Air Conditioning Packs

The air conditioning pack is the heart of a jet transport's environmental control system (ECS). It takes bleed air from the pneumatic system, which arrives far too hot for the cabin, cools it in heat exchangers and an air cycle machine (ACM), removes the water that condenses as it cools, and delivers it at a controlled temperature to be mixed and distributed. The cabin is pressurised by the same air, so the packs are also the source of cabin pressure: lose both and the aircraft can no longer stay pressurised.

Jet transports normally carry two packs, each fed by its own engine, and on many types either can keep the aircraft pressurised alone. Pack operation is largely automatic. The crew select the flow, set the temperatures, and must recognise and handle pack failures, including the dual pack failure that turns into a depressurisation.

Air conditioning system overview

Air conditioning (AC) on an aircraft means supplying the air the occupants breathe at a controlled temperature, with excess moisture removed. Bleed air leaves the precooler at around 200 °C on a type such as the A320 (see bleed air and pneumatic systems); ATPL texts give a cabin temperature target of 18 to 24 °C (65 to 75 °F). The pack closes that gap without any refrigerant other than the air itself: an air cycle system.

Duplication is a design rule. ATPL texts, quoting the British Civil Airworthiness Requirements, require that no single component failure reduce the fresh air supply below 0.5 lb per seat per minute, which is met by two packs supplied from separate engines. The packs sit in air conditioning bays outside the pressure hull; the 737 NG has a left and a right bay.

On the A320 the air passes through the following stages:

View full schematic

Pack flow control

Each pack is fed through a pack valve, or pack flow control valve. On the A320 it is pneumatically operated and electrically controlled, and a spring closes it when there is no air pressure. It closes automatically if the upstream pressure is too low, if the compressor outlet overheats, during an engine start on its side (both pack valves close if the crossbleed valve is open), when the related engine FIRE pushbutton is pushed and when DITCHING is selected. On the ground the pack valves reopen 30 seconds after the second engine's N2 passes 50 per cent, to avoid a second closure cycle.

The valve also meters the flow. ATPL texts describe the mass flow controller as a calibrated variable-orifice valve that holds a constant mass flow to the pack whatever the upstream pressure, so engine power changes do not disturb the cabin supply.

The crew choose the flow level:

• A320: the PACK FLOW selector gives LO (80 per cent), NORM (100 per cent) and HI (120 per cent). HI is selected automatically in single-pack operation or with APU bleed supply, whatever the selector shows, and if LO cannot meet a temperature demand the system reverts to normal flow. If the bleed pressure is too low to satisfy cooling demand, the controllers ask the engine interface units to raise the minimum idle.
• 737 NG: the PACK switches are OFF, AUTO and HIGH. In AUTO with both packs operating, each regulates to low flow; with one pack operating it regulates to high flow in flight with the flaps up. On the ground or with flaps extended that automatic switch to high flow is inhibited, so that engine power is kept for single-engine operation, unless the pack is running on APU air with both engine bleed switches off. With either engine bleed switch ON, the packs must not be operated in HIGH for take-off, approach or landing, except when the cargo fire or smoke and fumes removal checklists call for it.
• E190-E2: a "smart" ECS adjusts its performance to the number of passengers.

Pack high flow costs bleed air and therefore fuel; it is used for hot conditions, full cabins, smoke removal or single-pack operation.

Heat exchangers and ram air

The primary heat exchanger pre-cools the incoming bleed air with ram air, outside air flowing through the heat exchanger. The secondary heat exchanger (the main heat exchanger in the illustration) sits between the ACM compressor and turbine and removes the heat added by compression, so that the air enters the turbine as cool as possible. A heat exchanger can cool air only towards the temperature of the ram air, never below it; ATPL texts quote a thermal efficiency of at least 80 per cent of the temperature difference. Getting below ambient is the turbine's job.

The ram air system supplies the cooling air through inlet and outlet doors that the pack controller modulates: more open for more cooling, more closed to reduce drag. The ram air doors close for take-off and landing so that the heat exchangers do not ingest debris, water or slush.

• A320: the ram air inlet flap closes when take-off power is set with the main gear struts compressed. On landing it closes when the struts compress at 70 kt or more, and reopens 20 seconds after the speed drops below 70 kt.
• 737 NG: the ram air inlet doors go fully open on the ground and in slow flight with the flaps not fully retracted, and modulate in cruise. Deflector doors ahead of the inlets extend on the ground to keep slush out, and a blue RAM DOOR FULL OPEN light shows when the ram air inlet door is fully open.

On the ground and at low speed there is too little ram airflow, so a ground cooling fan draws air across the heat exchangers. It may be electrically driven or a fan on the ACM shaft, driven by its turbine, like the A320's cooling air fan.

Air cycle machine and bootstrap system

The air cycle machine, or cold air unit, carries a compressor, a turbine and usually a cooling fan on one shaft. In the bootstrap air cycle system:

1. the primary heat exchanger pre-cools the bleed air;
2. the ACM compressor raises its pressure further, heating it;
3. the secondary heat exchanger removes that heat;
4. the air expands through the ACM turbine, doing work that drives the compressor and the fan.

Energy taken out of the air as work leaves it with less internal energy, so its temperature drops sharply, well below the outside air. The greater the pressure drop across the turbine, the colder the output, which is why the compressor first raises the pressure. Bootstrapping describes this use of the bleed air's own energy to drive its cooling, which lets the pack achieve deep cooling from modest bleed pressures.

If the A320's ACM seizes, the pack can still run in a heat-exchanger cooling mode at reduced flow, with temperature still regulated by the bypass and trim air valves.

Water separation

Cooling air below its dew point condenses its moisture. Left in the supply, the water would enter the cabin as fog or droplets and promote corrosion, so a water separator removes it, most importantly at low altitude and on humid days on the ground.

In the classic arrangement described in ATPL texts, a low-pressure water separator sits downstream of the ACM turbine and spins or coalesces the droplets out of the air. Because the turbine outlet can be below freezing, the separator can ice up. A temperature sensor therefore controls a water separator anti-ice valve, an anti-ice bypass that admits hot air between the turbine and the separator to keep it above freezing; alternatively a safety valve lets the air bypass a blocked separator, losing water separation but keeping the cabin supply.

Some packs extract the water before the turbine instead. The A320's water separator system dries the air before it enters the turbine, so the very cold air downstream of the expansion carries little water to freeze, and the 737 NG's pack likewise uses a high-pressure water separator.

Pack temperature control

The pack's outlet temperature is set by mixing cold air from the cooling stages with hot air that has bypassed them. The hot air bypass valve, or turbine bypass valve, routes some bleed air around the heat exchangers and ACM: more bypass gives warmer air, less gives colder. ATPL texts distinguish manual control, in which the pilot positions the valves, from automatic control, in which a controller compares the selected and sensed temperatures and drives the valves. On the A320 each pack's controller modulates the bypass valve and the ram air inlet flap; the ECAM BLEED page shows the bypass valve as H (hot) or C (cold).

Both packs produce the temperature needed by the zone demanding the most cooling; warmer zones receive hot trim air added downstream (see cabin air distribution).

Temperature sensors also protect the pack. On the A320 the ECAM shows the compressor outlet temperature amber above 260 °C and the pack outlet amber above 90 °C, and the PACK pushbutton's FAULT light comes on for a compressor or pack outlet overheat or a valve position disagreement. On the 737 NG an overheat closes the pack valve and lights the amber PACK light; once the fault has cleared, the TRIP RESET switch resets the light.

Pack controllers

Each pack is governed by a pack controller, an electronic line-replaceable unit (LRU) that monitors the pack's temperatures, pressures, ACM speed and ram air flow and controls its bypass valve, ram air doors, ground cooling fan and water separator anti-ice valve. Controllers are duplicated internally:

• A320: temperature is regulated by a zone controller and two pack controllers or, depending on the aircraft, by two air conditioning system controllers (ACSCs). Each is a dual-lane controller: one lane controls while the other stands by. A single lane failure has no effect; losing both lanes loses that pack.
• 737 NG: each pack has a primary and a standby control; if the primary fails, the standby control in the opposite controller takes over. If both fail, the PACK, MASTER CAUTION and AIR COND lights come on and the pack runs on without control unless an overheat trips it off.
• E190-E2: a two-channel air management system controller, either channel able to run the whole system.

Single and dual pack failures

Single-pack operation is a normal capability. A single 737 NG pack in high flow can maintain pressurisation and acceptable temperatures up to the maximum certified ceiling, and on the E190-E2 one pack can supply all the air conditioning and pressurisation, though single-side pneumatic operation with one pack inoperative limits the E190-E2 to 31,000 ft. For a take-off with one A320 pack unserviceable, the failed pack is selected OFF and the other may be left ON, supplied by its own engine's bleed, with the take-off N1 limited to the bleed-on value.

When performance is limiting, the take-off may be made with the packs off, or supplied by the APU, so that the engines carry no air conditioning bleed. On the A320 pack 1 is selected ON after thrust reduction and pack 2 at least 10 seconds later, for passenger comfort; selecting a pack before thrust reduction would raise the exhaust gas temperature. Packs are also selected OFF for engine start on the 737 NG.

A dual pack failure stops all conditioned air. The cabin can no longer be pressurised, so air leaks out, the cabin altitude climbs and the temperature drifts towards the outside value. The crew descend, normally to 10,000 ft or the minimum safe altitude, and divert (see decompression). Once the differential pressure is low enough, an emergency ram air inlet can ventilate the cabin. On the A320 the RAM AIR pushbutton opens it, but a check valve admits air only when the differential is below 1 psi, and the outflow valve then opens about 50 per cent under automatic control. The E190-E2's emergency ram air ventilates the aircraft below 25,000 ft when both packs have failed or are off.

On the ground, air conditioning can also come from an LP ground air conditioning unit connected to the mixing unit. The A320 prohibits using it at the same time as the packs.

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.