The Nimrod Review - Official Documents

The Nimrod Review
23.35
556
I consulted the United States Air Force (USAF) on this specific question of lifing seals. I had the benefit of advice
from a leading USAF expert in the field, from AFRL/RXSA (System Support), Wright-Patterson Air Force Base. 16
The expert explained that seals contain a lot of entropy17 and there are a large number of variables that can
affect them, including stress, heat, and drying out. He told me that “there are no current methods that can
reliably predict seal life”. For this reason, the USAF does not life seals. He said that there was currently a lack of
sufficient knowledge about the materials from which seals were made and the constituent properties had not
been translated into a Finite Element (FE) model. He said that this topic was still in the “too difficult” box and
further research was required. I fully accept all his advice and views.
Prevailing attitude that fuel leaks ‘inevitable’
23.36
There was, and remains, a prevailing attitude that leaks in aviation fuel systems are an inevitable fact of life.
23.37 This is reflected in, e.g. the DE&S “Review of Lifing and Maintenance Policy for Aircraft System Seals,” 18 dated 22
November 2007, which stated that “ultimately, leaks from seals are inevitable and the system design principles
used for aircraft in both the civil and military environments to mitigate against leak hazards (sic)”. The term ‘leak
tolerant’ (which seems to have sprung up since the loss of XV230) appears intended to convey the sense that fuel
leaks in themselves should not pose a significant hazard because aircraft should be designed and constructed in
such a way that leaks would never reach a point of ignition. This is undoubtedly true in theory: any competent
designer should naturally do everything to eliminate the risk posed by sources of ignition. Nonetheless, as the
TWA 800 case19 and XV230 have starkly illustrated, however careful you think you have been, you cannot be
sure you have anticipated all potential ignition sources.
Ignition/Leak Philosophy
23.38 As I state in CHAPTER 5 and repeat, any philosophy which relies primarily on a belief that all potential ignition
sources have been eliminated is, in my view, unsound. Further, it is not generally a good approach to tolerate
recurrent defects, even minor ones, they might have unexpected, unforeseen or cumulative consequences. It is
also a well known adage that ‘fuel will tend to find a source of ignition’. Liquid paths are eccentric. Accordingly,
good practice, and the principle of ALARP, require that the risk of both parts of the ignition source/fuel sources
equation be equally carefully addressed.
‘To disturb or not to disturb’?
23.39 As stated in Chapter 5 there has been a widespread policy in the MOD, and other air forces and the civil sector,
that fuel seals should generally be left undisturbed. There has been a prevailing and widely-held belief that
pro-actively replacing seals might actually increase the number of fuel leaks by disturbing the system and fuel
couplings up and down the line and/or because of early ‘failures’ 20 of newly assembled couplings.
23.40
23.41
Whether this belief is justified in all circumstances is, however, not clear. It is noteworthy that the recent Nimrod
Fuel Seal Replacement Programme (FSRP), whereby 42 fuel seals located in specific areas deemed to present
a particular risk have been replaced, has not led to an early ‘rash’ of infant mortality leaks; indeed, the high
quality of work has been encouraging and has given cause for optimism as to the benefits of such targeted seal
replacement programmes. 21
Equally, the Nimrod teardown exercise has yielded valuable data and the opportunity for careful sampling and
testing of systems and their component parts.
16 NADCAP Quality Products Group for Sealants and Elastomeric Seals; voting member of SAE International Aerospace Technical Committees G9
(Sealants) and CE (Elastomeric Seals); chair G9 sub-committee for Sealant Removal Technology.
17 Thermodynamic/mechanical disorder and randomness.
18 DE&S(WYT)/365/8/2/CASD, page 18.
19 TWA 800 exploded on a flight from Paris to New York on 17 July 1996 over Long Island. Investigators determined that fuel vapour in the almost
empty central fuel tank had been ignited by an unknown source, most probably an electrical short.
20 Probably due to incorrect assembly.
21 It is still too early to tell the medium and long-term effects of the FSRP.