Jan 17

Cabin temp hot NNC or PACK NNC why R recirculation fan AUTO and L recirculation fan OFF

Hi guys,

In these NNC, boeing asks for this configuration. FCOM says that R FAN is ON and L FAN is OFF if a pack is in High mode in flight and that’s the case because we have switched a pack to off a few steps before in these NNC.

So I wonder what is the goal of these two steps.

thanks a lot for your help.



Hi Adrien,

Thank you for your post. The FCOM 2.10.5 states that when either PACK switch (not PACK mode) is in HIGH, the left recirculation fan shuts down. So the L RECIRC is operating until you switch it OFF.

- Petr

Jan 18

Thank you Petr. I had not got this detail.

I still don’t understand why the L recirc OFF and the R recirc AUTO.

how Is that improving cabin air ?



The left and right recirculation fans are drawing air into the mix manifold. If you have a look at the mix manifold diagram, it shows up as a hollow container. In reality, it is not completely hollow; it forms sections. The L RECIRC fan is providing more air into the flight deck than the R RECIRC.

This is also why when you switch the L RECIRC off during the turnaround, the noise in the flight deck decreases significantly.

As to why the QRH has the following configuration, I can only speculate:)

- Petr