What happened
On 13 December 1997, a Hughes 269 helicopter, registration G-BXMY, was conducting a private flight over the River Severn near Bristol. While cruising at approximately 3,000 feet, the pilot heard a loud bang, after which the aircraft became unstable and began a descent. The pilot declared a Mayday and initiated an autorotation, heading toward the coast.
As the aircraft descended to roughly 500 feet, the pilot attempted to apply power. Although the engine remained capable of producing power, it was accompanied by intense vibrations. By adjusting the collective control, the pilot managed to clear a sea wall and perform a successful forced landing in a field between Clevedont and Portishead. There were no injuries to the pilot. Due to poor radio reception, another helicopter was requested to overfly the site to confirm the safety of the crew.
The investigation
Following the incident, a licensed engineer inspected the aircraft and identified a hole in the engine cooling air scroll. The investigation established that one of the twelve bolts used to attach the cooling air impeller to the engine starter ring gear was missing. This bolt had struck the underside of a lip on the main gearbox. Additionally, the remaining attachment bolts were found to be loose.
Maintenance records showed that the helicopter had recently undergone a Star/Annual check at a maintenance organisation in Chichester/Goodwood. During this check, the engine cooling impeller had been replaced with a used, serviceable unit after cracks were found in the previous component. The investigation revealed that the specific nut-tightening sequence required by the maintenance instructions had been omitted during the installation of the new impeller.