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BOEING 737 SYSTEMS REVIEW
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AIR CONDITIONING & PRESSURIZATION
1. GENERAL
¡ Conditioned air comes from either the aircraft air-conditioning system or a preconditioned ground
source. (engines, APU, ground cart)
¡ Air from the preconditioned ground source enters the air-conditioning system through the mix
manifold. The air-conditioning system provides temperature controlled air by processing bleed air
from the engines, APU or a pneumatic ground source through two air-conditioning packs.
¡ Conditioned air from the left pack flows directly to the cockpit. Excess air from the left pack, air from
the right pack and air from the recirculation system is mixed in the mix manifold. The mixed air is
then distributed to the passenger cabin.
¡ Recirculation Fan(s) control the recirculation system which maintains proper ventilation while
economizing the use of bleed air. It collects air from the aircraft cabin, filters it and returns it to the
mix manifold to be mixed with fresh conditioned air supplied by the packs.
737-300/-500
¡ Pack control is provided by the two temperature selectors and cabin temperature sensors.
¡ Pack temperature control is unbalanced. The Control Cabin (cockpit) Temperature Selector controls
the pack outlet temperature of the left pack.
¡ The Passenger Cabin Temperature Selector and Cabin Temperature Sensor control the pack outlet
temperature of the right pack. Since excess air from the left pack is mixed with the air from the right
pack in the mix manifold, changing the pack outlet temperature of the left pack (via the Control Cabin
Temperature Selector) will also change the outlet temperature of the right pack to obtain the same
temperature demand for the passenger cabin.
737-400/-800/-900
¡ Pack temperature control is balanced. The outlet temperature of both packs is normally controlled at
the same temperature by two electronic controllers.
¡ The pack outlet temperature is determined by the zone that requires the most cooling (Control Cabin,
Forward Cabin or Aft Cabin).
¡ A three-zone trim air system provides individual zone temperature control by adding high temperature
air from the pneumatic system to those zones that have a higher temperature demand than the pack
outlet temperature. Any trim air failure will cause the packs to revert to independent operation.
¡ Only in case of failure of the trim air system, pack temperature control will become unbalanced.
2. MAIN COMPONENTS & SUBSYSTEMS
BLEED AIR SUPPLY
¡ The pack valves control the flow of bleed air from the aircraft pneumatic system to the air conditioning
packs. A single pack is capable of maintaining pressurization and acceptable temperature
throughout the aircraft.
¡ Two pack operation from a single engine bleed air source is not recommended due to excessive
bleed requirements.
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PACK TEMPERATURE CONTROL 737-300/-500
¡ Cockpit and Cabin temperature is controlled by two controllers, controlling respectively the LH and
RH Pack temperatures. This is automatic using an auto temp. controller with a manual control as
back up.
PACK AND ZONE TEMPERATURE CONTROL 737-400/-800/-900
¡ There are three zones : control cabin (cockpit), forward cabin and aft cabin.
¡ Desired zone temperature is set by adjusting the individual Temperature Selectors.
¡ The packs produce an air temperature which will satisfy the zone which requires the most cooling.
Zone temperature is controlled by adding the proper amount of trim air to the air leaving the mix
manifold through the zone supply ducts.
¡ The quantity of trim air is regulated by individual trim air modulating valves.
¡ If air in a zone supply duct overheats, the associated amber ZONE TEMP light illuminates, and the
associated trim air modulating valve closes. The trim air modulating valve may be reopened after the
duct has cooled by pushing the TRIP RESET Switch.
Electronic Controllers
¡ Control is performed by two electronic controllers, the left and right electronic controller which have
the following functions:
Left controller
Right controller
-
- FWD CABIN zone
- primary control of CTL CABIN zone
- RH pack temp control valve
- LH pack standby temp control valve
- RH ram-air door
AFT CABIN zone
back-up control of CTL CABIN zone
LH pack temp control valve
RH pack standby temp control valve
LH ram-air door
Zone Temperature Control Modes
¡ The left electronic controller controls the aft cabin zone and provides backup control for the cockpit.
The right controller controls the forward cabin zone and provides primary control for the cockpit.
¡ Failure of the primary cockpit temperature control will cause an automatic switch to the backup
control and will illuminate the CONT CAB amber ZONE TEMP Light upon Master Caution Recall.
Failure of both the primary and standby controls will illuminate the light automatically.
¡ Failure of the forward or aft cabin temperature control will cause the associated trim air modulating
valve to close. The Temperature Selectors operate normally, but the Temperature Selector settings
of the two passenger cabin zones will be averaged. The amber ZONE TEMP Light will illuminate
upon Master Caution Recall to indicate failure of the associated zone control.
¡ During single pack operation, the Temperature Selector settings of all three zones will be averaged.
3. AIR CONDITIONING PACK 737-300/-500
¡ The flow of bleed air from the main pneumatic duct through each air conditioning pack is controlled
by the respective pack valve.
¡ The left and right packs are completely independent. (Left pack supplies 20% to cockpit & 80 % to
mix manifold while Right pack supplies 100 % to manifold â smoke into the cockpit has a great
chance to come from the Left pack)
¡ Cabin altitude can be maintained with only one pack operating.
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¡ Normally the left pack uses bleed air from engine no. 1 and the right pack uses bleed air from
engine no. 2.
¡ The output of the packs is combined in the mix manifold.
Air Mix Valves
¡ Air that flows through the cold mix valve is processed through a cooling cycle and then combined with
hot air flowing from the hot mix valve in the mixing chamber.
¡ For each pack, the two air mix valves control hot and cold air according to the setting of the Control
Cabin or Passenger Cabin Temperature Selector. In the automatic temperature mode, the mix
valves are operated by the temperature controller.
¡ The temperature controller uses inputs from the respective Temperature Selector and cabin
temperature sensor. The automatic temperature controller is bypassed when the Temperature
Selector is positioned to MANUAL.
¡ Anytime the pack valve closes, the air mix valves are automatically driven to the full cold position.
This aids start up of the cooling cycle and prevents nuisance hot air trips when the air conditioning
pack is switched on.
Cooling Cycle
¡ The flow through the cooling cycle starts by passing through a primary heat exchanger. Air flows to
the compressor of an air cycle machine where the air is compressed and temperature increased.
Next, the air circulates through a secondary heat exchanger for additional cooling.
¡ The airflow then passes through the turbine of the air cycle machine where it is cooled by expansion.
The cold air flows to a water separator which removes moisture that has condensed out of the air by
operation of the air cycle machine. The moisture extracted from the air is injected in the ram air duct
to aid in cooling the heat exchangers.
¡ To prevent icing in the water separator, a temperature sensor signals the water separator anti-ice
valve to provide some warming air automatically.
¡ The processed cold air is then combined with hot air in the mixing chamber. The conditioned air
flows into the mix manifold and distribution system.
¡ Overheat protection is provided by temperature sensors located in the air cycle machine. An
overheat condition in the compressor outlet duct or turbine inlet duct causes the pack valve to close
and the PACK TRIP OFF Light to illuminate.
Airflow Control
¡ With both Air Conditioning Pack Switches in AUTO and both packs operating, the packs provide
"normal air flow". However, with one pack valve closed, the other pack automatically switches to
"high flow" in order to maintain the necessary ventilation rate. This automatic switching is inhibited
on ground, or in flight with the flaps extended, to insure adequate engine power for single engine
operation.
¡ If both Engine Bleed Air Switches are OFF and the APU Bleed Air Switch is ON, the working pack is
fixed in "high flow".
¡ High flow mode can be manually selected by positioning the Pack Switch to HIGH.
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4. AIR CONDITIONING PACK 737-400/-800/-900
¡ The flow of bleed air from the main pneumatic duct to each air conditioning pack is controlled by the
respective pack valve. The packs are independent and normally operate in parallel. Normally, the
left pack uses bleed air from Engine No. 1 and the right pack uses bleed air from Engine No. 2. The
output of the packs is combined in the mix manifold. (Left pack supplies 20% to cockpit & 80 % to mix
manifold while Right pack supplies 100 % to manifold â smoke into the cockpit has a great chance
to come from the Left pack)
¡ Cabin altitude can be maintained with only one pack operating.
¡ Two pack operation from a single engine bleed air source is not recommended due to excessive
bleed requirements.
Cooling Cycle
¡ Flow through the cooling cycle begins as bleed air passes through the primary heat exchanger. Air is
then routed to the compressor section of the air cycle machine where as it is compressed the
temperature increases. Next the air circulates through a secondary heat exchanger for additional
cooling. The air then passes through a high pressure water separator where air is dehumidified. The
moisture extracted from the air is injected into the ram air duct to aid in cooling the heat exchangers.
The air then passes through the turbine section of the air cycle machine, where final cooling by
expansion occurs.
¡ To prevent icing in the water separator, a temperature sensor signals the electronic controllers to
increase the Air Cycle Machine turbine outlet temperature.
¡ The processed cold air is then combined with hot air which has bypassed the air cycle machine
through the pack temperature control valve. This conditioned air then flows back through the high
pressure water separator and into the mix manifold and distribution system.
¡ Overheat protection is provided by temperature sensors located in the air cycle machine and pack
outlet. An overheat condition in the compressor outlet duct, turbine inlet duct, or pack discharge
outlet will cause the PACK Light to illuminate and the pack valve to close resulting in a pack
shutdown.
Pack Temperature Control
¡ Electronic controllers command the pack temperature control valve toward open or closed to satisfy
pack discharge requirements.
¡ If a primary pack control fails, the affected pack is controlled by the standby pack control in the
opposite controller. A primary or standby pack control failure causes the PACK, Master Caution and
AIR COND annunciator Lights to illuminate upon Master Caution recall.
¡ If both the primary and the standby pack controls fail for the same pack, the PACK, MASTER
CAUTION and AIR COND annunciator Lights illuminate. The pack will continue to operate without
control unless excessive temperatures cause the pack to trip off.
Airflow Control
¡ With both Air Conditioning Pack Switches in AUTO and both packs operating, the packs provide,
normal air flow. However, with one pack valve closed, the other pack automatically switches to "high
flow" in order to maintain the necessary ventilation rate. This automatic switching is inhibited on
ground, or in flight with the flaps extended, to insure adequate engine power for single engine
operation.
¡ Automatic switching also occurs if both Engine Bleed Air Switches are OFF and the APU Bleed Air
Switch is ON, since the working pack is then fixed in "high flow". High flow mode can be manually
selected by positioning the Pack Switch to HIGH.
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5. RAM AIR SYSTEM
¡ The ram air system is used to provide cooling for the heat exchangers in the air conditioning system.
Operation of the system is automatic.
¡ During flight, the ram air modulating system automatically regulates airflow through the system. A
temperature sensor in the air cycle machine (ACM) compressor discharge duct commands, (737300/-500) a ram air controller (737-400/-800/-900) the left and right electronic controllers, which
controls airflow through the system.
¡ The controller modulates the mechanically linked ram door and (737-300/-400/-500) exhaust louvers
to maintain the required cooling airflow across the heat exchangers. In normal cruise, the ram doors
modulate between open and closed.
¡ On ground, or in flight with the flaps not up, the ram door will move to the full open position for
maximum cooling. The RAM DOOR FULL OPEN Light illuminates whenever the ram door is fully
open.
737-300/-400
¡ A turbofan is located in each ram air exit duct just upstream of the exit louvers. It augments the ram
airflow on ground or in flight with the flaps not retracted.
¡ The fan operates pneumatically using bleed air. It is activated electrically, when the pack is on, by
the air-ground safety sensor or flap limit switch.
737-600/-700/-800/-900
¡ A turbofan, mechanically driven by the air cycle machine, augments the ram airflow.
Deflector Door
¡ A deflector door is installed forward of the ram air inlet doors to prevent debris ingestion prior to liftoff
and after touchdown.
¡ The deflector door extends electrically when air-ground safety logic is in the ground mode.
6. AIR CONDITIONING DISTRIBUTION
¡ Conditioned air is collected in the mix manifold. The temperature of the air is directly related to the
setting of the Temperature Selectors.
Cockpit
¡ Since the cockpit requires only a fraction of the air supply provided by the left pack, most of the left
pack output is routed to the mix manifold.
¡ Conditioned air for the cockpit branches into several risers which end at the floor, ceiling and foot
level outlets. There are air diffusers on the floor under each seat. They cannot be controlled and air
flows continuously as long as the manifold is pressurized.
¡ Overhead diff users are located on the cockpit ceiling, above and aft of the no.3 windows. Each of
these outlets can be opened or closed as desired by turning a slotted adjusting screw.
¡ There is also a dual purpose valve behind the rudder pedal of each pilot. These valves provide air
for warming the pilot's feet and for defogging the inside of the no. 1 windshields. Each valve is
controlled by knobs located on the Captain's and First Officer's panel.
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Passenger Cabin
¡ The passenger cabin supply distribution system consists of the mix manifold, sidewall risers, and an
overhead distribution duct.
¡ Sidewall risers go up the right and left wall of the passenger cabin to supply air to the overhead
distribution duct. The overhead distribution duct routes conditioned air to the passenger cabin. It
extends from the forward to the aft end of the ceiling along the aircraft centerline and also supplies
the sidewall diffusers.
Forward Cargo Compartment
¡ The forward cargo compartment is warmed in flight when more than 1.0 psi pressure differential
exists. Air from the E & E compartment flows up and around the forward cargo compartment lining.
The recirculation fan maintains this warming air flow.
¡ With the recirculation fan(s) off, the forward outflow valve remains open to ensure this warm air flow
(except when the main outflow valve is nearly closed in order to maintain pressurization).
Conditioned Air Source Connection
¡ A ground air conditioning source may be connected to the mix manifold.
Recirculation Fans 737-300/-500
¡ The recirculation fan system reduces the air conditioning pack load and the engine bleed air
demand. The air for the recirculation fan is exhaust air from the main cabin and electrical equipment
bay collected in a shroud located above the forward cargo compartment. (basic principle : it reduces
bleed demand by the packs by increasing the volume of air into the mix manifold; it reduces also air
conditioning system pack load & creates low pressure area to pull EE compartment hot air to radiate
around forward cargo bin)
¡ This air is filtered and recirculated to the mix manifold (full cabin air exchange every 2 or 3 hours).
The fan operates with the switch in AUTO except with both packs on and one or both in HIGH. The
fan is driven by an AC motor.
Overheat Detection 737-300/-500
¡ Overheat detection is provided by temperature sensors located downstream of the packs.
¡ An overheat condition causes the appropriate mix valves to drive to full cold and the DUCT
OVERHEAT Light to illuminate. A temperature higher than the duct overheat causes the appropriate
pack valve to close and the PACK TRIP OFF Light to illuminate.
Recirculation Fans 737-400/-800/-900
¡ The recirculation fan system consists of two fans and reduces the air conditioning pack load and the
engine bleed air demand. The air for the recirculation fans is exhaust air from the main cabin and
electrical equipment bay. (basic principle : it reduces bleed demand by the packs by increasing the
volume of air into the mix manifold; it reduces also air conditioning system pack load & creates low
pressure area to pull EE compartment hot air to radiate around forward cargo bin)
¡ This air is filtered and recirculated to the mix manifold. (full cabin air exchange every 2 or 3 hours)
¡ The fans are driven by AC motors. Each recirculation fan will operate only if the respective RECIRC
FAN Switch is selected to AUTO. In flight, if both packs are operating and either pack is switched to
HIGH, the left recirculation fan will shut off. The right recirculation fan will operate in flight unless
both packs are in high.
¡ On ground, the right recirculation fan will operate even if both Pack Switches are in HIGH. The left
recirculation fan will operate on ground except when both packs are in HIGH.
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Overheat Detection 737-400/-800/-900
¡ Overheat detection is provided by temperature sensors located downstream of the packs and the mix
manifold. An overheat condition causes the appropriate trim air modulating valve to close and the
ZONE TEMP Light to illuminate.
7. EQUIPMENT COOLING
¡ EFIS equipment, circuit breaker panels in the cockpit and electronic equipment in the E & E
compartment are cooled by the equipment cooling system. Warm air from the equipment is routed to
the forward cargo compartment or dumped overboard.
¡ Flight Instrument panels and circuit breaker panels in the cockpit and electronic equipment in the E &
E compartment are cooled by the equipment cooling system. Warm air from the equipment is ducted
away by an AC powered exhaust fan.
¡ On ground, or with the cabin differential pressure less than 1.0 psi, the exhaust fan air is routed
through a flow control valve (737-300/-400/-500) or overboard exhaust valve (737-600/-700/-800/900) and dumped overboard at the bottom of the aircraft.
¡ With increasing airflow at greater cabin differential pressures, the flow control valve closes. Warm air
from the electronic equipment is then diffused around the forward cargo compartment.
¡ Failure of an equipment cooling fan illuminates the respective Equipment Cooling OFF Light.
Selecting the alternate fan should restore airflow and extinguish the OFF Light.
¡ The supply fan draws cool air from the passenger cabin and furnishes it to the Flight Instrument
panels. The exhaust fan draws air from the Flight Instrument panels, CB panels and E & E
compartment. Additional thermal switches are located in the E & E compartment.
¡ If an overtemperature occurs on ground or if the equipment cooling fails, alerting is provided through
the crew call horn in the nose wheel well.
¡ Normal power = Main Bus No.1 / Alternate power = Main Bus No.2 (if APU is INOP or you try to taxiin on 1 engine without APU, you will get the lights & horn !
For B737s NG, if the EQUIP COOLING SUPPLY OFF light illuminates during flight indicating a
subsequent failure of the remaining associated fan, continued flight > 30 minutes can result in a loss of
Captain's Display Units & the Lower Center Display Unit.
if the EQUIP COOLING EXHAUST OFF light illuminates during flight indicating a subsequent failure of
the remaining associated fan, continued flight > 30 minutes can result in a loss of F/O's Display Units &
the Upper Center Display Unit.
8. PRESSURIZATION
¡ The aircraft is pressurized by bleed air supplied to and distributed by the air conditioning system.
Pressurization and ventilation are controlled by varying the opening of outflow valves. A proportional
relationship is maintained between ambient and cabin pressure in climb or descent, and a maximum
differential is normally maintained in cruise.
¡ Cabin pressurization is controlled by regulating the discharge of conditioned air through the outflow
valves.
¡ Pressurization control is provided by the Electronic Cabin Pressure Controller which controls the
main outflow valve. The main outflow valve controls the air flow out of the airplane fuselage & it is
actuated :
- 737-300/400 : by an AC or a DC motor
- 737-600/700/800/900 : by 3 DC motors. (all use the same actuator mechanism)
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Pressure Relief Valves
¡ Two pressure relief valves provide maximum safety pressure relief by limiting the differential pressure
to a maximum of 8.65 psi (737-600/-700/-800/-900 : 9.1 psi).
¡ A negative relief valve prevents external atmospheric pressure from exceeding internal cabin
pressure. (you can check it during the walkaround by pushing door inward)
Pressurization Control 737-300/-400/-500
¡ Pressurization control in provided automatically by a single pressurization controller.
¡ The pressurization system controls cabin altitude in anyone of four modes:
¡ AUTO - Automatic; the normal mode of operation. Uses AC motor.
STBY - Semi-automatic; a standby system in the event of AUTO failure. Uses DC motor.
MAN AC - Manual control of the system using AC motor.
MAN DC - Manual control of the system using DC motor.
¡ In the automatic mode of operation, aircraft altitude is sensed directly from the static ports. In the
standby mode, aircraft altitude is sensed electrically from the Air Data Computer (ADC). Barometric
corrections to these pressures come from the Captain's altimeter in AUTO and the First Off icer's
altimeter in STBY. The controller receives additional information from the air/ground safety sensor
and cabin pressure altitude sense port.
¡ The main outflow valve can be actuated by either an AC or a DC motor. The AC motor is used
during AUTO and MAN AC operation. The DC motor is used during STANDBY and MAN DC
operation. The forward outflow valve closes automatically to assist in maintaining cabin pressure
when the main outflow valve is almost closed or when the recirculation fan is operating.
¡ On 737-400 aircraft, the system considers the operation of the right recirculation fan.
Pressurization Control 737-600/-700/-800/-900
¡ Pressurization control is provided automatically by two pressurization controllers.
¡ The pressurization system controls cabin altitude in anyone of 3 modes.
¡ AUTO - Automatic; the normal mode of operation. If the active controller fails, automatic switching
will occur to the remaining controller.
ALTN - Cabin pressure control is controlled by the remaining controller.
MAN - Manual control mode if AUTO and ALTN modes are unserviceable.
¡ Barometric altitude is provided by the Air Data Inertial Reference Units (ADIRUS) that receive
barometric correction inputs from the Captain's and First Officer's BARO Set knobs.
¡ The 737-800 is equipped with one (main) outflow valve, actuated by DC motors only.
9. PRESSURIZATION OUTFLOW
737-300/-400/-500
¡ Cabin air outflow is controlled by the main outflow valve, the forward outflow valve, and the flow
control valve. During pressurized flight, the flow control valve is closed, and the majority of the
overboard exhaust is through the main outflow valve. Passenger cabin air is drawn through foot level
grills, down around the aft cargo compartment, where it provides heating, and is discharged
overboard through the main outflow valve. A small amount is also exhausted through the toilet and
galley vents, miscellaneous fixed vents, and by seal leakage.
¡ The flow control valve opens to exhaust the cooling air from the E & E compartment overboard during
ground operation, unpressurized flight, and pressurized flight below a cabin differential pressure of
approximately 1.0 psi.
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¡ When the flow control valve closes, air is directed around the forward cargo compartment liner for
inflight heating.
¡ The forward outflow valve is the overboard discharge exit for air circulated around the forward cargo
compartment (located approximately under first cabin window on Left side of the aircraft). The valve
is closed whenever the recirculation fan is operating. You can "refresh" the aircraft (for example with
Full PAX load) by placing R pack to HI (RECIRC fan is then turned OFF & the forward outflow valve is
opened adding more fresh air…)
¡ On 737-400 aircraft, the valve is closed whenever the right recirculation fan is operating.
737-600/-700/-800/-900
¡ Cabin air outflow is controlled by the main outflow valve and the overboard exhaust valve. During
pressurized flight, the overboard exhaust valve is closed, and the majority of the overboard exhaust is
through the main outflow valve. Passenger cabin air is drawn through foot level grills, down around
the aft cargo compartment, where it provides heating, and is discharged overboard through the main
outflow valve. A small amount is also exhausted through the toilet and galley vents, miscellaneous
fixed vents, and by seal leakage.
¡ On ground and in flight with low differential pressure, the overboard exhaust valve is open and warm
air from the E & E bay is discharged overboard. In flight, at higher cabin differential pressure, the
overboard exhaust valve is normally closed and exhaust air is diffused to the lining of the forward
cargo compartment. However, the overboard exhaust valve remains open if both pack switches are
in high and the recirculation fan is off. This allows for increased ventilation in the smoke removal
configuration as there is no forward outflow valve installed.
10. PRESSURIZATION CONTROL 737-300/-400
Auto Mode Operation
¡ In AUTO, the pressurization control panel is used to preset two altitudes into the pressure controller :
- FLT ALT (flight or cruise altitude).
- LAND ALT (destination airport altitude).
¡ Take-off airport altitude (actually cabin altitude) is automatically fed into the pressurization
controller at all times when on ground. The air/ground safety sensor signals whether the aircraft is on
ground or in flight.
¡ On ground, the FLT/GRD switch is used to keep the cabin depressurized by driving the main outflow
valve full open when the switch is in the GRD position. With the switch in the FLT position, the
controller modulates the main outflow valve toward close, pressurizing the cabin to .1 psid.
¡ This ground pressurization of the cabin makes the transition to pressurized flight more gradual for the
passengers and crew, and also gives the system better response to ground effect pressure changes
during take-off.
¡ In flight, the pressure controller maintains a proportional pressure differential between flight and cabin
altitude. By climbing the cabin altitude at a rate proportional to the aircraft climb rate, cabin altitude
change is held to the minimum rate required.
¡ Maximum cabin rate is 500 sea level ft/min during climb and 350 sea level ft/min during descent.
¡ Approximately 1 000 feet below flight altitude (actually when outside air pressure is within .25 psid of
the pressure that will exist when the aircraft is at selected FLT ALT) a cruise relay trips. The
controller schedules a constant cabin altitude during cruise using a 7.45 psid (7.80 psid with FLT ALT
> 28000 ft) between flight and cabin altitudes.
¡ An amber OFF SCHED DESCENT Light illuminates if the aircraft begins to descend without the
cruise relay being tripped; for example, a flight aborted in climb and returning to the take-off airport.
The controller programs the cabin to land at the take-off field elevation without further pilot inputs.
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If the Flight Altitude Indicator is changed or the Flight Altitude Selector is depressed during climb, the
automatic cabin abort capability to the original take-off field elevation will be lost.
During isobaric cruise, minor excursions from flight altitude may cause the pressure differential to go
as high as 7.90 psid to maintain a constant cabin altitude.
Beginning descent, approximately 1 000 feet below cruise altitude (.25 psid), a descent relay trips,
scheduling the cabin to begin a proportional descent to the selected LAND ALT. The controller
programs the cabin to land slightly pressurized (.1 psid), so that rapid changes in altitude during
approach result in minimum cabin pressure changes.
Taxiing in, the controller drives the main outflow valve slowly to full open when the FLT/GRD Switch
is positioned to GRD, thereby depressurizing the cabin. Having the main outflow valve full open also
prevents the equipment cooling exhaust fan from depressurizing the aircraft to a negative pressure.
An amber AUTO FAIL Light illuminates if any one of three conditions occur :
- Loss of AUTO AC power.
- Excessive rate of cabin pressure change (exceeding 1800 sea level feet/minute).
- High cabin altitude (exceeding 13,875 feet).
With illumination of the AUTO FAIL Light, the pressure controller automatically changes to the
STANDBY mode; however, the Pressurization Mode Selector remains in AUTO. Positioning the
Mode Selector to STBY extinguishes the light.
Standby Mode Operation
¡ A green STANDBY Light will be illuminated when the pressure controller is in the STANDBY mode.
In the STANDBY mode the controller allows maximum 7.90 psid.
¡ On ground, the GRD position of the FLT/GRD Switch drives the main outflow valve full open. The
FLT position drives the main outflow valve to attempt to pressurize the cabin to the selected CAB
ALT. CAB ALT should be set 200 feet below the take-off airport altitude to pressurize the cabin
properly when the FLT/GRD Switch is placed to FLT prior to take-off.
¡ In flight, by referring to the placard below the pressurization control panel, the Cabin Altitude Indicator
is set to the isobaric cabin altitude, based on the flight altitude. Cabin rate of climb or descent is
controlled by the Cabin Rate Selector. In descent, the Cabin Altitude Indicator is set 200 feet below
landing field altitude to insure a pressurized cabin during landing.
Manual Mode Operation
¡ A green MANUAL Light illuminates with the Pressurization Mode Selector in MAN AC or MAN DC.
Manual mode allows the pilot, by using the Outflow Valve Switch, to modulate the main outflow valve
while monitoring the Outflow Valve Position Indicator.
¡ MAN AC mode uses the AC motor to control the main outflow valve; MAN DC uses the DC motor.
Full travel of the outflow valve in MAN AC requires approx. 2 seconds, in MAN DC approx. 6
seconds.
11. PRESSURIZATION CONTROL 737-600/-700/-800/-900
Auto and Alternate Mode Operation
¡ The AUTO system consists of two identical controllers, with one controller alternately sequenced as
the primary operational controller for each new flight, The other automatic controller is available as a
backup.
¡ In the AUTO or ALTN mode, the pressurization control panel is used to preset two altitudes into the
auto controllers:
- FLT ALT (flight or cruise altitude);
- LAND ALT (destination airport altitude).
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¡ Take-off airport altitude (actually cabin altitude) is fed into the auto controllers at all times on ground.
¡ The air/ground safety sensor and engine N1 (from the Stall Management Computer) are inputs for the
controllers. On ground and at lower power settings, the cabin is depressurized by driving the outflow
valve to the full open position.
¡ The cabin begins to pressurize on the ground at higher power settings. The controller modulates the
outflow valve toward close, slightly pressurizing the cabin. This ground pressurization of the cabin
makes the transition to pressurized flight more gradual for the passengers and crew, and also gives
the system better response to ground effect pressure changes during take-off.
¡ In flight, the auto controller maintains a proportional pressure differential between aircraft and cabin
altitude. By increasing the altitude at a rate proportional to the aircraft climb rate, cabin altitude
change is held to the minimum rate required.
¡ An amber OFF SCHED DESCENT light illuminates if the aircraft begins to descend without having
reached the preset cruise altitude; for example, a flight aborted in climb and returning to the take-off
airport. The controller programs the cabin to land at the take-off field elevation without further pilot
inputs. If the FLT ALT indicator is changed, the automatic return to the original take-off field elevation
is lost.
¡ The cruise mode is activated when the aircraft climbs to within 0.25 psid of the selected FLT ALT.
During cruise, the controller maintains a pressure differential of 8.35 psid (above 37,000 feet), 7.80
psid (between 28,000 or 37,000 feet) or 7.45 psid (less than 28,000 feet). Deviations from flight
altitude may cause the pressure differential to go as high as 8.45 psid to maintain a constant cabin
altitude.
¡ The descent mode is activated when the aircraft descends 0.25 psi below the selected FLT ALT. The
cabin begins a proportional descent to slightly below the selected LAND ALT.
¡ The controller programs the cabin to land slightly pressurized so that rapid changes in altitude during
approach result in minimum cabin pressure changes.
¡ While taxiing in, the controller drives the outflow valve slowly to full open to depressurize the cabin.
¡ An amber AUTO FAIL Light illuminates if any of the following conditions occur :
- Loss of DC power.
- Controller fault in the operational controller.
- Excessive rate of cabin pressure change (exceeding 2000 sea level feet/minute).
- High cabin altitude (exceeding 15,800 feet).
¡ With illumination of the AUTO FAIL light, pressurization control automatically transfers to the other
auto controller (ALTN mode).
¡ Moving the pressurization mode selector to the ALTN position extinguishes the AUTO FAIL light,
however the ALTN light remains illuminated.
Manual Mode Operation
¡ A green MANUAL Light illuminates with the pressurization mode selector in the MAN position.
¡ In the MAN mode, the outflow valve position switch is used to modulate the outflow valve by
monitoring the cabin altitude panel and valve position on the outflow valve position indicator. A
separate DC motor, powered by the DC standby bus, drives the outflow valve at a slower rate than
the automatic modes. Outflow valve full range of motion takes up to 20 seconds.
12. AIR CONDITIONING PANEL
AIR TEMPERATURE SOURCE SELECTOR (737-300/-500/-600/-700)
¡ SUPPLY DUCT
selects main distribution supply duct sensor for temperature selector
¡ PASS CABIN
selects passenger cabin sensor for temperature indicator
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AIR TEMPERATURE SOURCE SELECTOR (737-400/-800/-900)
¡ SUPPLY DUCT = CONT CAB, FWD CAB and AFT CAB
¡ PASS CAB = FWD, AFT
selects FWD or AFT passenger cabin temperature
¡ PACK = R & L
Selects the pack discharge temperature
¡ AUTO : automatic temp control for the associated zones rotating toward C (cool) or W (warm) sets
the desired temperature (18°C to 30°C)
¡ OFF : closes the associated trim air modulating valve > the left pack will maintain a fixed temperature
of 750F (24°C) the right pack will maintain a fixed temperature of 650 F (18°C).
------------------------------------------------------------------------------------------------------------------------------------------TEMPERATURE SELECTORS (737-300/-500/-700) (L & R)
¡ Flight deck (left) and passenger cabin (right)
AUTO : temperature controlled automatically for passenger cabin controlled thru a temp sensor in
cabin ceiling &controller in electronic equipment bay
MANUAL (springloaded to center): moves mix valves manually (automatic temp controller bypassed)
TEMPERATURE INDICATOR (all 737s)
¡ indicates temperature at location selected with the AIR TEMP source selector
AIR MIX VALVE INDICATORS (737-300/-500/-600/-700)
¡ indicates position of air mix valves left for cockpit, right for passenger cabin
¡ controlled automatically with temperature selector in AUTO, controlled manually with temperature
selector in MANUAL
¡ air mix valves go full cold when pack valve closes, duct overheat, or pack trip off
TRIM AIR SWITCH (737-400/-800/-900)
¡ ON : trim air pressure regulating and shutoff valve signaled open allows bleed air from upstream of
the packs to be directed to the three trim air modulating valves
¡ OFF : trim air pressure regulating and shutoff valve signaled closed left and / or right pack primary
control will control both packs independently the left pack primary control will position the left pack
temp control valve in response to the CONT CAB selector to establish the temp for the flight deck the
right pack primary control will position the right pack temp control valve in response to the cooler of
the AFT CAB or FWD CAB selectors to establish the temp for the passenger cabin.
----------------------------------------------------------------------------------------------------------------------------------------RECIRCULATION FAN SWITCH (737-300/-500)
¡ AUTO :
- Fan is ON, except with both packs ON & 1 or both in HIGH
- 1 pack HIGH or AUTO = fan ON
- Both packs OFF or AUTO = fan ON
- Both packs ON & 1 or both HIGH = fan OFF
¡ OFF : fan OFF
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RECIRCULATION FAN SWITCH (737-400/-800/-900)
¡ an additional recirc fan improves air circulation in the aft fuselage.
¡ AUTO
In flight :
- Left fan is ON when both packs are operating & at least 1 pack is in high flow
- Right fan is ON except when both packs are in high flow.
On ground :
- Left pack will run except when both packs are operating in high flow.
- The right fan will continue to run when both packs are operating in high flow
¡ OFF : fan OFF
RECIRCULATION FAN SWITCH (737-600/-700)
¡ Same except : if the recirc fan is turned off in flight, the overboard exhaust valve (OEV) opens.
-------------------------------------------------------------------------------------------------------------------------------------AIR CONDITIONING PACK SWITCH
¡ AUTO :
- With both packs operating, each pack regulates to normal flow.
- With 1 pack operating, regulates to high flow when in flight with flaps UP
- When operating 1 pack from the APU (both engine bleed switches OFF) regulates to high flow.
¡ HIGH :
- Pack regulates to high flow.
¡ OFF :
- Pack is OFF.
TRIP RESET SWITCH
¡ PRESS :
If the fault condition has been corrected, reset :
- BLEED TRIP OFF
- PACK TRIP OFF (737-300/-400/-500)
- PACK (737-600/-700/-800/-900)
- DUCT OVERHEAT (737-300/-500)
- ZONE TEMP (737-400/-800/-900)
¡ Lights remain illuminated until reset.
13. PRESSURIZATION PANEL
¡ CABIN ALTIMETER / DIFFERENTIAL PRESSURE INDICATOR
- inner scale: indicates cabin altitude in feet
- outer scale: indicates differential pressure between
- cabin and ambient in psi
- connected to alternate static system (only B737-600/-700/-800/-900)
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¡ Air Conditioning & Press, Max Differential pressure (limitation)
- Max differential pressure limit : 8.65 psi (B737-300/-400/-500)
- Max differential pressure limit : 9.1 psi (B737-600/-700/-800/-900)
- Max cabin differential for Takeoff/Landing .1 25 psi
¡ Air Conditioning & Press, Max Differential pressure (operations)
Max operating differential psi :
- 7.8 ±.1 psi above FL280
- 7.45 ±.1 psi at or below FL280
- 8.35 psi above FL370 (only B737-600/-700/-800/-900)
¡ CABIN RATE OF CLIMB INDICATOR
- indicates rate of climb or descent in fpm, normally on descent the auto mode uses a maximum of
350 fpm; on climb, its maximum is 500 fpm
- detects pressure changes from a port on the back of the indicator
¡ ALT HORN CUTOUT SWITCH
- cuts cabin altitude warning horn (goes off when cabin reaches altitude = 10 000 ft)
¡ Two types of pressure controllers : Electronic & Digital
- Non-EFIS aircraft use the Electronic pressure controller
- EFIS and B737-600/-700/-800/-900 aircraft uses the Digital pressure controller
¡ FLIGHT ALTITUDE INDICATOR
- shows selected cruise flight altitude (set before takeoff)
¡ LANDING ALTITUDE INDICATOR
- indicates altitude of destination airport
¡ CABIN RATE SELECTOR (ONLY ELECTRONIC PRESSURIZATION CONTROLLER)
- DECR : cabin altitude rate of change equals 50 ft/min
- INCR : cabin altitude rate of change equals 2000 ft/min
- r index : cabin altitude rate of change equals 300 ft/min.
¡ CABIN ALTITUDE SELECTOR (ONLY ELECTRONIC PRESSURIZATION CONTROLLER)
Rotate :
- to set desired cabin altitude
- Outer knob sets 1000 feet increments
- Inner knob sets 10 feet increments
For standby use :
- Cruise : use placard chart
- T/O & Landing : use landing field elevation minus 200 ft
¡ OUTFLOW VALVE POSITION INDICATOR
- indicates position of aft outflow valve (operates in all modes)
¡ OUTFLOW VALVE SWITCH
(spring loaded to center) â Full travel 7 sec. in manual AC & 14 sec in manual DC
- OPEN : opens main cabin outflow valve electrically
- CLOSE : closes main cabin outflow valve electrically
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¡ FLIGHT GROUND SWITCH (ONLY ELECTRONIC PRESSURIZATION CONTROLLER)
- GRD : On the ground drives outflow valve open at a rate selected by Cabin Rate Indicator
Inhibited after takeoff (functions the same as FLT position)
- FLT : pressurizes aircraft at rate selected by Cabin Rate Selector to cabin altitude selected on
Cabin Altitude Indicator.
¡ PRESSURIZATION MODE SELECTOR (ONLY ELECTRONIC PRESSURIZATION CONTROLLER)
- CHECK : tests auto feature function of AUTO system
- AUTO : airplane pressurization system controlled automatically using CAPTAIN's altimeter
- STBY : 1) pressurization system controlled thru the standby mode (using F/O's altimeter)
2) requires cabin altitude rate of change & cabin altitude selections (auto mode is bypassed)
3) allows maximum 7.90 psi.
- MANUAL : airplane pressurization controlled manually by outflow valve switch (all AUTO & STBY
circuits are bypassed.
1) AC : - Outflow valve operates from AC power which is at a faster rate
- Used by AUTO mode (full travel in 7 sec.)
2) DC : - Outflow valve operates from DC power which is at a slower rate (half speed)
- Used by STBY mode (full travel in 14 sec.) & for emergency evacuation.
¡ DIGITAL PRESS MODE SELECTOR (ONLY DIGITAL PRESSURIZATION CONTROLLER)
- AUTO : airplane pressurization system controlled automatically using CAPTAIN's altimeter
- ALTN : 1) pressurization system controlled automatically using alternate controller
2) AUTO FAIL amber light goes off when you select the ALTN position
3) the active controller changes with every flight or in case of an AUTO FAIL event.
- MAN : - Both cabin pressure controllers are bypassed
- Pressurization system is controlled manually by outflow valve switch
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BOEING 737 SYSTEMS REVIEW
14. FAULTS & INDICATIONS
VALID FOR SERIES
LIGHT
ALTN
AUTO
FAIL
AUTO
FAIL
BLEED
TRIP OFF
DUAL
BLEED
DUCT
OVERHEAT
MANUAL
OFF
OFF SCHED
DESCENT
PACK
INDICATION
- illumination of both ALTN & AUTO FAIL indicates
a single controller failure & automatic transfer to
ALTN mode
- Pressurization mode selector in ALTN position.
if any one of these 3 conditions occur :
- Loss of AUTO AC power
- Excessive rate of cabin pressure change
(> 1800 SL ft/min).
- High cabin altitude (> 13,875 ft).
if any of these 4 following conditions occur :
- Loss of DC power.
- Controller fault in the operational controller.
- Excessive rate of cabin pressure change
(> 2000 SL ft/min).
- High cabin altitude (> 15,800 ft).
excessive engine bleed air temperature or pressure
is sensed :
- related engine bleed air closes automatically
- requires reset.
whenever there is a potential risk for backpressuring the APU+ APU bleed air valve open &
engine No.1 Bleed air switch ON, APU Bleed air
valve & isolation valve open.
Indicates passenger cabin duct overheat (temp mix
valves drive full cold).
300
X
X
X
X
X
X
- EQUIPMENT COOLING EXHAUST OFF LIGHT
= no airflow from the selected cooling exhaust fan
- EQUIPMENT COOLING SUPPLY OFF LIGHT
= no airflow from the selected cooling supply fan
Aircraft descended before reaching the planned
cruise altitude set in the FLT ALT indicator.
X
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X
600
700
800
900
X
X
X
X
X
X
X
X
X
X
X
X
Ludovic ANDRE / version 00
X
500
X
Pressurization system operating in the manual
mode
- Indicates pack trip off due to overheat or indicates
failures of both primary & standby pack controls
(Pack continue to operate)
- During Master Caution recall, indicates failure of
either primary or standby pack control. Will
extinguish upon Master Caution reset.
400
X
X
X
X
X
X
X
X
X
X
X
X
X
X
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BOEING 737 SYSTEMS REVIEW
VALID FOR SERIES
LIGHT
INDICATION
PACK
TRIP OFF
indicates a pack trip off. Packs valve automatically
closes & mix valves drive full cold. Trips are caused
by pack temperatures exceeding limits or duct
overheat.
Ram door in full open position.
X
X
X
Pressurization system operating in standby mode.
X
X
X
300
400
500
X
600
700
800
900
X
X
X
X
X
X
RAM DOOR
FULL OPEN
MANUAL
ZONE
TEMP
- CONT CAB indicates a duct overheat, or failure of
the cockpit primary and standby temperature
control.
- FWD CAB/AFT CAB indicates duct overheat.
- During Master Caution recall, the illumination of
the CONT CAB Light indicates failure of the
cockpit primary or standby temperature control.
Illumination of either FWD or AFT CAB Light
indicates failure of the associated zone
temperature control. Will extinguish upon Master
Caution reset.
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X
X
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EQUIPMENT COOLING
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