noise in cockpit descending through about 5000 ft B737 [Archive]

EightsOnPylons

25th Apr 2004, 10:59

hello,

when descending through about 5000 ft I have noticed that a special sound starts in the cockpit of the B737, it almost sounds like hard rain hitting the nose, or ice crystals hitting the nose. I have also heard that the noise is simulated and comes on in the simulator when descending through about 5000ft.. what noise is this?? does anyone know what I mean ??

thanks,
eightsonpylons

chuck yeager

25th Apr 2004, 13:54

Overboard Exhaust Valve

On the ground and in flight with low differential pressure, the overboard exhaust valve is open and warm air from the E & E bay is discharged overboard. In flight, at higher cabin differential pressures, the overboard exhaust valve is normally closed and exhaust air is diffused to the lining of the forward cargo compartment.

However, the overboard exhaust valve is driven open if either pack switch is in high and the recirculation fan is off. This allows for increased ventilation in the smoke removal configuration.

The noise you hear when descending through 5000 feet is the cooling airflow with the overboard exhaust valve open. Next time you hear the noise start, look at the cabin differential pressure and you will note that it is about 2.0 psi (“in flight with low differential pressure”).

You may also note the same phenomenon in reverse during the climb. When cabin differential pressure reaches about 2.0 psi the overboard exhaust valve closes and the cockpit noise reduces significantly.