The Nimrod Review - Official Documents

6.34
6.35
6.36
Chapter 6 – Overflow or Pressure from Air-to-Air Refuelling
investigations be carried out by way of in-flight dye tests. Unfortunately, it appears this was never followed up
following the subsequent shelving of the Nimrod AEW3.
This was regrettable. Such dye tests would very probably have led to a realisation of the risks posed by the
starboard blow-off valve. In this regard, I agree with the comment on the BOI by the Air Member for Materiel:
“It is particularly disturbing that the undesirable overflow characteristic of the AAR system design appears to
have been identified during development trials in the mid 1980s for the Nimrod Airborne Early Warning (AEW)
aircraft, yet (although corrective action was taken for No 5 Tank) no corrective action was taken for the No 1
Tank despite recommendations that it be investigated further.” 21
If the dye test had been carried out, it is likely that, not only would the risk of fuel possibly tracking back into
No. 7 Tank Dry Bay have been identified and addressed, but it is also possible that the ignition risk posed by
the SCP elbow would have been highlighted by the dye penetrating the ‘muff’ insulation and, thus, the danger
posed by the bleed-air system exposed.
In my view, responsibility for failure to follow up the question of dye tests after 1985 or read across this issue to
the MR2 and R1 fleets must be jointly shared by BAE Systems as the Design Authority (DA) and the MOD.
2006: Dye tests for MRA4
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6.38
6.39
Dye tests were, however, carried out by BAE Systems in 2006 in relation to the MRA4 programme,
these have provided clear visual evidence of the manner in which liquid expelled from an aircraft can cover a
significant area.
22 and
Flight tests were conducted on the MRA4 prototype PA3 in June 2006 to check whether water discharged from
the forward and aft waste water drain masts would track back and impact on the wings and fuselage causing
an ice problem. The test was carried out by pouring 0.5 gallons of coloured fluid into both the toilet and galley
waste water systems during flight and evacuating it. The colour of the fluid, ‘Desert Sand’, 23 provided a strong
contrast with the standard Nimrod grey external paint scheme. The test on the forward mast was carried out
just prior to landing at a speed of 192 knots for the forward mast and 170 knots for the rear mast. This speed
range was calculated as most likely to allow fluid to impact the aircraft fuselage. The aircraft avoided cloud and
rain following the trial to prevent the dyed fluid being washed off.
After landing, a photographic record was made of all the areas where the impingement of fluid on the wings
and fuselage had occurred. The results are instructive. They show the fluid fanning out and tracking back
over and under the wing and entering parts of the fuselage. The toilet mast protrudes several inches from the
aircraft. A boundary layer of air exists close to the aircraft structure. The air immediately proximate to the aircraft
structure will travel at an extremely slow speed because of surface friction, relative to the aircraft, but this speed
will increase as the distance from the aircraft structure increases, until the air is travelling at the same speed
relative to the aircraft as the surrounding free-flow air. The thickness of this boundary layer varies, but is of
minimal width. The significance of this is that the mast from the forward toilet protrudes directly into this freeflow
air. Despite this, the fluid had clearly impacted the aircraft fuselage, as can be seen from the photograph
below. The blow-off outlet of No. 1 tank is flush with the aircraft fuselage. Fuel exiting the blow-off will have to
negotiate the boundary layer before it reaches the free-flow air. Thus, it would appear even more likely to track
along the fuselage. It should be noted that there would be significantly more fuel involved in blow-off than
liquid in this experiment. Furthermore, the higher speeds which the aircraft would be flying during AAR would
make it more, not less, difficult for the fuel to reach the free-flow air. As BAE Systems’ witnesses have made
clear, the diagrams supplied to the BOI24 reflect the flow of air around a particular section of the fuselage, rather
than any likely fluid flow. That the fluid flow is likely to be more complex is suggested by the wide dispersal of
the dyed fluid in the photographs below.
21 Comment on the BOI by the Air Member for Materiel, Air Marshal Sir Barry Thornton dated 8 October 2007.
22 BAE/D/NIM/RP/173496 and BAE/P/NIM/RP/162811.
23 The fluid used was temporary camouflage coating (1314GB0361E) diluted with water.
24 BOI Report, Exhibit 60.
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