B737 Air Conditioning
The Boeing 737 air conditioning system cools engine or APU bleed air in two packs and delivers it to three temperature zones: the flight deck, fed directly by the left pack, and the forward and aft cabin, fed from a mix manifold with filtered recirculated air, with trim air adjusting each zone.
The Boeing 737 air conditioning system turns hot bleed air into the conditioned air that ventilates, heats, cools and pressurises the cabin. Two packs cool the air, a mix manifold blends it with filtered cabin air, and three temperature zones receive it. Fans cool the electronics, warm exhaust air heats the forward cargo hold, and most of the air finally leaves through the outflow valve.
The crew's controls are simple: two PACK switches, two recirculation fan switches, a trim air switch and three temperature selectors. The logic behind them is what type rating examiners ask about: when a pack goes to high flow by itself, when HIGH is not allowed, what happens to temperature control when a controller or the trim air fails, and how the system keeps smoke from the forward hold out of the cabin. This article follows the 737 Next Generation (NG) FCOM and notes where the 737 MAX family differs. The air cycle machine itself is described in air conditioning packs, and the air supply in B737 bleed air and anti-ice.
Air conditioning overview
Normally the left pack uses bleed air from engine No. 1 and the right pack from engine No. 2; the packs sit in a left and a right air conditioning bay. Conditioned air from the left pack, taken upstream of the mix manifold, flows directly to the flight deck. The rest is collected in the mix manifold, where its temperature follows the settings of the temperature selectors, and is distributed to the passenger cabin.
Three zones have their own temperature selector: the flight deck (CONT CAB), the forward cabin and the aft cabin. The lower cargo compartments are sealed and pressurised but have no fresh air circulation or temperature control of their own.
On the ground the packs can be fed by the APU, or by an external air cart; a ground air conditioning source can also be connected straight to the mix manifold to distribute preconditioned air through the aeroplane. The APU can supply two packs on the ground or one pack in flight, and no more than one pack is operated from one engine.
Packs and pack control
In each pack the bleed air passes through a heat exchanger, is cooled further in the air cycle machine, is mixed with hot bypass air to set its temperature, and passes through a high-pressure water separator before reaching the mix manifold. Temperature sensors in the cooling cycle provide pack overheat protection: an overheat closes the pack valve and lights the amber PACK light. The pack valve is operated by bleed air, so it also closes when an engine fire switch shuts that engine's bleed valve.
Each pack has a primary pack control and a standby pack control, housed in two controllers so that the standby control of each pack sits in the opposite controller. If a primary control fails, the standby control in the other controller takes over. If both the primary and the standby control of the same pack fail, the PACK, MASTER CAUTION and AIR COND lights come on, and the pack continues to run without temperature control unless excessive temperatures make it trip off.
The amber PACK light therefore means a pack trip off or the failure of both controls of a pack when it comes on by itself, and the failure of either control when it shows only on a master caution recall, going out when the master caution is reset. A tripped pack is reset with the TRIP RESET switch once the fault has cleared.
Pack high flow modes
Each PACK switch has three positions, OFF, AUTO and HIGH. In AUTO with both packs operating, each pack regulates to low flow. A pack regulates to high flow in AUTO when it is the only one operating in flight with the flaps up, and when it is the only pack and is supplied by the APU with both engine BLEED switches OFF. HIGH gives the maximum flow on the ground with the APU BLEED switch ON.
The pack high flow mode also switches in automatically. In the air, with the flaps up, both PACK switches in AUTO and both engine bleeds ON, the remaining pack goes to high flow if one pack or one engine fails or a pack is selected OFF. On the ground or with the flaps down, this automatic switching is inhibited so that full engine power remains available for single-engine operation. With the APU as the only source (APU running, APU BLEED ON, both engine BLEED switches OFF, both packs in AUTO), a single pack failure brings high flow regardless of flap position or air/ground state, and selecting either or both PACK switches to HIGH forces an APU high air flow rate for maximum ventilation.
A single pack in high flow can maintain pressurisation and acceptable temperatures throughout the aeroplane up to the maximum certified ceiling.
Exam tip: the pack high flow limitation: with one or both engine BLEED switches ON, the packs are not operated in HIGH for take-off, approach or landing. The CARGO FIRE and SMOKE/FUMES REMOVAL non-normal checklists override it, because they need HIGH for ventilation.
The PACK switches are set OFF for engine start and to AUTO in the before taxi procedure.
Ram air system
Each pack's heat exchangers are cooled by outside air taken in through a ram air inlet. On the ground, and in slow flight with the flaps not fully retracted, the ram air inlet doors move fully open for maximum cooling, and a blue RAM DOOR FULL OPEN light shows each door fully open; the lights are expected to be on during the preflight. In normal cruise the doors modulate.
A ram air deflector door is fitted forward of each ram air inlet to keep slush out of the system before lift-off and after touchdown. The deflector doors are extended electrically through the air/ground safety sensor.
Zone temperature and trim air
The temperature selectors cover roughly 18 °C (65 °F) to 30 °C (85 °F). The packs produce air at the temperature needed by the zone that requires the most cooling, and the trim air system warms the air for the other zones: a trim air modulating valve for each zone adds trim air to its supply, and the TRIM AIR switch is ON in normal operation. Setting a zone's selector to OFF closes its trim air modulating valve, and the control system then ignores that selector. During single pack operation with TRIM AIR OFF, the pack aims at the average temperature demand of all three zones. If a zone supply duct overheats, that zone's amber ZONE TEMP light comes on and its trim air modulating valve closes; the TRIP RESET switch reopens it once the duct has cooled.
Two electronic controllers share the zones:
| Controller | Controls | Backs up |
|---|---|---|
| Right | Forward cabin; primary control of the flight deck | |
| Left | Aft cabin | Flight deck |
Failures degrade the control in steps:
- Primary flight deck control fails: the backup control takes over automatically and the CONT CAB ZONE TEMP light comes on only on a master caution recall. If both fail, the light comes on by itself.
- Forward or aft cabin control fails: that zone's trim air modulating valve closes; the selectors still work, but the two passenger cabin settings are averaged.
- All zone controls and the primary pack controls fail: the standby pack controls command the packs to meet the average demand of the two cabin zones, the trim air valves close and the flight deck selector has no effect.
- All temperature selectors OFF: the left pack holds a fixed 24 °C (75 °F) and the right pack 18 °C (65 °F), measured at the pack temperature sensor.
When trim air is lost, the system uses the unbalanced pack temperature mode. If the flight deck trim air is lost, the left pack supplies the flight deck at its selected temperature and the right pack satisfies the passenger zone needing the most cooling. If a passenger zone's trim air, or all trim air, is lost, the forward and aft cabin demands are averaged to control the right pack.
The AIR temperature source selector chooses what the temperature indicator displays: SUPPLY DUCT (the supply duct of a zone), PASS CAB (the forward or aft passenger cabin) or PACK (the left or right pack temperature).
Flight deck and cabin air distribution
Conditioned air for the flight deck branches into risers that end at floor, ceiling and foot-level outlets, with overhead diffusers on the ceiling above and aft of the No. 3 windows. A dual-purpose valve behind each pilot's rudder pedals provides foot air and windshield air: pulling the FOOT AIR control sends conditioned air to the pilot's leg position, and pulling the WINDSHIELD AIR control sends it to the inside of the No. 1 windows for defogging. In the passenger cabin the air is distributed from the mix manifold through sidewall risers and an overhead distribution duct.
Two AC-powered recirculation fans reduce the pack load and the engine bleed demand. Air from the passenger cabin and the electrical equipment compartment is drawn through the lining of the forward cargo compartment, filtered and returned to the mix manifold. Each fan runs only with its RECIRC FAN switch in AUTO, and its logic depends on the PACK switches:
| Fan | On the ground | In flight, both packs operating |
|---|---|---|
| Left | Runs unless both PACK switches are in HIGH | Runs unless both are in HIGH, or L PACK is in AUTO with R PACK in HIGH |
| Right | Runs even with both PACK switches in HIGH | Runs unless both are in HIGH |
Equipment cooling and smoke penetration
The equipment cooling system has a supply duct and an exhaust duct, each with a normal and an alternate fan. The supply duct cools the flight deck displays and the electronic equipment in the E&E compartment. The amber EQUIP COOLING OFF light means no airflow from the selected fan; selecting the alternate fan should restore the airflow and extinguish the light within about 5 seconds. On the ground, an equipment cooling overtemperature is signalled by the crew call horn in the nose wheel well.
The overboard exhaust valve decides where the warm equipment cooling air goes. On the ground and in flight at low differential pressure it is open, and the air is discharged overboard. In flight at higher differential pressure it is normally closed, and the exhaust air is diffused into the lining of the forward cargo compartment, which gives the hold extra heating. The valve is driven open if either PACK switch is in HIGH and the right recirculation fan is OFF, for more ventilation in the smoke removal configuration, and in pressurised flight a forward cargo fire alarm commands it to a smoke mode.
The forward cargo lining carries both recirculated air and equipment cooling exhaust air. Smoke penetration prevention for the forward cargo compartment works through a smoke control relay: with the RECIRC FAN switches OFF and the PACK switches in HIGH, it stops the equipment cooling supply fan for 5 minutes, inhibits the EQUIP COOLING supply and exhaust OFF lights for 5 minutes and stops the exhaust fan for the rest of the flight. Moving any RECIRC FAN or PACK switch out of those positions restarts the fans. The cargo fire procedure itself is covered in B737 fire protection.
Pack and zone lights
| Light | Normal meaning | On master caution recall |
|---|---|---|
| PACK (amber) | Pack trip off, or both pack controls failed | One pack control failed |
| ZONE TEMP, FWD or AFT CAB (amber) | Duct overheat in that zone | Zone temperature control failed |
| ZONE TEMP, CONT CAB (amber) | Duct overheat, or primary and standby flight deck control failed | Primary flight deck control failed |
| RAM DOOR FULL OPEN (blue) | Ram air inlet door fully open | |
| EQUIP COOLING OFF (amber) | No airflow from the selected fan |
737 MAX family
Two items in the EASA and FAA documents on the MAX family concern this system. The EASA type certificate data sheet limits the 737-8200 to 207 passengers and cabin crew in total because of the environmental control system's ventilation rate per occupant. The FAA's draft revision 21 of the Flight Standardization Board report lists a smoke vent valve on the 737-7, with revised non-normal checklists. The figures in this article are those of the NG FCOM.
Frequently asked questions
When can the Boeing 737 packs not be used in HIGH?
With one or both engine BLEED switches ON, the 737 NG's operational limitations say the packs are not operated in HIGH for take-off, approach or landing. The CARGO FIRE and SMOKE/FUMES REMOVAL non-normal checklists override this, because they call for HIGH to increase ventilation. HIGH gives the maximum flow on the ground with the APU BLEED switch ON.
What happens if one pack fails on the 737?
In flight with the flaps up, both PACK switches in AUTO and both engine bleeds ON, the remaining pack switches automatically to high flow if one pack or one engine fails or a pack is switched off. On the ground or with flaps down this switching is inhibited, to keep engine power available for single-engine operation. A single pack in high flow can maintain pressurisation and acceptable temperatures up to the maximum certified ceiling.
What does the ZONE TEMP light mean on the Boeing 737?
For the forward or aft cabin, an amber ZONE TEMP light means a duct overheat in that zone; its trim air modulating valve closes and can be reopened with TRIP RESET once the duct has cooled. The CONT CAB light means a flight deck duct overheat or failure of both the primary and standby flight deck temperature controls. On a master caution recall, a ZONE TEMP light shows a zone control failure, for the flight deck the primary control.
What is the unbalanced pack temperature mode on the 737?
It is the mode used when trim air is lost. If the flight deck trim air is lost, the left pack supplies the flight deck at its selected temperature and the right pack satisfies the passenger zone that needs the most cooling. If a passenger zone's trim air or all trim air is lost, the forward and aft cabin demands are averaged to control the right pack.
What is forward cargo smoke penetration prevention on the 737?
It keeps smoke from the forward cargo compartment out of the cabin and flight deck. With the RECIRC FAN switches OFF and the PACK switches in HIGH, a smoke control relay stops the equipment cooling supply fan for 5 minutes, inhibits its OFF light for the same time and stops the exhaust fan for the rest of the flight. Moving any of those switches restarts the fans.
Test yourself on B737 Air Conditioning
The v1prep banks cover this topic in the B737 type-rating bank, with a worked explanation for every answer. EASA ATPL, PPL, IR and CPL, the FAA written tests and A320/B737 type ratings.
Start practising →Sources and further reading
- EASA Type Certificate Data Sheet IM.A.120, Boeing 737
- FAA Flight Standardization Board Report, Boeing 737
- EASA Easy Access Rules for Large Aeroplanes (CS-25), CS 25.831 Ventilation
- 14 CFR 25.831, Ventilation
- FAA Aviation Maintenance Technician Handbook, Airframe (FAA-H-8083-31B), Chapter 16, Cabin Environmental Control Systems
Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.