ATA 21
Air Conditioning & Pressurisation
Two packs supplied by left and right bleed ducts. Recirculation fans. Digital pressure controller with two auto channels, alternate and manual modes driving the outflow valve.
Where it is
What it looks like
Technical data
| Item | Value |
|---|---|
| Max differential | approx. 8.35 psi (relief 8.65) |
| Cabin altitude at FL410 | approx. 8,000 ft |
Components
1.Packs (2)
Air-cycle machines.
Two air-cycle machine (ACM) packs mounted in the wing-to-body fairing below the center fuselage, one left and one right, each fed by bleed air from its respective engine, the APU, or the opposite side via the isolation valve.
- Hot bleed air is cooled by ram-air heat exchangers, then expanded across a turbine that drives a compressor and fan, producing cold, moisture-laden air.
- A water extractor removes condensation before the air enters the mix manifold.
- Pack flow and temperature are modulated by the pack temperature control valve and ram air modulating doors, both commanded by the Equipment Cooling/Pressurization logic or, on later 737s, the ADIRU-fed environmental control system.
- Each pack can be selected AUTO, HIGH, or OFF from the cockpit AIR COND panel.
| Pack flow (AUTO) | approx. 0.4-0.8 lb/min per passenger equivalent |
| Bleed air supply pressure | approx. 30-45 psi regulated |
| Turbine inlet temperature limit | approx. 450 F max |
| Pack discharge temperature range | 0 C to 90 C |
- Pack overheat/trip: PACK light illuminates, pack automatically shuts off, crew selects remaining pack or APU bleed
- Pack valve fails closed: Loss of conditioned air flow from that side, duct pressure drops
- Water extractor clogging: Reduced cooling performance, possible moisture ingestion into cabin
- Inspect pack bay for bleed air leaks and heat damage during line checks.
- Functional test of pack temperature control valve and ram air doors.
- Check water extractor and sock filter for contamination at scheduled intervals.
2.Recirculation fans
Two electrically driven centrifugal fans located in the forward cargo compartment ceiling area, each supplying one side of the mix manifold by drawing cabin return air through HEPA-equivalent filters.
- Each fan is a 115V 3-phase AC motor-driven impeller controlled by its own relay/contactor from the P6 panel.
- Return air from the cabin is mixed with pack discharge air in the mix manifold to balance temperature and reduce total bleed demand.
- A smoke detection or pack failure will automatically trip the associated recirc fan off to prevent smoke recirculation.
| Fan motor power | approx. 0.75 hp, 115 VAC |
| Flow contribution | approx. 50% of total mix manifold airflow |
| Filter type | high-efficiency particulate filter elements |
- Fan failure: RECIRC FAN light on, reduced total airflow, slightly warmer cabin temperature
- Filter clogging: Reduced flow, increased fan motor load, possible overheat trip
- Inspect/replace recirculation filters per maintenance interval.
- Check fan bearing noise and current draw during ground run checks.
3.Mix manifold
A sheet-metal plenum located above the forward cargo compartment where conditioned air from both packs and recirculated cabin air combine before distribution to the cabin risers and flight deck.
- Pack discharge air and recirculation fan output merge in the manifold, equalizing pressure and temperature before being ducted upward through riser ducts to overhead distribution nozzles.
- Manifold pressure balance ensures even flow split between zones even if one pack is inoperative.
| Manifold location | station area forward cargo compartment ceiling |
| Distribution zones | flight deck, forward cabin, aft cabin |
- Duct leak/rupture in manifold: Uneven cabin temperature, possible duct overheat warning
- Blocked riser duct: Localized zone loses airflow
- Visual inspection of manifold ducting and clamps for cracks or leaks.
- Check duct overheat loop wiring routed near the manifold.
4.Outflow valve
A single butterfly-type outflow valve mounted in the lower aft fuselage, just forward of the aft cargo compartment, that regulates cabin pressure by metering air overboard.
- Commanded by the digital cabin pressure controller (CPC), which schedules cabin altitude based on flight altitude and a programmed landing field elevation.
- The valve is driven by a dual electric actuator (primary and standby controllers) with a manual backup rotary control in the cockpit.
- Valve position modulates continuously to maintain the scheduled cabin rate of climb/descent, typically limited to 500 fpm or less.
| Max differential pressure | approx. 8.35-8.65 psi |
| Normal cabin rate of change | up to 500 fpm, higher on emergency depress |
| Actuator type | dual redundant electric motor driven |
- Valve fails full open: Cabin cannot pressurize, altitude horn/warning, emergency descent may be required
- Valve fails closed/stuck: Cabin overpressure risk, pressure relief valves activate
- Functional check of CPC auto/standby modes and valve travel.
- Inspect valve seals and linkage for wear or corrosion.
5.Pressure relief valves
Spring-loaded negative and positive pressure relief valves mounted in the aft fuselage near the outflow valve, protecting the fuselage pressure vessel from over- or under-pressurization.
- The positive relief valve opens if cabin differential pressure exceeds a set limit above normal operating differential, venting air overboard independent of the CPC.
- A negative pressure relief valve (often combined with the forward or aft valve) allows outside air in if cabin pressure drops below ambient, preventing structural implosion on the ground after a hot/cold soak.
| Positive relief setting | approx. 8.65-8.9 psi differential |
| Negative relief setting | approx. -0.5 to -1.0 psi differential |
- Relief valve leaks/sticks open: Cabin cannot build normal differential pressure
- Relief valve fails to open: Structural overpressure risk in a pressurization control malfunction
- Ground functional test of relief valve cracking pressure.
- Inspect valve seat and gasket for leakage during pressurization checks.
Operation
- Set FLT ALT and LAND ALT before flight; the controller schedules cabin altitude.
Controls
| Control | Function |
|---|---|
| PACK L/R | OFF/AUTO/HIGH. |
| Pressurisation mode selector | AUTO/ALTN/MAN. |
Indications
| Indication | Meaning |
|---|---|
| CABIN ALTITUDE warning | Intermittent horn above approx. 10,000 ft cabin. |
| PACK light | Pack trip. |
Failures & crew actions
| Failure | Effect / action |
|---|---|
| Cabin altitude warning | Masks, emergency descent. |
| Pack trip | Single pack ops. |
Maintenance
- Heat exchanger cleaning.
- Outflow valve operational check.
Deep dive · ATA 21
Air Conditioning & Pressurisation — in depth
Packs & zones
Two packs (air cycle machines) in the wheel well area. Each pack takes bleed air via the pack valve; outlet air goes to the mix manifold, with recirculation fans. NG has three temperature zones (flight deck, forward and aft cabin) with trim air.
PACK switch AUTO/HIGH: in AUTO, high flow only when one pack operates in flight with flaps up.
Pressurisation
Two automatic controllers (AUTO and ALTN) and a MAN mode drive the outflow valve (aft fuselage, lower right) plus an overboard exhaust valve. The pilot sets FLT ALT and LAND ALT on the pressurisation panel before takeoff.
Safety relief valves limit differential to about 8.65 psi; negative relief prevents inward pressure.
| Max differential | 8.35 psi normal, 8.65 relief |
| Cabin altitude at FL410 | approx. 8 000 ft |
| CABIN ALTITUDE warning | 10 000 ft (intermittent horn) |
| Masks deploy automatically | approx. 14 000 ft cabin |
Lessons — Helios 522
The pressurisation mode selector was left in MAN after maintenance; the outflow valve stayed open. The cabin altitude warning horn was misidentified as a takeoff configuration warning. Lesson: always check the pressurisation panel on the before-start checklist and react to the cabin altitude warning with oxygen masks first.
Warning · Intermittent warning horn in climb = cabin altitude. Masks on, establish communication, descend.
Educational summary — verify against the official AMM/FCOM/POH for your aircraft.