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packs are feeding all three zones. Whichever zone is commanding the coldest
temperature will drive BOTH packs to that temp. Hot air is then added to any
other zone that is commanding a higher temp. This hot air is called trim air and is
how the zone temp system controls temperatures in three zones with only two
packs.
There are three air conditioning zones: Cockpit, FWD Cabin and AFT Cabin.
The zones are controlled by having the packs deliver all air at the lowest temp
requested by any of the three zones. Then hot air is added through the trim air
valves to the other two zones as needed to meet temp requirements. A/C zone
temp selectors have a range of: Cold 18°C/64°F, 12 o’clock 24°C/76°F, Hot
30°C/86°F
The AC pack can bypass bleed air around the air cycle machine (ACM) if the
ACM fails and run the bleed air through the primary heat exchanger directly. This
allows the pack to operate as a simple heat exchanger with reduced pack flow.
Pack flow will revert to HI during single pack operation or APU bleed source
regardless of selector position.
US Airways Airbus A319, A320, A321 Notes
20
The Zone controller can override pilot selected pack flow (HI, NORM and LOW)
as needed to meet demands. It can also command higher APU speed or engine
idle as needed.
One Zone controller with two channels. Failure of the primary channel will result
in fixed temperature at 76° F with no optimization. Failure of the secondary as
well will result in a fixed temp of 68° F pack 1 and 50° F pack 2.
One Pack controller per pack. Two channels per controller. If primary fails the
secondary pack air flow will be fixed at the pre-failure setting. No further
optimization is available. Further failure of the secondary will result in a fixed
pack outlet temp of 59° F.
Pack controllers also regulate the cooling air flow through the ACM. During
takeoff and touchdown the controllers close the ram air inlet flaps to prevent
ingesting debris.
Note: The Airbus 319/320/321 can be dispatched with one pack INOP up to
FL310 or below as per MEL 21-5201A
When sitting on the gate with AC established (APU or EXT PWR ON) the PACK
1 & 2 amber FAULT lights will be on when the packs are not supplied (no APU
bleed or external high pressure air).
RAM air
RAM air is available for cabin ventilation in the event of loss of pressurization or
smoke removal. When the RAM AIR pb is selected the RAM air inlet opens.
When pressurization differential is less than 1 psi. the outflow valve will open to
50% to allow exhaust. If above 1 psi. then the outflow will remain normal.
Pressurization
There are two identical independent pressurization systems. Control is normally
fully automatic. The system has one control panel, two controllers, one outflow
valve and two safety valves. The outflow valve has three DC motors: Primary,
Backup and Manual. Controllers can operate in automatic, semi-automatic and
manual modes.
Automatic: Controller automatically takes the destination field elevation from the
aircraft database. The entire pressurization schedule is optimized by the system.
US Airways Airbus A319, A320, A321 Notes
21
Semi-automatic: If the database is not available for some reason the pilot can
select the landing elevation from the LDG ELEV knob by pulling the selector out
of the AUTO detent and turning to the needed value.
Manual: Normally, the controllers take turns controlling by swapping after each
leg. If the active controller fails the backup automatically takes over. If both
automatic systems fail the pilot may control manually by pressing the CABIN
PRESS MODE SEL to MAN. The primary and backup outflow valve motors are
depowered and the manual motor is activated. Now the pilot can select vertical
speed on the cabin using the MAN V/S CTL switch.
Abort mode: If the aircraft returns after takeoff the system will reset to departure
field elevation.
Ditching pb: The Ditching pb will close all exterior openings below the flotation
line. This pb is also used during deicing to prevent deicing fluid from entering the
aircraft.
 Airbus Gotcha: on ground with Ditching pb ON and all doors closed & external
low pressure connected a pressurization differential will build.
Note: If the pilot suspects that pressurization is not performing normally but has
not yet failed press the MODE SEL pb to MAN for 10 secs. then return to AUTO.
This will cause the systems to swap.
Depressurization: When cabin exceeds about 11,000’ the cabin may illuminate
and Exit and all cabin signs illuminate automatically. Masks will automatically
drop at 14,000’ cabin altitude.
　
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