Vapour Seal/Fuel Tank Interspaces Ventilation System

There are three independent ventilation systems that provide a fuel vapour barrier between the fuselage fuel tanks and the pressurized passenger and baggage compartments. Each of these systems is also capable of draining or syphoning overboard, fluids which may collect as the result of condensation or a fuel leak from the fuselage tanks.

The system comprises of air inlet grilles, filter, non-return valves, seal membranes and ducts, which facilitate an airflow from the passenger compartment and the underfloor space, to the enclosed but ventilated airspace formed between the seal membrane and the tanks. Air bleeds from this airspace are connected to vent pipes which duct the ventilating air overboard in association with the drains.

Seal Membranes (Picture 1)

These membranes, which are fabricated from a Viton coated fabric, extend over the top of tanks 6, 8, 10 and 9, and the forward bulkheads of tanks 9 and 11. The membrane is attached and sealed to the aircraft and tank structure by brackets, clamps and membrane supports, and covered with insulation blankets. The air supply duct has been fabricated as an integral part of the seat membrane.

Airtight zip fasteners are incorporated in the membranes to provide limited access to the catenary floor.

Non-return Valve (Tank 11 Vapour Seal Membrane)

A tow pressure non-return valve (NRV) is attached with binding wire to the vapour seal end of the air inlet duct to the vapour seat. This NRV is a sphincter type of valve, made of a nitrite/PVC compound with a differential pressure of 0.1 in WG between the passenger compartment and the vapour seat. The NRV opens and allows the air to flow from the LH rear baggage compartment wall duct into the vapour seal air space.

Non-return Valve (Tanks 6, 8, 9 and 10 Vapour Seal Membranes)

A low pressure non-return valve (NRV) has been fitted to the air inlet of each vapour seal air supply duct. The NRV is a simple gravity action; single flap valve, with a piano wire hinge, that has a differential pressure of 0.1 in WG between the underfloor space and the vapour seal, the NRV opens, and allows the air to flow from the underfloor area into the vapour seal airspace.

Non-return Valve – Tank 9 Vapour Seal Forward Drain (Picture 2)

A pressure operated flap drain valve is fitted in the vapour seal interspace forward of fuel tank No.9, to permit fuel and moisture drainage at this point when the aircraft is on the ground and to augment the syphon action in flight.

Debris Guards

Ingress of small particles into the underfloor fuel vapour ventilation system is prevented by a fine mesh wire grille at the entry to each duct.

Vents and Drains

The vapour seals are vented by rigid steel pipes which connect each air bleed to atmosphere. The air bleeds are bolted to the top of the vapour seal membrane and the pipe of each bleed extends into the vicinity of the vapour seal air space where leaked fuel would collect. A restrictor, which is calibrated at the build stage, is set in the outlet vent from tank 11 vapour seal.

The drain pipes are set at low point of the vapour seal air space, and a restrictor similar to that of the vent restrictor is set in the drain from the tank 11 vapour seal.


The air supply duct to the vapour seal on tank 11 is fed by a branch from the LH rear baggage compartment wall cooling duct in the roof of the rear baggage compartment, and terminates at the NRV in the vapour seal. The duct, which is impervious to fuel vapour, is made of rigid resin impregnated glass cloth.

Air supply ducts to the underfloor vapour seals, are partly integral with the seal membranes, and partly made from rigid metal pipes. The metal ducts which contain a NRV at the inlet are connected to the membrane ducts by corrugated flexible pipes and clips. Holes through the duct and membrane, which allows the air into the seal airspace, are pitched at regular intervals along the ducts.

Operation (Picture 1)

Passenger compartment discharge air is ducted from the underfloor space and the rear galley area, to flow through the systems and then discharge overboard, as shown in the schematic airflow diagram. Cabin differential pressure causes the air to flow, and there are no manual or automatic controls of the system.

Fuel leaking into the vapour seal air space is passed overboard either by gravity feed through the drain pipes, or siphoned off by the air bleeds into the vent pipes from the low points of the catenary floor, and the lower part of tank 11 vapour seal.

In the event of the forward bulkhead of tank 9 leaking, fuel which would accumulate to the top of the siphon tube when the aircraft is grounded and unpressurized, is drained to atmosphere via a 1/4 in (6.35 mm) dia. drain hole in the bottom of the vapour seal.

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