The Nimrod Review - Official Documents

The Nimrod Review
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18. Targeted fuel seal replacement programmes such as that recently undergone by the Nimrod fleet,
teardowns, and seal sampling, are useful tools in the management of fuel seal reliability (see
Chapter 23).
19. It is important in the future that active thought is given to the appropriate management of all fuel
seals.
Alternative theories
20. The alternative fuel source theories are not realistic and can be discounted.
Introduction
5.1
5.2
A distinction needs to be drawn between three types of fuel leak: (a) fuel leaks from integral wing tanks; (b)
fuel leaks from main fuselage tanks; and (c) leaks from fuel couplings and pipes located in the main body of
the aircraft. Wing leaks (a) are rarely a threat to an aircraft, as fuel readily disperses into the airflow behind
the aircraft. The Nimrod’s fuselage tanks (Nos. 1, 5 and 6) are, at a minimum, double skinned1 and designed
such that any fuel leaks are vented to atmosphere. However (b) and (c) leaks from fuselage tanks, fuel pipes
and couplings within the main fuselage are potentially a much greater problem: their leak path is difficult to
predict and they can drip and/or run through the fuselage allowing fuel to reach many areas of the aircraft and
accumulate in hidden voids. Such leaks, therefore, present a potential fire risk in the event that they contact
an ignition source. For these reasons, aircraft are required to be designed such that any potential single-point
ignition risks are mitigated by inter alia fire detection and protection systems.
I set out first a description of the Nimrod fuel system and the various couplings used within it.
Description of Nimrod Fuel System
5.3
The Nimrod has 13 fuel tanks and can carry over 85,000lbs of fuel. There are three integral tanks in each wing
(Nos. 2, 3 and 4 – port and starboard) and a wing pod tank (No. 4A port and starboard), three tanks in the
central fuselage (Nos. 1, 5 and 6), and two tanks located in each trailing edge fillet adjacent to the fuselage
(No. 7 tank port and starboard). The interior surface of the wings is covered in a sealant and fuel is contained
within the wing structure; the wings’ surfaces effectively form the fuel tank. The fuel tanks within the fuselage
have a more complex structure. The No. 1 tank, outside the pressure hull, was part of the original Comet design
and consists of a metal structure divided into four cells, within which the fuel is held in rubber bags attached to
the cell walls. The Nos. 5 and 6 tanks are within the aircraft pressure hull and, for this reason, they have double
skins. They are divided into three cells and, again, the fuel is held in rubber bags. The No. 7 tanks (in the port
and starboard wing fillets) are also of metal construction, although rubber bags are not used to contain the fuel.
Refuelling is controlled by high-level float switches in the tanks, which automatically curtail refuelling when the
tank is full. 2 Fuel is pressure fed round the fuel system by immersed booster pumps in each tank. The tanks are
ventilated by ram air taken from the wing inlets and vented to atmosphere. Blow-off valves are fitted to each
tank except the No. 4A tanks; these are safety devices to prevent damage to the structure of a tank if the highlevel
float switches fail and a full tank continues to be refuelled. 3
1 Fuel is held in a rubber bag, which in turn is contained within a metal structure.
2 Although it is important to note, as the XV230 BOI pointed out, the fitting of two independent float switches in No. 1 tank meant that the tank was
prone to asymmetric filling (XV230 BOI Report, Annex N).
3 No. 5 tank had the blow-off valve blanked off on fitting of AAR Mod 715 in 1989 and a restrictor fitted in the fuel line to limit pressure build-up.
This was because of concern that a blow-off from that fuel tank might allow fuel to be ingested by the aircraft engines.