The Nimrod Review - Official Documents

The Nimrod Review
48
4.24
Figure 4.7: Old Section of Cross Feed Duct
Although, in theory, the Nimrod’s hot air duct insulation was intended to be liquid-proof, there were a number
of weaknesses in its design. These weaknesses stemmed from the fact that the Refrasil was not continuous
and did not cover every inch of the ducting; therefore, parts of the duct, in particular some expansion bellows,
were left unlagged or covered in ‘shrouds’ or ‘laced muffs’, which were vulnerable to fluid ingress (see further
below).
Unlagged Bellows
4.25
4.26
There are a number of un-insulated bellows within the Cross-Feed/SCP duct system. The BOI was unable to
determine any recorded reason for leaving the bellows in the upper No. 7 Tank Dry Bay un-insulated. Trials
conducted by the BOI recorded that these bellows reached a temperature of between 343°C and 399°C when
the Cross-Feed duct was open. 29 As noted above, these trials were limited by restrictions placed on the operation
of the SCP after the loss of XV230 and full operating temperatures could not be reached. The temperature
of the bellows when the SCP operated in the air would be higher. Indeed, from the BAE Systems and AAIB
evidence quoted previously, the duct could have reached temperatures in excess of 400°C.
BAE Systems has been unable to trace a positive design decision to leave the No. 7 Tank Dry Bay bellows
un-insulated. This is perhaps not surprising in view of the passage of time since the original design was
completed. BAE Systems submitted to the Review, however, that the decision not to insulate the bellows was an
“understandable and a reasonable engineering judgement”. 30 This was based on five main assertions: 31
(1) As the bellows are situated above the fuel pipework in the No.7 Tank Dry Bay leaking fuel is unlikely to
impinge on them;
(2) Although the bellows are below elements of the aircraft’s vent system, this system will only hold limited
amounts of fuel in the event of a system ‘abnormality’;
(3) The AIT for AVTUR is likely to be higher than the surface temperature of the bellows;
(4) Any leak of fuel reaching the bellows would be unlikely to be sufficient for a sustained fire; and
(5) Fitting a detachable insulating muff would have added a potential loose article hazard to an area crowded
with flight control cables and pulleys.
29 BOI Report, Exhibit 36.
30 BAE Systems written submissions to the Review.
31 BAE Systems written submissions to the Review.
Example of compressed insulation
found in the No 7 Tank Dry Bay