Boeing 737 ground operations question

[Matts28]

First of all, hello all. Aside from having a deep admiration and love of aviation, I am not a professional in anything regarding the industry. I do however have a deep fascination with how aircraft work. My apologies in advance if this question is dumb in anyway.


I have been doing some research on this topic but I cannot really find a solid answer anywhere. I understand how bleed systems work. I also have a really BASIC understanding of how the air conditioning system works off of bleed air supplied to the packs. My confusion comes from the recirculation fans.


My questions are as follows:


1.) What exactly is the function of these fans?
2.) If an aircraft is parked and is being fed pre conditioned air directly into the mix manifold, do these fans need to be on? Or do these fans only get turned on during pack/bleed operations?


Thanks so much for your time!

[737aviator]

The fans save fuel by recycling cabin air back into the mix manifold. With the fans off the packs conditioned air requirement would be higher which would thus use more bleed air and burn more fuel.

Its my company SOP to have the fans on whenever PAX are on the A/C. Pre conditioned air would still cool the cabin without them but the fans will help increase the circulation of air. Some pilots will switch off one though due to the noise it makes in the flight deck!

[vapilot2004]

Additional info to 737aviator's response: For the stretch versions (-400, -800, -900) there are two recirculation fans while the non-stretch aircraft use one. The fan intake air is filtered. During normal operation no recirculated air comes forward to the flight deck ducts, fans on or off.

We discovered on hot days at the gate out west, particularly after a long period on stand, preconditioned air seems to cool better with the RECIRC fan(s) AUTO just as 737aviator noted above. The fan(s) give the illusion of more cooling by increasing the airflow, but in fact the temperature of that airflow can actually go up when the fan(s) are on during ground operations. In humid environments back east on a quick turnaround, RECIRC OFF seems to be the better configuration until off stand with both packs running.

[Otto Throttle]

The recirculation system draws air from the cabin and flight deck under the floor and it passes around the lining of the cargo compartments, where it partially aids in heating the compartments (the forward cargo compartment gets an extra boost of warm air from the exhaust air from the avionics cooling system). This "used" air then flows back to the mix manifold where it gets redistributed and reconditioned.

On very hot days, if the cargo compartment doors are open on the ground, as they often are, the cargo compartment gets hot, and so does the air circulating around the compartment lining. Because this means that hotter than "normal" air is being recirculated back to the mix manifold, the packs have to work much harder to recondition the air. Aircraft cooling can be more effective in these circumstances with the fans off, but in doing so it obviously reduces the volume of air flow in the cabin and flight deck, which slightly reduces the ability of the packs to cool the air (less airflow = less cooling). The baggage handlers probably won't thank you though ;)

[Matts28]

Thanks guys!

[ImbracableCrunk]

The recirc fans do give the impression of better cooling when hooked up to PCA. However, I'd prefer to have the PCA unit push the stale are out through the L1 door than have it recirculated if it's a good unit.

I guess people prefer a fan of stale air over a dribble of cool air.

[vapilot2004]

Otto, you must be used to transiting airports with no pre-conditioned air? Are you running on APU or using high pressure air from the stand to run the packs? Just curious - fewer and fewer US airports have HP air available except that which comes from the start truck.

[JammedStab]

Perhaps going out on a limb here as it was quite a while ago that I flew it, but I don't remember a recirc fan on the -200 model.

[vapilot2004]

None fitted as I recall either, JS, we're on the same limb now, you and I.

There was the little gasper fan which only fed cold air from the packs to the individual outlets, which could be on or off or somewhere in between. At full on (all outlets fully open) I seem to remember being told the flow was about 1/5 of the total. I don't think the fan was much of a fuel saver since its air source is pack supplied.

...I'd prefer to have the PCA unit push the stale are out through the L1 door than have it recirculated if it's a good unit.

I guess people prefer a fan of stale air over a dribble of cool air.

Same sentiment IC for us in hot and humid areas and as to your last observation, apparently!

[Otto Throttle]

Vapilot,

I'm working for a Middle East operator. Ground conditioned air is hit and miss out here, despite the ridiculous summer temperatures, so we generally run from the APU unless we can get a really good ground unit. There's still only so much you can do when it's pushing 50C outside though :)

[Matts28]

I guess I was just surprised to hear that the air pressure from the air conditioning carts is enough to cool the plane without needing the fans on all of the time. I thought (like normal AC units in a house or car) that the air would need "pushed" through the plane somehow. I didn't know it could go straight into the manifold and find its way around. Very interesting indeed!

[JammedStab]

I guess I was just surprised to hear that the air pressure from the air conditioning carts is enough to cool the plane without needing the fans on all of the time. I thought (like normal AC units in a house or car) that the air would need "pushed" through the plane somehow. I didn't know it could go straight into the manifold and find its way around. Very interesting indeed!

Different aircraft type for me but we are required to turn the recirc fans off with ground air being used.

[vapilot2004]

I guess I was just surprised to hear that the air pressure from the air conditioning carts is enough to cool the plane without needing the fans on all of the time. I thought (like normal AC units in a house or car) that the air would need "pushed" through the plane somehow. I didn't know it could go straight into the manifold and find its way around. Very interesting indeed!

Some ground supplies are better than others to be sure M28. For us, humidity makes a huge difference on how effective PCA units are, either that or all the ones back east are knackered. When off stand, air at the pack inlets gets 'pushed' through the packs and beyond by some pretty big fans hanging off the wings. :ok:


I'm working for a Middle East operator. Ground conditioned air is hit and miss out here, despite the ridiculous summer temperatures, so we generally run from the APU unless we can get a really good ground unit. There's still only so much you can do when it's pushing 50C outside though

My hat's off to you Otto! An older friend used to fly that region in the 1970s and 1980s and he often remarked at the profound disparities between place to place. He also said on flights to places like Djibouti, livestock would sometimes be roaming the aisles in the back!