NGjockey

It seems to be a widely spread practice on the 737-800 to turn off the left recirculation fan in order to keep the noise level down in the cockpit. If I remember correctly, the purpose of the recirculation fans is to reduce bleed air demand, hence reducing fuel consumption.

Can anyone tell me if there really is a noteable difference in fuel consumption when the left fan is ON instead of OFF? Are there any concrete numbers (percentage or kgs per hour)?

Anxiously awaiting enlightenment...!

RYR-738-JOCKEY

Run a BITE test if you really want the figure. More interestingly, though...if you switch off one pack the consumption goes up.

NGjockey

That may have something to do with the configuration of the system. With the pack switches in AUTO and both packs operating, each pack regulates to a low flow rate. With only one pack operating in flight with the flaps up, the flow rate is regulated to HIGH, which increases fuel consumption. This also happens on the ground with APU bleed air ON and only one pack in AUTO: You will see a higher APU EGT ( = more fuel burned) than with both packs in AUTO.

Going back to the left recirc fan: I've been told that on a flight of four hours having that fan ON saves up to 100kg of fuel. Now that's just "hearsay", but with all those really efficient fuel saving programmes in operation these days (removal of ashtrays etc.) I keep wondering why that recirc fan never came up for discussion until now if it really saved that much fuel?