MD-11 Flight Crew Operations Manual
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Fuel -
Description and Operation
Fuel.10.13
Tank 1 and 3 inboard and outboard sections are separated by a vent box. The vent
box is a sealed compartment that forms a trap between the vent manifold and the
standpipe overboard outlets. A small quantity of fuel can enter the static and climb
vent lines during various maneuvers, but the vent box size (about 379-530 liters)
traps this fuel and prevents spillage overboard. Fuel can also enter the vent box
during refuel or transfer overfill conditions. Fuel drains from the vent box back
into the tank through the drain valves.
The two compartments of tanks 1 and 3 are interconnected by a vent line which
also serves as the fill line for the inboard compartment. This line has an open vent,
a float type climb vent valve, and a check valve to prevent back flow. The left and
right compartment vent spaces of tank 2 are connected with a simple vent line
through the center wing tanks.
A cross vent system is installed. The vent spaces in the left wing are connected to
the vent box in the right wing and vice versa. The open vent in tank 1 and the tank
2 open vent are joined and routed to the right wing vent box. Similarly, the vent
spaces of tank 3 and the center aux tanks are connected and routed to the left wing
vent box. Alternate vent inlets in each tank are used to vent the tanks when fuel
covers the open outlet during aircraft maneuvers. These alternate inlets are fitted
with float valves to prevent fuel flow when they are submerged. Drain valves
connected to the vent lines permit the fuel that may slosh into the vent lines to
re-enter the tank whenever the fluid level in the line is approximately 1 inch or
more above the fluid level in the tank.
A flame arrestor is installed in the vent box stand pipe to eliminate flame
propagation through the vent system as the result of an external fire source. There
are pressure relief and vacuum relief valves in the stand pipe which bypass the
flame arrestor to vent the tank if the flame arrestor becomes iced or otherwise
blocked.
The tank 2 fill rate is higher than that of tanks 1 and 3. This creates the potential
for greater vent back pressure in tank 2 during fill. A relief valve located in tank
2 interconnects the two vent systems. This relief valve opens when the combined
tank 1 and tank 2 vent system pressure exceeds the combined tank 3 and center
aux tank vent pressure by more than 0.4 psi.
Fuel Vent System - Tail
The tail fuel tank vent system is an open system similar to the wing fuel tank vent
system.
The tail fuel tank vent system is completely isolated from the wing fuel tank vent
system.
October 02, 2006
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