B757 Recirc Fan Noise

[haughtney1]

Please can someone tell me....why is it when you turn the right recirc fan off in the cruise, the ambient noise level drops by a significant amount ?

[ICEMAN757200]

Interesting question , we operate the B-757SF without the left pack in order to minimize the ambient sound , you can however turn off the right recirc fan to provide a more rapid exchange of air.May be the noise stops because Air moving from the cabin to the lower deck exhausts now exclusively thru the outflow vlv so air stops being suck by the R Recirc vlv to return to the mix manifold, as well of course the noise caused by the elec motor stops as well.We should ask an mechanic to be sure .

[haughtney1]

The thing I've noticed is that if you leave the recirc off for about 30 seconds or so, then turn it back on....the ambient noise level (fan noise etc etc) remains at the lower level.
I'm not sure as to why this is (but it works!), I'd guess it has something to do with the air system plumbing..and perhaps by turning off the R recirc (hence increasing the flow rate) it somehow reduces the amount backpressure or stagnant air in the system...and hence when you turn the R Recirc back on..it doesnt have to work quite as hard to maintain the massflow..and is quieter. But I'm guessing.

Anyone?

[Blighty Pilot]

This was a nice little trick I found out about in my previous company and we used to use it a lot. People seem to be nervous of doing it in out present company. I'm sure Dennis could give you an answer.

[THOMAS DUCK]

Guys,
The reason why the ambient sound level is reduced is that the R recirc fan in addition to the R pack supplies the cooling air for the overhead cb panel (that annoying icy blast when you are on the jumpseat) and forward instruments in addition to windscreen demisting.
On the ground with the APU supplying air the pack will already be in Hi flow and therefore the noise difference will be quite marked. In the air turning the R recirc fan off THEN puts the pack into Hi flow to compensate and the noise difference will not be so marked.
Hope that helps

[chemical alli]

a few maint tips

recently our company went through the same problem with the 767 as the systems are the same and after alot of testing we came to a few answers.first check the recirc filters if they are old and full they increase the noise ,second some aircraft depending on line number actually have a noise attenuator or muffler fitted which can be retro fitted ,also depending on recirc fan p/n there are noisier fans ,most of the noise was actually airflow noise throught the overhead cb panel fitting the attenuator and ENSURING ALL DUCTING WAS PROPERLY CLIPED HELPED DRAMACTICALLY

[Swedish Steve]

Originally Posted by chemical alli

recently our company went through the same problem with the 767 as the systems are the same

Be careful. The systems are not the same. On the B767 you can turn off both recirc fans and all you get is an increase in fuel flow as the packs go to max mode and take more air from the engines.
On the B757 the right recirc fan is the same, but the left recirc fan is used as the extract fan for the fwd equipment cooling system. Turning it off will produce equip cooling messages, and open the exhaust valve to induce flow in the system. This fan is no-go for ETOPS flights.

[THOMAS DUCK]

S.S.
Its exactly as you say, its worth pointing out that if you do switch off the L Recirc fan the overboard exhaust valve opens but then cannot be closed again until you are back on the ground. It is stuck open for the remainder of the flight!!