Pneumatic noise in 737 cockpit below 4000'

[TypeIV]

One thing that for a long time has haunted me is the noise that you have in the cockpit below 4000ft approximately. It resembles the sound of pneumatic friction in a tube and also a running tap.

I've tried to figure it out by asking our captains and searching in the FCOM but now I have to face it and ask here. One captain told me it has something to do with a specific valve opening and closing. I've kept an eye on the ram door lights but they don't seem to have any correlation to this noise.

[172_driver]

It's the overboard exhaust valve opening at low differential pressures (around 4000 ft) ditching air from the E&E bay straight out on the street. At higher differential pressures I believe this air leaves "the normal way" via the outflow valve.

[STBYRUD]

Yup, correct! Opens when the differential pressure is less than two psi if memory serves me right.

[Spanner Turner]

Correct, although my notes say closed at 1psi diff or flow through valve of more than 30/lbs of air a minute. This is the valve that will be electrically driven open for smoke clearance mode. Next time you hear the noise stop, have a quick glance on the overhead panel and see what the diff press reading is on the pressurisation panel.

On ground this is the exit for the equip cooling exhaust. When it closes in flight that warmish equip cooling air is directed to the forward cargo compartment to keep the dogs and cats from turning into popsicles.

[STBYRUD]

Eeep, very true Spanner_Turner!

Spring force opens the valve. When the airplane starts to
pressurize, airflow through the valve increases. The valve
closes when the airflow through the valve is more than 30 lbs/
min (14 kg/min). When the valve is closed, 1 psi differential
pressure keeps the valve closed.

[de facto]

1psi in descent,2 psi in climb...and dogs are heated as well in the back via cabin air recirculation ,it all depends how long your flight will be and if you prefer they get more heat and less O2 or less heat and more 02.

At higher differential pressures I believe this air leaves "the normal way" via the outflow valve.

Correct...thanks to the Right recirc fan.

[president]

Under each cockpit sliding window there is a small drain hole the thickness of a pencil. It is the drain from condensation formed behind the overhead panel. You will see the clear plastic tubes leading in the corner of the sliding windows. This drain hole opens and closes by diff pressure (ball on a spring). I believe that's the noise you hear and not the overboard exhaust valve several meters behind you and under you.

[Skyjob]

Under each cockpit sliding window there is a small drain hole the thickness of a pencil. It is the drain from condensation formed behind the overhead panel. You will see the clear plastic tubes leading in the corner of the sliding windows. This drain hole opens and closes by diff pressure (ball on a spring).

This drain hole is not the source of the sound, agreed as per prior posts though. Thanks for detailed responses to those involved

[president]

Because everyone says it belongs to the overboard exhaust valve it doesn't make it right. Why would that sound be heard on the flight deck at all? And not at all on row 1 in the cabin which undoubtedly is much closer. I was told the sound came from the opening of the drain valve by a 737-licensed mechanic.

[criticalmass]

Page 45 in Chris Brady's excellent "The Boeing 737 Technical Guide", second paragraph down, in the section entitled "Equipment Cooling" sheds some light on this.

Basically, supply-fans and exhaust fans push (and pull) cooling air from the cabin through the flightdeck panels, display units, circuit-breaker panels and E&E bay...and this cooling air has to go somewhere after it has done its job.

In the Classics, this cooling air is dumped overboard by the flow control valve. But in the NGs, it is vented via the overboard exhaust valve. (quote) "The valve closing can be heard as a sudden hiss on the flightdeck on climb out or during final approach at 2psi." (unquote)



(My copy is version 65, Feb 2013)

[Skyjob]

Because everyone says it belongs to the overboard exhaust valve it doesn't make it right.

The pencil sized drain you describe is incapable of producing the sound due to its dimensions. The rush of air heard is of large quantity and would't be possibly to flow through such a small tube and make such noise without additional physical signs. Although you are correct in that it is fitted it and its use it is not the source of the sound.

On occasion a pilot can observe a droplet of water in said tubes, but this doesn't move when the sound is occurring during descent, meaning at that time there is no air rushing though the tube as you described as a possibility.

I'm sure the boys in Boeing explained it properly in our Type Rating course, many years ago, when a pilot asked that same question having been on an observation flight prior to conversion training. If not, ...

[No Fly Zone]

Interesting Question and (mostly)...

Wow. An interesting question; one that includes (mostly) spot-on responses. It is always gratifying to know that some folks thoroughly understand their airplanes and systems - and that others want to understand them.. . Whoopie!!

[yotty]

Strange Noise.

I wonder if it might be the fwd outflow valve?