B777 Fuel tank vent system

[extricate]

Hi,

I was just reading through the BA B777-200 incident where thrust are lost on both engines due to ice accretion, the report talked about the fuel tank vent system. I can't seem to find detailed explanation other than the surge tank in FCOM. Can someone with the knowledge care to share and educate the purpose of the vent system?

Quote: Fuel tank vent system

Each tank is vented to atmosphere through channels in the roof of the fuel tanks, which are connected to surge tanks mounted outboard of each of the main tanks. The surge tanks are vented to atmosphere through a flame arrestor and a scoop mounted on the lower surface of each wing. Should the flame arrestor or scoop become blocked, a pressure relief valve will operate and prevent the tanks from becoming over, or under, pressurised.

Thanks

[TURIN]

Pressure changes due to altitude, also if the tanks were perfectly sealed you would never get the fuel in as the air pressure inside would build up. May even cause structural damage.
Also, when the fuel is being used by the engines you need to ensure a positive air pressure in the tank.

Hope this helps.

[Capt Quentin McHale]

extricate,

The FCOM description is very close to the money without going into too much detail. Here's a little bit more for you.

Fuel tank venting is accomplished with sealed hat-shaped stringers attached to the upper wing skin inside the tanks. These stringers are interconnected with drain and vent tubes and terminate at the surge tank.

The stringers vent via the surge tank to atmosphere through a flame arrestor and ram air NACA scoop on the lower surface of the wing near the wingtips. The NACA scoop provides a positive pressure head through the vent system to the tanks during flight.

Pressure relief valves (can't remember if there's one or two) located near the NACA scoops on the under surface of the wing protect against tank damage due to excessive positive or negative pressure if the flame arrestor becomes blocked. The valves are operated by either fuel or vapour pressure and open at a positive pressure of 4.0psi or a negative pressure of 1.25psi. The valve poppet will move upwards into the surge tank skin surface (approx 1 inch) therefore indicating a differential pressure exists either positive or negative due to a blocked vent system (more than likely the flame arrestor) Hope this helps.

[extricate]

Thanks for these guys