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The Hedge
4th Jul 2007, 19:47
Hi,

I have asked around and no one has been able to answer so far:

At approx 2psi diff or around 4-3000ft on descent an airflow noise starts from which I think is the fwd toilet area.

Can someone smarter than me please explain why it happens and where its coming from.

Cheers

alexban
4th Jul 2007, 22:16
Same happens on the 737,it's the opening of the forward outflow valve commanded at 2 psi during descent.

IFixPlanes
5th Jul 2007, 10:32
Maybe the noise come from an damaged drain valve that acts like a "flute".
AMM:
DRAINS - DESCRIPTION AND OPERATION
1. General
Drains are installed in the fuselage structure of the aircraft at or near to the lowest points in an area. The drains let water and condensation drain out so that they do not cause corrosion.
2. Description:
Drain valves are installed in the fuselage shell. When the cabin pressurizes, the valves, preset to operate when cabin pressurization reaches 2 psi (0.1378 bar) , close to prevent loss of cabin pressure. As the cabin pressure decreases below the preset value, the valves open (under spring tension) to allow any water that has collected to drain away. You can remove the drain valves, for inspection or replacement, with an allen key from outside the aircraft.
In other structural components (doors, door frames, stringers, brackets and fittings) holes are drilled at the lowest points to make sure that water and condensation drain to the drain valves.
FWD Area:
http://img182.imageshack.us/img182/191/drainvalveeu5.th.jpg (http://img182.imageshack.us/my.php?image=drainvalveeu5.jpg)

FE Hoppy
5th Jul 2007, 11:40
IFixPlanes has it.
It's not a defect, just the way the "sink plugs" are designed. When the diff pressure is low the plug opens and the air flows through. If the plug is unseated by some fod and remains open throughout the flight the noise can be quite loud at high diff pressure.

IFixPlanes
5th Jul 2007, 12:15
...It's not a defect, just the way the "sink plugs" are designed...
I think that they are normally not so loud.
By choosing the word "damaged" i had a bended valve housing at the drain/air path or a foreign object in mind, which let the drain valve act like a "flute" and make more sound then usual.

javelin
5th Jul 2007, 22:12
No fellas, this is something else and is related to the changeover of the avionics cooling system but I am not sure what. We had this as part of a quiz a while ago.

Dream Land
5th Jul 2007, 23:44
No fellas, this is something else and is related to the changeover of the avionics cooling system but I am not sure what. We had this as part of a quiz a while ago Like going from closed to the intermediate position due to OAT?