On 16 May 1995, the No 4 engine of a BAe Nimrod R1 (XW666) being operated out of RAF Kinloss in day VMC by the UK Royal Air Force on an airworthiness function flight caught fire about 45 nm north east of Kinloss and the fire then spread very rapidly. An emergency diversion to RAF Lossiemouth was initiated but when the fire appeared likely to lead to a loss of structural integrity, the aircraft was successfully ditched some 4.5 nm north east of the originally-envisaged runway. Crewmen were able to deploy a dinghy which all seven occupants then boarded before being picked up by helicopter. Three of the occupants sustained minor injuries. The aircraft subsequently sank.
An Investigation was conducted by an RAF Board of Inquiry. The full Report of the Inquiry was not placed in the public domain but a 'Military Aircraft Accident Summary' was and this, together with some additional information contained in an edited transcript of an interview given by the aircraft Captain to the RAF Leadership Centre in 2004 on his decision making in response to the fire which was subsequently published, have been used to summarise the Findings.
It was noted that the aircraft was on an airworthiness function flight following a period of six months 'major servicing' at the Nimrod Major Servicing Unit at RAF Kinloss by RAF personnel. The flight took place over a calm sea in "excellent weather conditions". Some 35 minutes into the flight tracking seaward at 15000 feet about 45 nm from Kinloss, a scheduled test of the aircraft anti-icing system was followed by annunciation of a No 4 engine fire warning. An emergency diversion to the nearest useable runway at RAF Lossiemouth was commenced but whilst the drill for the extant fire warning was still in progress, a No 3 engine fire warning was also annunciated. Neither response procedure had any effect on the visually evident wing fire. Two explosions were heard and one of the rear crewmen advised the Captain that "the aircraft was on fire and that panels were falling away from the right wing". Although in just a few minutes, the aircraft had recovered to a position only a few miles from Lossiemouth, the Captain decided, on the basis that structural failure as a result of the effects of the fire may be imminent, to ditch the aircraft whilst he was still able to control it. A controlled flapless ditching (a fire-related hydraulic system failure had disabled them) was achieved with two bounces before the aircraft settled on the surface. All occupants took to a dinghy and were subsequently rescued by helicopter. Six minutes had elapsed from awareness of the fire to the ditching. Soon afterwards the fuselage, which had broken in two, sank.
A "substantial proportion" of the wreckage was recovered from the sea bed to assist the Investigation. It was found that when the anti-icing system had been selected on, a short circuit in the DC electric loom attached to the No 4 engine, which was for undetermined reasons mechanically damaged, had led to the powering up of the air cross-start system for the engine. Because this engine was already intentionally at Idle as part of the test schedule, the unloaded starter turbine ran almost instantly up to full speed. The nut holding the turbine disc in place then failed, allowing the disc to move rearwards on its shaft and exit the protective casing before striking and puncturing both the engine bypass casing and the no 2 fuel tank. Damage to the latter led to a "massive" fuel leak which was subsequently ignited by either the arcing within the faulty loom or by the heat of the adjacent engine - note that on this aircraft type, the engines were mounted within the wing structure. Thereafter, the fire spread rapidly in an area not covered by the engine fire extinguishing systems.
Safety Recommendations were made in respect of:
- improving maintenance procedures for electrical looms
- the need to isolate electrical circuits in the vicinity of the engines
- replacement of the nuts holding the starter turbine in place with ones of higher quality.
The UK Ministry of Defence Military Aircraft Accident Summary on which this summary article is mainly based was published in May 1996.