B738 Air Systems (Recirculation fans, Temperature control, Press controller)

[unintentionallyblank]

Hello, I am studying in preparation for a B738 TR, and I have come across a few questions while reading about the Air Systems. I believe the answers to these questions will not have to be expanded enough to warrant four individual new threads, so I hope no one minds I'll put them all in the same thread.

1) The FCOM lists conditions for when L/R recirculation fans will be operating. All of these conditions stated include that both packs are operating. Will any of the recirculation fans ever be used during single pack operations? If not: why not?

2) For the Left/Right Electronic Controllers (Temperature controllers), are the designations "left" and "right" in any way related to the "left" and "right" designations of the packs, or is "Left/Right" just a way of keeping the two electronic controllers apart?

3) Without trim air being available, and both packs on the standby pack controls, is flight deck zone temperature selector always ignored (for both the left/right pack)?

4) From what I understand, the manual outflow valve control operates a dc motor separate from the automatic pressure controller. What about the alternate automatic pressure controller, does it also control a dc motor separate from both the normal pressure controller and the manual control?



Cheers!

[Callsign Kilo]

For single pack operation (bleed trip off, wing body overheat isolation, s/e flight etc) there is no reference to turning the recirculations fans off. With single pack the operating pack reverts to high flow which will increase bleed demand. The recirc fans will in turn reduce bleed demand as they are effectively recirculating conditioned air to the mix manifold. You would be making poor use of the system if you turned them off, unless of course you had a fire!!

I don't believe there is any direct link between left and right pack controllers and the left and right packs. It is simply a system with redundancy. I may be wrong.

A smarter man than me probably has a better answer to the above and q's 3&4

Best of luck

[unintentionallyblank]

Thanks for your answer.

What confuses me is that the recirc fans do not have an on-mode, only AUTO and OFF, and quoting the FCOM (my emphasis):

AUTO –
• in-flight–

• the left recirculation fan operates if both packs are operating unless either PACK switch is in HIGH
• the right recirculation fan operates if both packs are operating unless both PACK switches are in HIGH.

• on the ground–

• the left recirculation fan operates unless both PACK switches are in
HIGH
• the right recirculation fan operates even if both PACK switches are in
HIGH.

It does not specify any condition where the fan will be operating unless both packs are operating. (Except on the ground, where it makes no mention of it.)

[ctdesing]

if trim air isn't available, the pack #1 air temp is equal to cont cab temp selector, the pack #2 air temp is equal to the average fwd and aft cabin temp selectors. It's explained on the Flight Safety Boeing 737-800 CBT.

[aveng]

1)The flow control valve closed and auto relays and
air/ground relay form a logic circuit for the R429 fan
relay and are in the air conditioning accessory units
(ACAU). This logic looks at pack flow conditions. If
one (L or R) of the pack valve closed relays (pack off) is
energized, the recirculation fan operates to increase
cabin ventilation. If one (L or R) of the pack valves
have their auto relays energized, the recirculation fan
operates.
The air conditioning overboard exhaust valve reconfig
cont circuit breaker is for unpressurized dispatch. If
the circuit breaker is open, the bypass circuit enables
the recirculation fan to operate if the smoke control
relay R648 energizes. The smoke control relay is
energized during these conditions:
- Left or right pack switch is in HIGH
- Airplane is in the air
- R RECIRC FAN switch is off.
2) Related to the normal controller for each pack ie. L pack controller = normal control for l pack.
3) AIR CONDITIONING - TEMPERATURE CONTROL - UNBALANCED MODE
General Description
The unbalanced mode is when the flight compartment trim
air system fails or the trim air switch is OFF.
There are two unbalanced modes of operation:
- Unbalanced
- Unbalanced average.
Unbalanced
The right pack is controlled to satisfy the colder
demand of the two passenger compartment temperature
zones. The left pack is controlled to satisfy only the
temperature requirements of the flight compartment.
The backup flight compartment cabin temperature sensor
and cabin temperature selector provide the pack demand
signal to the left auto pack control.
The backup flight compartment duct temperature sensor
signal is used to provide a 35F(2C) limit in the flight
compartment duct.
The primary flight compartment cabin temperature sensor
and cabin temperature selector provide the standby pack
demand signal to the right auto pack control.
Unbalanced Average Mode
The unbalanced average mode of operation is activated
if the trim air switch is OFF or if either of the
passenger compartment temperature zones fail.
This mode is the same as the unbalanced mode except for
the right pack. The right pack is controlled to satisfy
the average temperature requirements of the two
passenger compartment temperature zones.
4) The outflow valve has these three motors:
- Two AUTO motors (dc) with electronic actuators controlled by different CPC’s.
- One MANUAL motor (dc).
Normal and standby pack control is identical and actually controller in charge swaps with alternate flights.

Hope you ace it