Summary of the BOI into the crash of XV230 on 2nd September 2006

NB: this is an HTML version of the MoD document Nimrod XV230 - Additional Briefing (PDF). The full (sanitised) BOI report is at Board of Inquiry (BOI) into the loss of NIMROD MR2 XV230

TIME-LINE

NB: all times are Zulu, or GMT. Local Afghan time is Zulu + 4.5 hours.

0913 Nimrod XV230 and its crew departed for a mission over Afghanistan, in support of coalition forces engaging the Taliban. No problems were raised during the �see-off� of the aircraft.

1100 Crew 3 rendezvoused with a Tri-Star tanker and received fuel in an air-to-air refuelling procedure that lasted 10 minutes. Air-to-air refuelling appeared to pass without incident and XV230�s crew prepared to turn east, towards their operational area.

1111:33 A bomb bay fire warning, either coincident with, or followed immediately by, an elevator bay underfloor smoke warning, was reported. The crew also reported smoke entering the aircraft�s cabin, from both elevator and aileron bays.

A �fire-wire� runs around the perimeter of the bomb bay (Fig. 1) and will activate visual and aural warnings in the aircraft cockpit if affected by heat.

Fig. 1

Within the aircraft pressure hull, but under the cabin floor, are 3 under floor bays (Fig 2) each of which holds elements of the aircraft�s hydraulic systems. As there is a fire risk inherent in any hydraulic system, each bay is fitted with a smoke detector and any can be connected to a fire extinguisher. All Nimrod crews practise dealing with smoke or a fire within these bays.

Fig. 2

The air flow under the cabin floor is forced from front to rear by fans. If smoke appears from 2 or more under floor bays, the forward bay is probably the source of the problem. Crew 3 appear to have reached this conclusion, as they began to conduct the drill for smoke in the aileron bay.

1112:26 The aircraft depressurised as the fire breached the aircraft�s pressure hull requiring the crew to don their oxygen masks.

Approximately 1114:00 A number of events occurred:

• A crew member reported a fire �from the rear of the starboard engines�. He was extremely precise and appeared to be indicating that the fire was not actually from the starboard engines.
• Another crew member, using a portable oxygen mask as part of the under floor drill, reported a fire within the aileron bay.
• The captain, who had already begun to turn the aircraft towards Kandahar Airfield, declared a MAYDAY and began a descent. The position and the rate of descent were correct for an approach to Kandahar.

1115:43 The aircraft�s accident data recorder and mission tape abruptly ceased recording data; probably because the fire had disrupted their power supplies.

1116:34 Marked the final transmission from the crew, who acknowledged the Kandahar Airfield weather for landing; the Board of Inquiry therefore concluded from this that at this time the crew clearly still believed that they were in control of their aircraft and that they would reach Kandahar Airfield. 1116:54 XV230 was observed, in what appeared to be a controlled descent, by a Harrier GR7 pilot, who was several thousand feet above. He reported an intense fire close to the starboard fuselage and stretching out between where the wing joins the aircraft fuselage to the starboard engine; there was a second fire that stretched behind the aircraft from a point on the side of the rear body of the aircraft. Very shortly after the Harrier pilot�s sighting, members of a Canadian army unit saw the aircraft as it passed their position and described a fire very similar to the Harrier pilot, but on the port side of the aircraft. Detailed analysis and expert consultation determined that the Canadians� range and viewing angle probably led them to misinterpret what was by now a fairly large fire on the Nimrod�s starboard side as coming from the side nearest to them.

1117:39 The Harrier pilot reported that he saw the Nimrod explode. Analysis of photographs of the crash site by the Air Accident Investigation Board indicated that XV230 had broken into 4 sections, at a height of between 750 feet and 1000 feet. The crash was not survivable. Action Immediately Following the Crash: A Combat Search and Rescue team deployed to the site and established that there were no survivors. The Canadian unit which had observed the crash reached the scene and secured the area; they were subsequently reinforced by members of 34 Squadron RAF Regiment based at Kandahar. The priorities for the 2 units were to recover the crew�s bodies, their personnel effects and any classified material. The following morning the Canadian unit was withdrawn to support other Coalition forces engaged in fighting with the Taliban, at which point several hundred locals began to enter the site. The security situation began to deteriorate rapidly and at 0910 the RAF Regiment patrol was withdrawn by air, some 21 hours after the initial arrival of ground forces. The security situation prevented any return to the crash site. The majority of the wreckage was removed within a short period of time, probably by local nationals.

A Board of Inquiry (BOI) was convened to investigate the crash with an experienced ex-Nimrod Wing Commander nominated as its President, supported by two Squadron Leaders from the Nimrod Force Headquarters at RAF Kinloss. The Board of Inquiry reached Kandahar the following day, but in view of the security situation was unable to deploy to the crash site.

RECONSTRUCTION OF EVENTS LEADING TO THE CRASH OF NIMROD XV230 ON 02 SEPTEMBER 2006

From its investigation the Board was now able to reconstruct what it believed probably happened to XV230 on the day of the crash. As air-to-air refuelling drew to a close, fuel escaped from one of two possible sources: the action of a pressure-relief device in the main fuel tank, which led to an overflow of fuel during Air to Air Refuelling, or a leaking fuel coupling. The escaped fuel entered the No 7 tank dry bay, accumulated on its lower panel and contacted the Supplementary Conditioning Pack (SCP) pipe, after entering a gap between 2 types of insulation. The fuel ignited in under a minute. Combustion products from the fire entered the bomb bay and triggered its fire wire. The fire also heated the aileron bay causing smoke which triggered the elevator bay alarm. After 50 seconds the fire under the aileron bay burnt through into the pressure cabin and the aircraft depressurised. The rush of escaping cabin air strengthened the fire, increasing the rate at which it burnt through the aircraft�s wing panels. After 5 minutes the fuel within No 7 tank (the small fuel tank in the wing fillet) began to boil; the fuel remained in liquid form at much higher temperatures than normal, as it was contained by the tank walls. Eventually the No 7 tank began to breach and fuel under pressure escaped; this fuel was ignited by the fire and was probably the second fire observed by the Harrier pilot. The fire would now probably have begun to damage the aircraft�s hydraulic systems and flight control system. The extent of the damage will never be precisely known, but the aircraft�s flight path in its final moments suggests that some damage had certainly occurred. Very shortly before the crash the No 7 tank suffered significant disruption, and the highly pressurised fuel within it expanded rapidly and ignited, in a phenomenon known as a Boiling Liquid, Expanding Vapour Explosion or BLEVE; this was probably the explosion observed by the Harrier pilot and the Canadians. Whether the BLEVE was provoked by the aircraft breaking up, or the BLEVE provoked the break-up is impossible to determine.

ACTIONS TAKEN TO REDUCE RISK TO NIMROD OPERATIONS

Following the loss of XV230 a number of measures were taken immediately by the Royal Air Force to reduce the risk to crews continuing to fly the Nimrod. These included:

• Turning off all possible ignition sources wherever possible.
• Increasing inspections of the area to detect any faults at as early a stage as possible; during this process no evidence of any fleet-wide problems was identified.
• Immediately after the crash, a fleet wide examination was conducted of areas where flammable materials such as fuel or hydraulic fluid may be placed in relative proximity to potential sources of ignition should a leak occur. These examinations include visual inspections of the inboard wing and bomb bay areas, and pressurised functional checks of the aircraft�s fuel system.

Shortly after:

• When the Board determined the most likely point of ignition it confirmed that the aircraft�s hot air system should remain off in the air.
• The Board worked with the Nimrod operating authorities to amend procedures to ensure that blow-off should not occur.

OVERVIEW OF THE BOARD�S FINDINGS

The Board noted that Crew 3 was faced with a series of complex and demanding emergencies. Throughout these events they acted with calm professionalism, conducting well-practised drills in an attempt to save their aircraft. The loss of 14 courageous Servicemen from all 3 Services is a tragedy, for the families and for their comrades throughout the United Kingdom. The Ministry of Defence takes seriously the duty of care it owes to its Servicemen and women, and will consider the lessons to be learned from the sad loss of this aircraft and its crew.

The Board made 33 recommendations as a result of its investigations. The main aim of these recommendations is to ensure safety measures are formally incorporated within Nimrod procedures and that all possible means to continue to ensure the safety of the Nimrod fleet and its crews are taken. These recommendations are being tracked to ensure that there is a clearly visible audit trail to highlight the means by which the Board�s aim is fulfilled.

As the Board was unable to inspect the wreckage of XV230 it used the evidence available, together with their aircraft system knowledge and logical analysis, to determine the most likely seat of the fire � the place where fuel had met a point of ignition.

The bomb-bay could not have been the seat of the fire: The Board ruled out the possibility that the fire on XV230 was initiated in the bomb bay. The equipment stored in the bomb bay appeared to be undamaged, militating against the possibility of a fire in the forward bomb bay, whilst the photographic evidence analysed by the Board showed no evidence that the rear bulkhead door had been subjected to a sustained fire.

The engines were not the seat of the fire: The closest engine to the aircraft�s fire was the No 3 engine. However, the aircraft�s accident data recorder indicated nothing amiss with any of the aircraft�s engines and no engine fire warnings were reported by the crew, making it most unlikely that any of the engines was the seat of the fire.

The seat of the fire was between the No3 engine and the fuselage: The Board determined that the seat of the fire must have been between the No 3 engine and the aircraft fuselage. The following areas were considered:

The Board believed that the No 7 tank dry bay was the most likely location for the seat of the fire.

No 7 dry tank bay relationship to bomb bay and other bays - Triggering of alarms: There is an opening between the No 7 tank dry bay and the bomb bay itself; the bomb bay�s fire wire routes around this opening and thus the hot products of a fire in the No 7 tank dry bay would naturally expand into this area and trigger the fire wire. Any fire in the dry bay would heat the adjacent aileron bay; all under floor bays contain some residual hydraulic fluid and this would have begun to boil and emit smoke, explaining the possibly simultaneous triggering of the bomb bay and under floor alarms.

Most Likely Ignition Point - A Gap Between 2 Types of Insulation Potentially Allowing Fuel to Contact the Bare Metal at Approximately 400 Degrees Celsius: A pipe leads from the main hot air pipe and exits the aircraft fuselage at the rear floor of the No 7 tank dry bay to feed the Supplementary Conditioning Pack, or SCP. On a number of Nimrods examined by the Board there was a gap between 2 types of insulation, which could potentially allow fuel to contact the bare metal at approximately 400 degrees Celsius. The insulation could have then held the fuel against the pipe enabling auto ignition. Fuel leaking into the dry bay would naturally gravitate to this pipe and its vulnerable insulation gap. The Board determined that this was the most likely ignition point.

Likely Sources of Fuel Leak: There are 2 likely means by which fuel could have reached the probable point of ignition. The Board decided that it did not have the evidence to determine which of the 2 means was the most likely and that they were equally possible. The 2 likely means considered were:

SUMMARY OF BOI RECOMMENDATIONS

POSSIBLE CONTRIBUTING FACTORS TO THE LOSS OF NIMROD XV3230

The Board of Inquiry identified a number of factors which possibly contributed to the loss of XV 230 as follows:

Age: While the Board considered that the Ministry of Defence had done all that was required of it to cater for the increasing age of the Nimrod, the Board was unable to discount the age of the seals as a possible cause of the increase in leaks from fuel couplings and of the condition of the hot air pipes� insulation. The Board acknowledged, however, that other factors might have been responsible for their condition.

Maintenance Policy. The Board also considered that maintenance policy in relation to the Nimrod�s fuel system was a possible contributory factor, as it did not detect a gradual increase in fuel leaks over time, nor did it prevent that increase. Maintenance policy for the aircraft�s hot air system was seen as a contributory factor, due to a lack of guidance on the allowable condition for hot air pipe insulation.

Failure of Hazard Analysis and Lack of a Fire Detection and Suppression System. The Nimrod Safety Case identified the hazards associated with the No.7 Tank Dry Bay but incorrectly quantified the SCP Duct hazard. Consequently, the risk was underestimated. Had the risk been correctly estimated it is almost certain that a range of risk-reduction options which would have reduced the likelihood of fire would have been considered and these may have included a redesign to fit fire suppression in the dry bay � although it is more likely that action to mitigate the risk by removing the potential ignition source would have been taken.

Not Identifying the Full Implications of Successive Changes to the Fuel System and Associated Procedures. The Board also felt that the formal incorporation of an air-to-air refuelling capability on the Nimrod in 1989 did not identify the full implications of successive changes to the fuel system and associated procedures.