Investigators found evidence of long-term fretting and elongation of the bolt holes in the fuselage structure. The direction of the elongation indicated that the nose landing gear had moved forward relative to the aircraft's axis. The investigation also noted that the aircraft was being moved using a powered tug, and that the tug's attachment method lacked a safety 'weak link' strap.
Findings
- The primary cause of the failure was bending fatigue in one of the attachment bolts.
- The failure of the first bolt increased the load on the remaining three bolts, causing their threads to strip from the anchor nuts.
- Evidence of fretting suggested the rear support had been loose for some time, potentially causing the forward spring support bushing to migrate.
- The investigation identified that sudden-start or 'snatch' movements by tug operators could apply significant forward loads to the nose landing gear rear support.
Safety action
- The AAIB made several safety recommendations regarding the maintenance and towing of Cessna 208 Caravan aircraft, including the need for the manufacturer to establish and publish maximum towing load limits.
- A recommendation was made to the UK Civil Aviation Authority to ensure that operators using powered tugs define and implement procedures to prevent excessive loading during ground handling.