Event
On 13 December 1997, at approximately 1545 UTC, a Hughes 269 helicopter (registration G-BXMY) was flying at 3,000 feet over the River Severn near Bristol when the pilot heard a loud bang. The helicopter became very unsteady and began to descend. The pilot declared a Mayday, immediately entered autorotation, and turned towards the coast. Realising he could not reach land, he attempted to open the throttle at about 500 feet and found the engine could still produce power, albeit with violent vibration. By raising the collective, the pilot cleared a sea wall and landed in an adjacent field.
After landing, the pilot reported to Bristol ATC. Due to poor radio reception, another helicopter overflew and confirmed the successful forced landing. A police unit attended shortly thereafter.
Investigation
A local licensed engineer examined the helicopter and discovered a hole in the top of the engine cooling air scroll. One of the twelve cooling air impeller attachment bolts was missing, and the remaining bolts were all loose. Evidence indicated that the missing bolt had struck the underside of a lip on the main gearbox.
The helicopter had been collected by the owner/pilot from a maintenance organisation at Chichester/Goodwood airfield earlier that day, following a Star/Annual check and issue of a new Certificate of Airworthiness. During that inspection, cracks were found in the engine cooling impeller, and it was replaced with a used but serviceable impeller.
Maintenance Findings
The maintenance instructions for installing the impeller onto the engine starter ring gear require that it be loosely fastened initially at four equally spaced positions using the appropriate bolts, washers, and nuts. After installing the remaining bolts, every fifth nut in a clockwise progression starting at the 12 o'clock position should be tightened to a torque of between 90 and 100 inch-pounds, followed by a re-check of all torques. The instructions also mandate that a minimum of two full threads on each bolt extend past the nuts, that the nuts have not bottomed-out, and that after 100 hours of operation the torque setting on all nuts be re-checked.
In a conversation with the maintenance organisation that installed the replacement impeller, it was revealed that the nut tightening sequence described above was inadvertently omitted and not detected during the Star/Annual check. Since this accident, the organisation has discussed their maintenance practices with the CAA and taken steps to improve their procedures.