Afghanistan 2nd September 2006 The Haddon-Cave Review of the terrible accident to XV230 mistakenly identified the source of the fire which almost certainly resulted in premature retirement of the Nimrod and the cancellation of the MRA4. Below is a note amplifying the details in the book.
Comment on Haddon-Cave Review (amended 14/12/14)
This note amplifies the comments in my book, Nimrod Rise and Fall, where I described the terrible accident on 2nd September 2006 of Nimrod XV230 in Afghanistan and is amended after hearing Sir Charles Haddon-Cave’s excellent Beaumont Lecture at the Royal Aeronautical Society on 19th November 2014. As mentioned in the book, the findings of the Haddon-Cave review in relation to the source of the ignition of the fire was incorrectly identified. This mistake undoubtedly affected the findings of the Review and almost certainly caused the premature end of the Nimrod programme and the cancellation of the MRA4. Sir Charles Haddon-Cave in his lecture said that the first warnings the crew had had were two fire warnings but clearly he and his advisors chose to ignore the warning of the Secondary Cooling Pack going off line over two minutes earlier because it suited the location for the source of the fire previously chosen by the RAF Board of Enquiry as discussed below. When writing the accident chapter in my Nimrod book I naturally looked at the accident reports on XV230, one by the RAF Board of Inquiry and one by the Haddon-Cave Review. These inquiries were extensive, exhaustive and extremely well researched and it was difficult not to feel overwhelmed by the amount of data. However, the more I examined the Haddon-Cave Review the more puzzled I became as the conclusions reached simply didn’t match the likely source of the initial fire nor did they agree with what actually happened. The Review in reality merely rubber stamped the RAF Board of Inquiry notwithstanding that it discussed more evidence which suggested a different source, see below. Luckily I had two retired BAe Nimrod experts to help me in my research and part of one of their analyses is attached to the end of this note. There is no way that this note can discuss any real detail but I intend to give the salient points which show that the Haddon-Cave Review got the fire source incorrect . The aircraft was being refuelled by a Tristar and the object was to refuel the aircraft to full tanks. The Haddon-Cave Review judged, almost certainly correctly, that during the six minute refuelling the blow-off valve in no 1 tank operated and fuel at 160 gallons per minute came out of the valve. Clearly the vital consideration at that point for the investigators should have been to determine where this fuel went but very surprisingly the Review made no attempt to carry out what would have been some very simple flight tests and, as mentioned, this omission in my opinion resulted in the premature demise of the Nimrod. The Review merely assumed that the fuel would somehow migrate back into the airframe through the skin of the aircraft in significant quantities, find its way onto the floor of the No 7 Tank dry bay, which incidentally had drain holes, and then find an unlagged pipe which would ignite the fuel as postulated by the earlier Board of Inquiry. However, what is so difficult to understand is that the Review and the Board of Inquiry ignored the fact that just aft of the bomb bay rear bulkhead and just 4 metres behind the No 1 tank blow-off valve, was a NACA intake taking in ram air to the Secondary Cooling Pack, SCP, intercooler which would have been at a temperature above 450°C; any fuel going into this inlet would immediately have been set alight, probably explosively.
This lack of realisation of the danger of fuel going into the NACA intake is particularly hard to understand since the Review itself emphasised a BAe 1985 report commenting on some Nimrod AEW refuelling trials which made it quite clear that there was a potential hazard on all marks of Nimrod while flight refuelling when the tanks were nearly full due to the operation of the blow-off valves. ‘. [There may be some cause for concern with regard to the wetted surfaces caused by a discharge of fuel and it is suggested that tests are made in flight using a coloured dye to study the behaviour of liquid in relation to the various ports and intakes, particularly the tail-pack pre-cooler in the bottom of the rear fairing. If the liquid is found to enter this intake, it may be necessary either to switch off the tail pack (SCP) before commencing an air-to-air refuel, or to carry out a modification on the blow-off valve outlets of Tanks 1 and 6 to prevent the fuel running down the skin.’ The significance of this report, which was sent to Resident Technical Officer as well as having an internal distribution, was not acted on by BAe despite its important flight safety implications. MoD in 1989 when carrying out the flight refuelling production modification 715, decided that no more test flying was necessary but unfortunately it is not possible to discover twenty five years later whether BAe tried to get the flight trials they recommended actually carried out. However, as already stated, the Review having focussed on this vital report, for some inexplicable reason and most unfortunately completely failed to understand its significance as the source for the fire; it is probably relevant to point out that no BAe experts were consulted in the formulation of the findings of the Review. The Review points out that Mod 715 increased substantially the instantaneous flow rate of fuel into the No. 1 tank during AAR (6.13) and discusses ground tests with bowsers, (6.14), but there is no evidence that these tests were carried out at the correct nose-up attitude of the aircraft simulating high weight. Incredibly nobody seems to have thought of actually conducting flights tests with coloured dye as BAe recommended. It can be seen that large amounts of fuel coming out of the No 1 tank blow-off valve would run down the outside of the bomb bay skin and also, because of negative pressure into the bomb bay through the gap in the side doors, straight into the NACA intake for the cooling air. Significantly, just over a year after the XV230 accident on 5th November 2007 and before the Review was finalised, Nimrod XV235 made an emergency landing due to fuel in the bomb bay after flight refuelling but the Review looked for a leak and didn’t appreciate that the fuelvcould almost certainly had come from the same place as XV230, namely the No 1 tank blow-off valve; no fire occurred on this occasion because by this time instructions had been given to switch off the SCP while refuelling as recommended by BAe in 1985. As a result of this incident air to air refuelling on the Nimrod was stopped and again no airborne tests were carried out to identify the problem(6.18). Considering the actual sequence of events which resulted in the disaster, the first significant event was the SCP tripping off line, almost certainly because of the fire at the intercooler; as mentioned this warning was discounted by the investigators because occasionally the SCP came off line for other reasons and it did not suit their preferred source of the fire. The moment that happened the hot air ducting, suspected by the Review for starting the fire, would have cooled down due to the automatic closure of the engine air feed cocks. The bomb bay warning came on 2 minutes and 10 seconds after the SCP closed down and the likely sequence of events was that the fire in the intercooler spread heat forward until it reached the rear bulkhead of the bomb bay and triggered the warning wire. It is important to notice that there was absolutely no evidence from photographs of heating in the bomb area as would have been the case if the fire had occurred in the No 7 dry tank bay as assumed by the Haddon-Cave Review. Incidentally Station 4, where the AV287 air sea rescue carrier was mounted in the bomb bay, was adjacent to the communicating gap with the Tank No7 dry bay and showed no signs of heat. Surely if there had been a raging fire on the floor of the dry bay, it would at least have scorched the material of the equipment mounted on the carrier? When the refuelling ceased the fire would have been kept going by the secondary drainage of residual fuel coming out of the bomb bay and going straight into the NACA intake. As the fire continued damage would have occurred immediately above the intercooler affecting the integrity of the connections joining the three cells which made No 6 tank; this is explained in detail in the attached paper explaining the sequence of events leading to the disaster. Nimrods had refuelled safely for many years without any problems but of course the Victor K2 and the VC10 were being used with a relatively low refuel rate. The accident occurred using the Tristar having a significantly higher refuel rate than the Victor. The fact that the aircraft was being refuelled to full tanks at maximum weigh exacerbated the situation due to the attitude of the aircraft; in this condition it was impossible to fill No 1 tank, which consisted of four cells, as the blow off valve on No 3 cell would operate before the other cells were full and there was no way that the Flight Engineer could tell when the blow-off occurred. Interestingly there was discussion of low contents shortly after the SCP warning suggesting that the crew had discovered that fuel had gone somewhere other than in the tanks; furthermore the Flight Engineer used the words ‘we’re full’ 27 seconds after the SCP went off line. Summing up, the accident was not due to inherently bad design of the Nimrod which had performed splendidly for forty years but due to the installation of the Air to Air refuelling modification without adequate flight testing, particularly at high fuel flow rates, at high aircraft AUW and using the TriStar as a tanker with its higher refuelling rate. Fuel from No 1 tank blow off valve went straight into the NACA intake for the SCP intercooler and the sad thing about the accident is that had the Secondary Cooling Pack been switched off, as suggested by BAe, the accident would not have occurred. The Review spent a lot of time highlighting the problems that had been found with fuel leaks and the unsatisfactory way the basic problem had been investigated with no fundamental recommendation for design improvement; it was on this point that the Review developed the need for the MAA to be introduced into the RAF. They postulated that on XV230 there was a sufficient fuel leak which not only found a bare hot duct but also to be set alight. One is left to speculate why the accident investigators did not carry out tests to prove where the fire actually occurred. Certainly the Review by Haddon-Cave would have to have been significantly different if the correct ignition source had been identified. Unfortunately as mentioned, the Review as written caused premature retirement of the Nimrod MR2 and cancellation of the MRA4 leaving the country without any long distance search and rescues capability and unable to track submarines, friendly or hostile. Tragically nothing can be reversed but it is felt important to understand all the issues, set the record straight and perhaps examine the whole of the Haddon-Cave Review more critically. As mentioned a fuller explanation of the actual sequence of events is attached below showing how the observed happenings in the Review are only consistent with the fire occurring in the SCP intercooler. However it can be more easily understood if read in conjunction with the full Review.
Events leading to the loss of aircraft Nimrod XV230 Evidence All the known evidence has been assembled in the BOI; evidence indicating the seat of the fire Part 2 para 21 copied from the BOI is as follow:-
* Mission tape at 1109:23. A remark from the air engineer as the Supplementary Conditioning Pack (SCP) trips off, causing a pressure change within the cabin. (para l. )
* Some discussion at the flight deck about low fuel contents in a tank,- which tank is not known.
a. Mission tape at 1111:33 hrs: report of a bomb bay fire warning, either coincident with or closely followed by an elevator bay warning.
b. Mission tape at 1111:33 hrs- 1112.26 hrs: an interlinked and at times, overlapping series of reports from the crew of smoke entering the cabin from the elevator and aileron bays.
c. Mission tape at 1112:26 hrs : a report that the aircraft had depressurised.
d. Mission tape at 1113:45 hrs: a report from the operator of flames coming from the rear of the engines on the starboard wing.
* Accident Data Recorder either stopped recording or ceased to function.
e. Report by Harrier GR7 pilot at 1116.54 hrs of XV230 descending with flames originating frm the starboard wing root and starboard fuselage.
f. Testimony from 3 members of A Sqn RCD who observed approximately the final 40 seconds of XV230’s flight. The Canadians reported a fire in many respects similar to that of the GR7 pilot, but on the port side of the aircraft.
g. Photograph of the AV287 carrier used to hold the Apparatus Sea Rescue (ASR) in the bomb bay, showing no smoke or flame damage. The ASR is a combination of a single inflatable dinghy and “ survival equipment packs, fitted to Station 4, at the mid-position of the bomb bay.
h. Photograph of 3 x No 4 MK1 fusing units, recovered from the crash site, which had been fitted to the ASR’s AV 287 carrier. None of these items display smoke or fire damage.
i. Photograph of the starboard rear bulkhead of the bomb bay showing probable scorching to the top third, but no warping due to heat.
j. Photograph of the starboard tailplane with paint discolouration caused by heat and flame.
k. Description of rear hinged fairing with no fire damage, although evidence of molten metal having dropped onto it.
l. See above.
m. Photographs taken of the interior of the aircraft tail section following the crash and smoke damage to the recovered sonar location beacon show clear evidence of internal burning in the compartments aft of the pressure shell. However the fire is of short duration and was probably ignited as a consequence of the principal fire further forward in the aircraft. It was not the initial scene of combustion.
Discussion
Reviewing the above , for reasons that will become apparent later, the only source for the fire that satisfies all the evidence is:-
The under floor in the starboard rear fuselage between the aileron and elevator bays.
The only necessary condition for this site to achieve compliance with all the evidence is:-
* the main cabin remains essentially intact during the incident.
The site has the potential for:-
* a significant source of supply of fuel
* A good flow of air for oxidation of the fire.
* a source of high heat load and spontaneous ignition
* a means of distributing the fire into the regions identified by observation of both the GR7 pilot and the Canadians.
* primary structure that if so damaged by fire can cause a catastrophic structural failure of the airframe in the manner believed to have occurred in the final stages of the flight.
There are 2 possibilities of how the incident may have started
1. A failure during the re-fuelling sequence in the filling of Tank 6 resulting in the spillage of fuel into the tank inter-space, and/or into the lower keel. At 1109:53 on the mission tape there was some discussion at the flight deck about low fuel contents in a tank- which tank is unknown.
There is no evidence to suggest this remark refers to Tank 6, notwithstanding however that it is possible to speculate how this leaked fuel can cause a cabin fire in accord with the evidence, this event will not be discussed further, in preference to 2 below.
2. There was a sequential series of events consequential of a blow off from the over filling of Tank1 during the re-fuelling.
The event of blow off from Tank1 has been given a great deal of thought by the BOI and the Review Board. XV230 was taking on fuel from a Tristar tanker re-fuelling essentially into the centre body tankage. It is believed that the Release to Service trials undertaken by the Establishments at high weight when the aircraft was air to air refuelled to maximum fuel capacity, was achieved at ’trickle charge’ fuelling rates. Boscombe Down had responsibility for Nimrod refuelling against the Tristar, so the release documentation from A&AEE needs to be consulted to see whether high flow rates at high fuel load were cleared by them. The peace time clearance of AAR was inadequate to cover the operation of the system at high flow rates at high AUW, as undertaken by the RAF in the Afghanistan incident.
Normally it is intended that Contractors Trials and Release to Service Trials are carried out by Test Pilots and Experienced Service Pilots close to the Design limits of the aircraft design envelope, and the C of A Release will be published with limit of operation by the RAF less than that accomplished by the test trials. This clearly has not been achieved in the clearances given for AAR. To put it rather crudely, the Release trials filled the aircraft with fuel, the RAF ’pumped up’ the aircraft to maximum fuel load.
Analysis
The BOI established a ‘timeline’ to illustrate how the known events occurred during the final phase of the flight up to when the aircraft was lost; we wish to copy this analogy to illustrate how we believe the accident happened.
The timeline is extracted from the BOI -- times are GMT -- Local time is plus 4.5 hrs.
09:13:00 XV230 takes off from operating base.
11:0:0 Rendezvous with Tristar tanker.
11:03:53 Connection made and fuel transfer from Tristar tanker commenced.
11:09:23 SCP tripped off line.
This would be observed by the crew feeling the cabin pressure pulse caused by the reduction of airflow into the cabin, even before the Flight Engineer could monitor the SCP control panel. We do not know why the pack shut down, whether it be the pre-cooler or the main pack?
A most probable reason would be overheat; caused by starvation of ram air into the pre cooler or main pack heat exchangers due to turbulent airflow in the wake of the Tristar ( unlikely since XV230 had been in the wake for at least 6 mins, and such an event would have occurred sooner), or the pre cooler was unable to cool itself due to burning fuel in the ram air side matrix.
At the time when the SCP shut down, the Nimrod was importing fuel from the Tristar from around 5.5mins coincident with the estimated time by the BOI modelling when the Tank 1 blow off could occur. As described later in the Review, the amount of fuel released and spilling overboard from a blow off was a significant quantity. It has been calculated to be approximately 120 litres every 10 seconds ( 160 gallons per minute). Blow off fuel would track the boundary layer of the pannier and starboard bomb door downwards due to gravity and aft wards in the airflow to flow into the collection area of the large NACA intake. There is numerous evidence that the gapping at the bomb door hinge line does collect fuel tracking back from the blow off. It is recalled that there was a negative internal suction field relative to ambient inside the rear weapons bay; this will assist fuel entering the bay. Fuel will collect in the heel of the upper door longeron, then overflow down the inside contours of the starboard door, and collect at door centreline. The doors are well sealed at centreline to reduce momentum drag, but unlikely to retain fuel. There will be a spillage of fuel for so long as the blow off occurs, plus a more intermittent feed from the secondary drain from inside the weapons bay.
With the SCP operating at high main engine power settings chasing the tanker, the bleed air temperature into the pre cooler and the metal fins of the matrix are around 450/470 deg .C. The NACA intake is feeding air into the pre cooler around 200/300 lb per minute. A fuel/air mixture on to the matrix would be quite dramatic, probably explosive in nature. As long as the bleed air flow is shut off,(by the pre-cooler overheat switch) the fire with the characteristic of a blowlamp will continue to burn due to residual heat in the cooler matrix,for as long as the stream of fuel is maintained, a very hot flame forced fed with oxygen from the ram air. The explosive effect may well cause damage to the light gauge pre cooler ducting, the pre cooler exhaust fire can then impinge directly on the lower keel of the rear fuselage pressure shell with the possibility that flames will lap outside the upper pannier skirt panels.
Post this event when the SCP tripped off line, there is a shutdown sequence when the SCP is shut down by both sets of PRV/SOV, being de-energised to shut, thus closing off the HP bleed air to the pre-cooler. In addition we believe that the Cross Feed Valves in the port and starboard engine bays are also selected to close, thus all HP air supply through the cross feed ducting are shut off.
The nett effect is that 5.5 minutes into the re-fuelling sequence, the SCP ducting and the Cross feed begins to cool down. There will be a finite time before the temperature of the ducting cools below the spontaneous combustion temperature for aviation fuel, but because there is no hot air flows maintaining the pipe temperature, depending upon the quantity of cold fuel making contact with the thin wall ducting might well quench the heat in the duct making combustion impossible.
The pre-cooler is unaffected by the above, the fire in the pre-cooler would be sufficient the maintain the matrix at high combustion temperatures.
Evidence g and h. Photograph of AV287 carrier used to hold the Apparatus Sea Rescue in the bomb bay showing no smoke or flame damage. The ASR is a combination of a single inflatable dinghy and survival equipment packs fitted to Station 4 at the mid position of the bomb bay .
Photograph of 3x No 4 Mk1 fusing units, recovered from the crash site, which had been fitted to the ASR’s AV287 carrier. None of these display smoke or fire damage.
We would not expect the fire in the aft rear pannier bay to show its effect in the forward bomb bay. The two bays are partitioned by the rear bomb bay bulkhead and the bulkheads on the hinged doors, and the fire would not encroach forward into the bomb bay.
Incidentally, Station 4 where AV287 was mounted in the bomb bay was adjacent to the communicating gap with the Tank No7 dry bay. Surely if there had been a raging fire on the floor of the dry bay, it would at least have scorched the material of the equipment mounted on the carrier
Evidence I Photograph of the starboard rear bulkhead of the bomb bay showing probable scorching to the top third, but no warping due to heat.
This is consistent with a fire in the upper regions of the hinged fairing bay, which is aft of the bomb bay rear bulkhead ie the position of the pre-cooler.
Evidence k Description of rear hinged fairing with no fire damage, although evidence of molten metal having been dropped onto it.
Consistent with a high temperature fire in the upper regions of the bay ie at the position of the pre-cooler close to the fuselage keel.. This is significantly important evidence of a hot fire in the area above the hinged fairing ie from the pre cooler, or the lower keel structure of the fuselage above the pre-cooler.
This is the only evidence reported by the post accident investigation of the crash remains of a site where clearly a high temperature fire had occurred. The pre-cooler installation is manufactured from thin gauge stainless steel; an investigation is necessary to determine whether al alloy component parts are a part of the installation, or whether the evidence of molten metal dropped onto the hinge fairing could be the melting of the fuselage pressure skin.
11:09:33 Some discussion at the flight deck about low fuel contents in a tank--- which tank it unknown.
11:09:53 Refuelling from Tristar ended and connection terminated after 22000lb of fuel transferred.
11:11:33 The crew were alerted that something was amiss by two almost simultaneous warnings: a fire warning in the bomb bay and a smoke/hydraulic mist warning in the elevator bay
At this time the fire in the pre cooler had been either continuous or intermittent for around 2:10 minutes depending upon the continuous or intermittent supply of fuel fed to the matrix. Because the pre cooler is operating at an instantaneous combustion temperature for a fuel/air mixture, fuel will ignite immediately a fuel/air mixture is present. There has been sufficient time for the fire to heat up the keel skin to a temperature where the contents of the bilge under Tank6 keel tanks is beginning to smoke and activate the fire and smoke warnings.
11:11:33- 11:12:26 an interlinked and at times overlapping series of reports from the crew of smoke entering the cabin from the elevator and aileron bay
11:12:26 The aircraft depressurised.
If we consider what effect the pre-cooler fire will have upon the rear fuselage keel, a number of significant events will be occurring:-
* Convection from the rise in temperature within the rear hinged fairing, and the fiercely hot flame playing directly on to the lower keel skin will raise its temperature, the keel structure expands as it heats up. A rough estimate of the total longitudinal expansion of the keel over a heat affected zone of say 200 inches in fuselage length could be as much as 2 inches for a temperature rise of 500deg.C.
Inside the pressure shell attached to the lower keel sit’s the 3 cells of Tank 6, each full of cold fuel (total contents of 3 cells around 5000 lb). The temperature difference between the heated keel structure and the cold soaked tank structure will induce severe straining at the tank mounting interface. The mountings maintain the correct separation of each tank cell and thus the integrity of the interconnecting fuel coupling. The interfaces have slotted holes at the attachments to cater for strains due to pressurisation and longitudinal straining due to flight loads, but cannot cater for such expansive increases due to a high temperature rise. Excessive straining will pull apart one cell from its adjacent cell, the resultant movement will fail the convoluted pressurisation shields and the fuel transfer passage between cells.
A breakdown in the integrity of Tank 6 cells is rather complex. Certainly if there was a gross fuel leak, fuel would spill directly on to the heated keel skin; whence ignition is instantaneous, an under floor fire is started and worsens as further leaking fuel feeds the fire. The under floor in the rear cabin is not the best place for a fire to occur; the flow of cabin air towards the rear cabin discharge valves give the fire a good supply of air. On the plus side, smoke and some of the heat will be partly discharged overboard and the true extent of the fire even at this early stage may remain under floor and not engulf the cabin with smoke.
The failure of the convoluted pressurisation seal will cause pressure cabin air to enter into the interspace between the bag tank and the tank structure. Fuel leaking into the Tank 6 cell interspace will be directed by drainage piping into the weapons bay, where it could be a source of an extra fuel feeding the pre-cooler fire from secondary drainage on to the weapon bay doors. Furthermore if pressurisation on to the cell bag tank causes the blow off to be activated, the Tank 6 blow off on the starboard skirt panel just forward of the NACA intake to the pre-cooler could be a further feeder of fuel.
11:12:26 The aircraft depressurised
* Mounted in the keel, below floor in the pressure shell, is the pipe run for the anti icing duct to the fin and tailplane (used also to feed APU air for main engine starting). The duct is thin metal about 6 inches diameter, connected together in separate pieces along the length of the fuselage with interconnecting sliding joints. Similar to the installation of the keel tanks these joints are not designed to cater for large expansions typical of heat damage. The ducts would be pulled apart and depressurisation into the ducts will occur.
11:13:45 The camera operator reported “ we have flames coming from the rear of the engines on the starboard side”
This is the Pre cooler fire outside the pannier skirt panel lapping up the fuselage side. It would appear to an observer in the cabin to be from the rear of the engines. Equally the pre-cooler fire could destroy the port upper skirt panel and lap on the port fuselage (see observation by the RCD below).
11:14:00 ‘Another crew member using a portable oxygen mask as part of the under floor drill reported a fire within the aileron bay’.
This is the standing fire in the under floor keel
11:15:43 Accident Data Recorder either stopped recording or ceased to function
The electrical wiring for the ADR runs in an electrics trough on the starboard side and would have been damaged by the under floor fire.
11:15:43 The mission tape ceased recording data.
At no time up to the loss of recording data did a crew member report that there was a fire in the No 7 dry bay. The top cover skin of the dry bay would have been clearly visible to an observer in the cabin.
11:16:54 The Nimrod was observed by a Harrier GR7 pilot in a descent with flames emitting from the starboard wing root and the starboard aft fuselage. Shortly thereafter, several members of ’A’ Squadron Royal Canadian Dragoons (RCD) observed the aircraft as it passed to the south of their position; the fire appeared to them to be on the port side of the aircraft, although it was in other respects similar to that seen by the GR7 pilot.
At this point in the timeline, we have postulated a continuous or intermittent fire in the SCP pre cooler, a heat damaged rear fuselage in the lower keel above the pre-cooler, a standing fire in the lower keel inside the pressure shell, and a cabin that is depressurising into the rear anti icing duct. The parting of the anti-icing duct creates two chimneys for the fire force fed by depressurisation; the flames, smoke and un-burnt gases are delivered along the ducts, forward towards the No 7 dry bay until blocked by NRV’s, and aft into the convolutions of the duct in the tail fuselage. If the fire inside the ducts is well oxygenated, the light gauge alloy ducts will be breached and the fire is released into each of the bays and then quickly melts the thin outer skin of each bay to be visible outside the airframe.
Evidence e. Report by the Harrier GR7 pilot of XV 230 descending with flames originating from the starboard wing root and the starboard fuselage.
The above are the two separate fires seen by the GR7 pilot.
The starboard wing root fire is a later fire melting through the upper skin cover in the 69 seconds since crew recording ceased.
It is conceivable that the pre cooler fire is still burning or flash fires occur as residual leaked fuel from the bomb bay bleeds back intermittently into the pre cooler.
Evidence f. Testimony from 3 members of A Sqn RCD who observed approximately the final 40 seconds of XV230’s flight. The Canadians reported a fire in many respects similar to that of the GR7 pilot, but on the port side of the aircraft.
This is the fire observed by the RCD.
Evidence m. Photographs taken of the interior of the aircraft tail section following the crash and smoke damage to the recovered sonar location beacon show clear evidence of internal burning in the compartments aft of the pressure shell. However the fire is of short duration and was probably ignited as a consequence of the principal fire further forward in the aircraft. It was not the initial scene of combustion.
This is the burning in the tail section of the flames from the rear anti-icing duct.
* However, examination of recovered equipment and the pathology report indicates that there is no evidence of the fire extending beyond the under floor bays into the crew compartment.
The fire caused by fuel leaking from Tank 6 on to the hot keel will be a rather dirty smoky fire, not hot enough to burn through the floor panels and enter the cabin. Furthermore, the two chimneys created by the anti-icing duct will be drawing the flames into the ducts during the de-pressurisation. We would not expect the underfloor fire to migrate into the crew compartment for so long as the cabin was depressurising.
11:17:51. ‘ at a height approximately 700 feet the aircraft broke up into at least 4 large sections (fuselage, starboard wing, port wing, and tail section), struck the ground at high speed, low incidence.
* ‘In the last few minutes of flight the aircraft’s average groundspeed was calculated as approximately 352 knots with an increase in the last few seconds of flight.’
By simple logic it is not possible to give an adequate explanation of the final events of the accident. Of primary importance is how much control the pilot had, and we have no evidence available, The most significant event in our postulation is the fierce fire in the pre cooler, the heat damage caused to the structural make up of the lower keel and the longitudinal expansion due to heat of the under surface rear fuselage. The expansion of the keel is significant enough to change the shape of the airframe particularly insofar as the empennage is concerned , to incidence of the tailplane. This will induce the aircraft to climb with an increase in the balance down tail load, which clearly in the desperate situation the crew are in the pilot would correct with elevator authority.
However, the degrading material properties in the aluminium alloy structure of the rear fuselage are becoming apparent: The material Modulus of Elasticity is reducing markedly with increase in temperature, making the fuselage more flexible to the inertia effects of the fuselage and the aerodynamic loading from the tailplane. The Moment of Resistance of the rear fuselage in bending will be rapidly affected by the reduced material properties and there will be a reduction in stability of the primary compressive elements of the lower keel due to heat damage.
The heat damage to the lower keel worsens, its effects are dominating to the integrity of the airframe, the flexibility of the fuselage increases, the compressive stability of the lower keel reduces, and the aerodynamic effects caused by changes in the balance tail load, become out of the control range for correction by the pilots. The final phase, the catastrophic loss of the bending strength of the rear fuselage causes the fuselage to fail and the aircraft is lost.
Conclusion
It is concluded that the events postulated were caused by the SCP operating during a high flow rate refuel into a receiver aircraft almost full of fuel, which unknown to the crew had created a period of prolonged blow off of Tank 1. Blow off fuel had tracked the outside skin of the pannier, and entered the NACA intake to the pre-cooler installation Coincidental with the air to air refuelling the SCP was operating at high bleed air temperatures. The coming together of these two effects within the pre-cooler of the SCP caused a sequence of events leading to the loss of aircraft XV230 and crew. The critical element of the event was the length of time the blow off overflowed, causing a prolonged period for the pre-cooler to be fed with fuel initially from the discharge and subsequently secondary drainage from the bomb bay. The heat damage to the lower keel pressure skin and then the compromising of the fuel connections between the tank 6 cells raised a fire within the pressure shell causing a catastrophic loss of strength and stability to the rear fuselage. Coincident with this loss of integrity, the crews attempt to reach safety as quickly as possible greatly increased the aerodynamic loading on the empennage and we believed that the airframe suffered a structural failure in bending, resulting in break-up and loss of the aircraft and crew.
Post 5.5 minutes into the re-fuelling sequence, the SCP shut down; all ignition sources along the Cross Feed and the SCP ducting were no longer fed with HP air and began to cool down. Any suggestion of individual fuel coupling leaks, doubts in the integrity of the fuel system piping or the SCP ducting questioned by the official inquiry, are not seen to be contributory factors.
It is inadmissible, in view of the evidence known throughout the inquiry, that whilst the Nimrod fleet was still flying, the opportunity was not taken to undertake a simple flight test verification to understand where fluid discharged from the Tank 1 and Tank 6 blow off migrates. Supposition and conjecture have always been necessary in this accident, but confirmatory tests would have made certain postulations more believable.
A link commenting on the Haddon-Cave review et al. All very sad:- https://sites.google.com/site/militaryairworthiness/
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