737-800 Recirc fans

[nick14]

Hi guys,

I have seen a few Capts turn off the Left Recirc fan off on the ground to reduce cockpit noise whilst using the AC. The other trick I have seen is operating off just the right pack although this does increase fuel consumption.

I was told by a trainer that this is not a recommended practice, however I didnt ask for a reason (which was my own fault for not questioning)

My question is, does it have any detrimental effect turning the left recirc fan off?

Thanks

[misd-agin]

Research limitations for dispatching with the left recirc inop. That might help understand any potential problems.

It's also quieter in cruise. ;)

[framer]

Misd agin, is the potential problem that if it's over 38 degrees you could have excessive temperatures in the E&E bay or the common display system because the fan isn't assisting in removing the hot air at the E&E end?

[misd-agin]

framer - I'm pretty sure it's instrument cooling related but I havn't done the research to prove it.

Years ago friend had an E&E cooling(???) failure on a 757. Checklist said "failure of main flight instruments can occur shortly"(or similar words).

Said he looked up at the instruments and they started to fade...:uhoh: Matter of minutes.

Luckily it was day CAFB.

[aviatorhi]

They're there for a reason, if you look back at the "history" of the way Boeing cooled the E&E bays on previous aircraft (707/720 and 727) you'll see that it was something that they took very seriously and there was a method of doing it any time the avionics busses were fully powered, in flight or on the ground. If you're running into people who insist on reducing the decibel level tell them they can either install earplugs while on the aircraft, if they refuse remind them to remove the radiators from their cars to save weight and increase fuel efficiency.

As far as the pack is concerned, to my knowledge they're both identical, not sure why anyone would rather run the right pack, though, I prefer the left because it keeps me cool.

[misd-agin]

Yes, they're there for a reason but even the MEL accepts that they're not necessary in all conditions.

[framer]

Without getting my books out I think that the equipment cooling fans do a good enough job if it's less than 38 degrees C. After that, it looks like they need a bit of help from the left recirc fan. Sound about right?

[aviatorhi]

Yes, they're there for a reason but even the MEL accepts that they're not necessary in all conditions.

True, but neither is my radiator when it's between December and February in Fairbanks, AK.

Realistically, I think, the inconvenience of the noise (which I would hardly call an inconvenience) does not outweigh the convenience/necessity/preference of cooling the E&E bay.

[STBYRUD]

...and the reduced load on the operating packs. I have even seen people switching the EE cooling fans to alternate and pull the standby altimeter vibrator CB - also because "that makes the flight deck quieter". My response was to suggest shutting down the engines and APU - that would certainly make the flight deck quieter :ugh:

[RMC]

The justification I have had for doing this is that it recycles poor quality (implication being virus infected) air from the cabin back into the cockpit so more likely to catch something from the great unwashed.
On the other side of the coin can someone confirm that with the fans off no recirculation would mean a greater bleed offtake (slightly higher fuel consumption).

[Yeelep]

RMC, that's a poor justification for a couple of reasons. One, even with the recirc. fans on, the left pack is the source of air to the flightdeck. Two,the recirc. fans draw air through HEPA filters.
On the other side of the coin can someone confirm that with the fans off no recirculation would mean a greater bleed offtake (slightly higher fuel consumption). Yes.

[eagleflier]

My FCOM says I can turn off the recirc fan below 10,000' to reduce FD noise which is exactly what I do. On the ground, if the packs are cooling sufficiently, the recirc fan goes off as well.
Only the Left one on the -800 tho as the right one ain't as noisy.

[de facto]

The justification I have had for doing this is that it recycles poor quality (implication being virus infected) air from the cabin back into the cockpit so more likely to catch something from the great unwashed.
Rmc,no recirculated air goes into the flight deck.:hmm:

For your question:fans off =fuel burn up.

[stallfail]

Description:
Operators have indicated the 737NG flight deck produces a higher level of noise than the 737 Classic flight deck. Boeing is actively working on reducing the noise generated in the flight deck by reworking the Environmental Control System including the recirculation fan noise and nose section boundary layer noise.

Portions of PRR 38592 have been implemented beginning at line number 1552, delivered 12 August 2004 (see incorporation line numbers under "Applicability" above). Service Bulletin 737-25-1543 which incorporates the intent of the PRR was released 9 November 2012.

PRR 38955 has been implemented beginning at line number 3770 delivered Sept 2011. Service Bulletin 737-21-1178 which will incorporate the intent of the PRR is anticipated for release Jan 2013.
Status
PRR 38592 "Flight Deck Noise Reduction" has been committed for production airplanes:

Part A - Add damping panels on skin interior between frames

Part B - Revise overhead lining for new ECS plenum
Add vortex generators to exterior skin below the #1 window
Add gap fill material to fwd edge of #2 window
New muffler build process to reduce noise
Re-route flex hose to reduce bends
Redesign plenum to reduce noise

Part C - Retrofit Service Bulletin - SB 737-25-1543

Part D - Increase density of insulation blanket.

PRR 38955 "Flight Deck Air Distribution Improvements" has been committed for production airplanes:

Part A - Overhead and shoulder plenums modified to reduce noise in the flight deck.

Part B - Retrofit Service Bulletin - SB 737-21-1178
Interim Action
Service Letter 737-SL-21-067, notified operators of a new muffler, Boeing P/N 65C25629-94 which was developed to eliminate the collapsed lining material. Additionally, the new muffler includes enhanced noise reduction features.

The original flight deck muffler, Boeing P/N 65C25629-85 is a component of existing line replaceable unit (LRU) muffler assemblies. The new muffler, Boeing P/N 65C25629-94 will be part of new LRU muffler assemblies. Boeing does not have retrofit solutions for installing the new muffler in the existing muffler assembly, but the existing muffler assemblies may be replaced with the new muffler assemblies as noted in the service letter.
Final Action
Part A, Part B and Part D of PRR 38592 have been incorporated in production at airplane line positions 1552, 1650, and 1696, delivered Aug 2004, Feb 2005 and April 2005 respectively.

Service Bulletin 737-25-1543 was released 9 November 2012 (Part C of PRR).

Part A of PRR 38955 (flight deck shoulder plenum and ducting redesign) was installed in production at line position 3770, delivered Sept 2011.

Service Bulletin 737-21-1178 will be published for incorporation of the modifications equivalent to production for Flight Deck Noise Reduction (Part B of PRR). ECD is late Jan 2013.

[Sciolistes]

framer,
Without getting my books out I think that the equipment cooling fans do a good enough job if it's less than 38 degrees C. After that, it looks like they need a bit of help from the left recirc fan. Sound about right? Sorry, to do this to you. Have a look at the Hot Weather supp proc. After eng start, eng bleeds off, APU bleed on, packs to high.

According to FCOM 2 packs to high maximizes the airflow with the APU. According to a later paragraph this turns off the left recirc fan.

You will notice with the APU and packs high, there is a reduction in FD noise that corresponds to the recirc being swtiched off.

Note the conditions for recirc fans are dependent on air/ground logic. In flight both recirc fans will be off with both packs in high. On the ground just the left recirc fan. So I figure the left fan is not essential for equip cooling. The description of the forward cargo compartment in Chap 2 and Equip Cooling in Chap 2 suggest the recirc fans have nothing to do with equip cooling. But given at least one recirc fan will be operating on the ground I guess it does that mean it is more difficult to cool people on the ground.