Boeing 737 ATA Chapter 21 covers the air conditioning, pressurization, and ventilation systems of the airplane. The Boeing 737 air conditioning system provides temperature-controlled air by processing bleed air from the engines, APU, or a ground air source in air conditioning packs. This system features a dual air cycle pack design, where the ram air system cools hot air in the pack.

Cabin pressurization is managed during all phases of airplane operation by the cabin pressure control system, which uses bleed air supplied and distributed by the air conditioning system. Pressurization and ventilation are regulated by modulating the outflow valve and the overboard exhaust valve.

COMPUTERS

  • IASC: 2 Integrated Air Systems Controllers – Pack Flow and Temperature Controller (PFTC) + Air Supply Controller (ASC).
  • ACAU: 2 Air Conditioning Accessory Units – interface for the airplane operational logic and the air system.
  • CPC: 2 Cabin Pressure Controllers – cabin pressure function.

CONTROL PANELS

COOLING

COOLING CYCLE – AIRFLOW

  • Hot bleed air
  • Pneumatic manifold
  • Ozone converter
  • Flow control and shutoff valve (FCSOV)
    • Primary heat exchanger for cooling cycle.
    • Trim Air PRSOV > 3 zone trim air modulating valves.
    • TCV & SBTCV (Temp Control Valve)
    • Tapping – hot bleed air to the turbine case to prevent icing.
  • Primary heat exchanger
  • Compressor of ACM
  • Secondary heat exchanger (through plenum/diffuser assembly: outer duct – plenum & inner duct – diffuser)
  • Reheater (Hot side) > Condenser (Hot side)
  • 2 water extractor ducts > Water spray nozzle > Ram Air Duct (cools the ram air stream by evaporation)
  • Reheater (Cold side)
  • Turbine of ACM
  • Condenser (Cold side)
  • Conditioned Air Check Valve > Mix manifold

FCSOV

  • Electrically-controlled and pneumatically-actuated.
  • Spring-loaded to the closed position.
  • Visual position indicator.
  • The latching solenoid controls the valve.
  • The control servo and torque motor control the airflow.
  • The latching solenoid has a plunger that lets you manually operate the solenoid.
  • IASC controls the FCSOV.

RAM AIR CONTROL

  • Ram air deflector door
  • Ram air actuator (Smart Ram Air Door Actuator (SRADA))
  • Ram air modulation panels
  • Ram air temperature sensor
  • Impeller fan
  • Fan bypass check valve

When the airplane is on the ground, the ACM impeller fan makes a low pressure zone. This pulls air through the heat exchangers and up through the plenum to the impeller fan. Then the impeller fan sends the air through the diffuser and out the ram air exhaust. The air pressure in the diffuser keeps the check valve closed. When the airplane is in flight, ram air pressure opens the fan bypass check valve.

PACK OVERHEAT PROTECTION

  • Compressor discharge overheat switch 199°C
  • Turbine inlet overheat switch 99°C
  • Pack discharge overheat switch 121°C

The switches are normally open. When an overheat condition occurs, the overheat switch closes. This energizes the pack overheat relay. When the pack overheat relay energizes, power is supplied to the close solenoid of the flow control and shutoff valve.

When a pack trip occurs – PACK amber light come on.

If there is an overheat condition, the ZONE TEMP light on the P5 panel comes on immediately. Push the TRIP RESET switch on the P5 panel after the duct has cooled to reset the zone temperature control system. TRIP RESET switch de-energize the overheat relay.

TEMPERATURE CONTROL

  • IASCs modulate these valves in response to temperature control requirements:
    • Pack temperature control valves (normal and standby TCV)
    • Zone trim air modulating valves.
  • The ram air control temperature sensor sends a temperature signal to the inlet SRADA. The inlet SRADA sends control signals to the exit SRADA. The exit SRADA sends fault signals to the inlet SRADA.
  • Temperature Selectors
    • C (cool) sets a temperature of 18°C
    • W (warm) sets a temperature of 30°C
    • Intermediate selector positions set proportionate temperature.
    • Turn the selector to the OFF position to close the related trim air modulating valve.
  • The LCD touchscreen temperature selectors on the cabin attendant panels can adjust the temperature approximately 1.7°C (3°F) of the flight crew temperature selectors.
RH IASCLH IASC
1. FWD CABIN ZONE TEMP CONTROL
2. PRIMARY FLT COMPT ZONE TEMP CONTROL
3. R AUTO PACK CONTROL
4. L STANDBY PACK CONTROL
5. R INLET SRADA BITE/FAULT DATA		1. AFT CABIN ZONE TEMP CONTROL
2. BACKUP FLT COMPT ZONE TEMP CONTROL
3. L AUTO PACK CONTROL
4. R STANDBY PACK CONTROL
5. L INLET SRADA BITE/FAULT DATA

PACK Light

  • The pack light turns on for over temperature conditions, and failures of the air conditioning temperature control system.
  • If a power interruption is sensed, the PACK light on the P5 panel comes on. The pack temperature control system is reset after power has been restored.
  • If an overheat condition is sensed, the PACK light on the P5 panel comes on. Push the TRIP RESET switch on the P5 panel after the cooling pack has cooled to reset the pack temperature control system.
  • A single failure of the pack control system causes a STATUS message with a MAINT light or a Scheduled Maintenance Task (SMT) on the MAX Display System (MDS). This shows a BITE check of the controllers is necessary. If a complete loss of temperature control for the pack control system occurs, the PACK light comes on and cannot be reset.

RECIRCULATION

  • Recycles approximately 50 percent of the cabin air for ventilation purposes. This reduces the quantity of fresh air from the pneumatic system for ventilation.
  • The recirculation system decreases the use of engine bleed air. This enables better thrust management and decreases fuel consumption
  • Recirculated air is filtered by High Efficiency Particulate Air (HEPA) filters before returning to the cabin. HEPA filters are highly effective at removing particulates such as viruses, bacteria and fungi from recirculated air.
  • The recirculation fans increase air flow to the passenger cabin in the main distribution system.
  • The recirculation fans are enabled when you move the recirculation fan switches to the AUTO position. Recirculation fan operation depends on air conditioning pack operation.
  • 2 recirculation fans (left and right) in the distribution compartment.
  • The check valves are between the recirculation fan and the main distribution manifold. >> To prevent the flow of conditioned air out of the main distribution manifold through the recirculation system.

AIR DISTRIBUTION

  • Moves conditioned air from the packs or ground air source to the temperature control zones.
  • Primary components:
    • Ground air connector
    • Mix manifold
    • Distribution ducts/risers.
  • The air distribution system provides filtered, recirculated air mixed with fresh air. This is approximately a 50/50 mix.
  • The usual control for the left pack makes sure that it supplies air at a temperature that gives the necessary cooling for the flight compartment.
  • The control for the right pack makes sure that it supplies air at a temperature that gives the necessary cooling for the mix manifold.
  • The flight compartment has an independent source of conditioned air. This provides a constant supply of fresh air circulation.
  • The flight compartment receives conditioned air from the right pack if the left pack is not operational.
  • The windshield and foot air outlet valves control airflow to the captain and first officer windshield outlets and foot outlets. You pull the knob to open the valve.
  • Conditioned air from the main distribution manifold flows through sidewall riser ducts.
  • The passenger cabin exhaust air goes through floor grilles to the recirculation system or overboard.

VENTILATION

  • The ventilation system uses differential pressure to pull air out of the airplane. The air moves through overboard vents from the cabin galley and the lavatory areas.
  • The flexible ducts connect the vent inlet opening in the galley ceiling to an exhaust nozzle in the airplane skin. The galley ventilation muffler reduces the noise of air being released from the pressurized cabin.

OZONE CONVERTER

Ozone control keeps ozone concentrations in the airflow to a satisfactory limit when the airplane is at high altitudes. The ozone converter is between the air supply manifold and the flow control valve of each pack. The ozone converter is a catalytic device that removes ozone concentrations. Air goes through the converter before it goes to the pack. A chemical reaction in the converter changes ozone molecules to oxygen molecules.

EQUIPMENT COOLING

  • The equipment cooling system uses fans to move air around equipment in the EE compartment and flight compartment.
  • There are two sets of fans (normal and alternate) for the supply and the exhaust systems.
  • One fan per system operates at a time.

The equipment cooling system has low flow detectors to give a warning when there is not sufficient cooling airflow. On the ground, the crew call horn sounds when low flow is detected in the supply system. This gives you warning and that you should shut down airplane electrical systems to prevent an overheat condition.

The amber OFF light comes on when low airflow is detected, or the two fans do not operate. The light can also come on if the low flow detector fails its startup BITE test.

The low flow sensors are in the forward equipment compartment. They are in the supply and exhaust ducts of the equipment cooling system.

The low flow sensor monitors the airflow and temperature of the equipment cooling air.

The ground crew horn automatically operates when the airplane is on the ground and these conditions are present:

  • High alarm or the ON DC signal from either ADIRU is active for 20 seconds or more
  • Left or right ADIRU switch is in the ALIGN or NAV position

The equipment cooling air filter removes small particles of dirt from the air before it enters the EE cooling system. This prevents contamination of the electrical and the electronic equipment. The equipment cooling air filter is in the EE compartment, on the right sidewall. It is upstream of the supply fans. Access to the air filter is from the forward cargo compartment right forward access panel.

OVERBOARD EXHAUST VALVE

The overboard exhaust valve is in the aft center section of the EE compartment.

The valve actuator has two positions, NORMAL and SMOKE.

The overboard exhaust valve has 3 modes of operation.

  • Normal Mode
    • L/R PACK switch – AUTO/OFF
    • R RECIRC FAN switch – AUTO
    • OEV open on ground, Closed in flight.
    • – valve actuator to the NORMAL position.
    • – the valve is open until the airplane pressurizes.
  • High Flow Mode
    • L/R PACK switch – HIGH
    • R RECIRC FAN switch – AUTO
    • Increases the ventilation of the cabin through increased air flow.
    • OEV open (SMOKE open postion) on ground, Closed in flight.
    • – valve actuator to the SMOKE (open) position.
  • Smoke Removal Mode
    • L/R PACK switch – HIGH
    • R RECIRC FAN switch – OFF
    • Opens the OEV to remove smoke from the flight deck and E/E compartment.

The amber SMOKE light comes on when smoke is detected in the equipment cooling supply or exhaust ducts.

Note: The cabin pressurization system supplies an open/close enable signal.

PRESSURIZATION

  • Pressurization control can be automatic or manual – 2 CPCs.
  • Each CPC has its own systems interface and valve motor system.
  • Only one CPC controls the outflow valve at any time. The other CPC is a backup.
  • The active controller changes for every flight or when there is an autofail event.
  • Both controllers run continuous BITE tests. If the active CPC becomes inoperative, the other CPC automatically takes control.
  • A BITE module is on the front face of each cabin pressure controller.

If the active CPC controller fails, the system changes pressurization control to the backup (alternate) CPC. and Amber AUTO FAIL light and Green ALTN light come on.

The ALTN light shows that the backup system is active. The AUTO FAIL light goes off when you select the ALTN position on the mode selector.

If both CPCs fail, the ALTN light does not come on. This indicates that the system cannot transfer control to an operative automatic controller.

  • AUTO FAIL light comes on, and FLT ALT and LAND ALT displays show five dashes (—–).

The manual control mode overrides and bypasses the two CPCs. The manual control system has its own valve motor system. A triple-redundant architecture.

OUTFLOW VALVE

  • The valve is a thrust recovery, double gate type valve.
  • The outflow valve has three motors:
    • Two AUTO motors with electronic actuators
    • One MANUAL motor
  • Only one motor drives the valve at a time.
  • Position transducer – show indication on P5. Also sends signals to CPCs (position feedback).
  • Each electronic actuator on the valve has a fail-safe aneroid switch.
  • Outflow valve switch is a three-position toggle switch – CLOSE, Neutral, OPEN.
  • The switch is spring-loaded to the neutral position.

Altitude switches in each electronic actuator override CPC signals and close the outflow valve if the cabin altitude is 14,500 feet. On airplanes that have the ultra high altitude landing option, the CPC will command the valve to close when the cabin altitude is above 15,500 feet. This function does not affect the manual mode of operation of the outflow valve. In the manual mode, the pilot uses the control module toggle switch to operate the outflow valve. The manual motor has no electronic actuator, and no pressure switch.

MANUAL light: The CPCs are deactivated and the MANUAL light comes on when the selector switch is in the MANUAL position.

CABIN ALTITUDE WARNING SWITCH

  • The cabin altitude warning switches, S128 and S1153, are on the ceiling in the forward EE compartment.
  • The two switches make sure that if the cabin altitude reaches 10,000, a warning will alert the crew of the condition.
  • When energized, the cabin altitude warning circuit causes an intermittent beep from the aural warning unit. The red CABIN ALTITUDE indicator lights on P1-3 and P3-1 will come on.
  • The ALT HORN CUTOUT push-button switch on P5 lets the crew deactivate the warning alarm until the next high cabin altitude event.
  • When the cabin altitude goes below 10,000 ft, the cabin altitude warning switch opens, and The red CABIN ALTITUDE indicator lights on P1-3 and P3-1 go off.

OFF SCHED DESCENT LIGHT

The off schedule descent feature works only in the AUTO and ALTN modes. It is not a feature of the MANUAL mode. If it is necessary to land immediately after takeoff, the pressurization control system programs the pressurization system for landing. The off schedule descent (OFF SCHED DESCENT) indication is part of this feature. The light tells you that the system will control cabin pressure for a return to the take-off field. An off schedule descent begins when the airplane starts to descend off schedule (before it gets to cruise altitude).

The OFF SCHED DESCENT light will go out if Airplane begins to climb again, or FLT ALT is reset to the current altitude, or Pilot selects manual (MAN) mode, or Airplane lands.

CABIN PRESSURE RELIEF

The cabin pressure relief system is a fail safe system. It protects the airplane structure from overpressure and negative pressure if the pressurization control system fails.

  • Two positive pressure relief valves – are fail safe devices that bleed fuselage pressure overboard if the aft outflow valve fails closed.
  • Negative pressure relief valve – prevent structure damage during a rapid descent. The valve opens when pressure outside of the airplane is 1.0 psi more than the pressure inside of the airplane (-1.0psid). 

CARGO COMPARTMENT BLOWOUT PANELS

During rapid decompression, differential pressure pushes the panels out of their frames. When the panels push out of their frames, the pressures in the upper and lower fuselage lobes equalize quickly. This equalization of pressure prevents damage to the airplane structure. A differential pressure of 1.0 psid will push the blowout panel out of the frame.

PRESSURE EQUALIZATION VALVE

  • The pressure equalization valves allow air to flow into or out of the cargo compartments to keep the cargo compartment pressure the same as cabin pressure.
  • The forward cargo compartment has a pressure equalization valve on the aft bulkhead. The aft cargo compartment has a pressure equalization valve on the forward portion of the vacuum waste bulkhead.
  • One valve lets air into the cargo compartment during airplane pressurization and one valve lets air out of the cargo compartment during airplane depressurization.

NOTE: The cabin pressure control module has integrated circuit electronics. It is an Electrostatic Discharge Sensitive (ESDS) device. Use proper care when you handle it. Flush operations of the vacuum toilet system can cause the cabin rate of climb indicator to momentarily show a rate of climb indication of 300-500 feet per minute. This is normal.

HEATING

  • Warm equipment cooling exhaust air flows under the forward cargo compartment floor and along the sidewalls. The air mixes with passenger compartment air in the main distribution manifold.
  • The aft cargo compartment air comes from the passenger compartment through the foot level grilles. The air goes into the sidewall area around and under the aft cargo compartment through the outflow valve.
  • The warm air on all sides of the cargo compartments is an insulator. It prevents the transfer of heat through the skin by conduction.
  • SUPPLEMENTAL HEATING: In the passenger compartment, door area heaters (Electrical heat elements) supply more heat around the two main entry doors to prevent cold zones around the doors. Also, heater blankets supply more heat around the emergency escape doors.
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