Definition

The environmental control system (ECS) in a modern transport aircraft controls heating, cooling, and ventilation of the flight deck and cabin. The ECS is integrated with the aircraft's pressurisation system. In addition to providing pressurised and conditioned air, ECS systems also normally provide cabin air recirculation, avionics cooling fans, emergency ram air ventilation, and conditioned air for gaspers at passenger seats and crew stations.

System Operation and Components

The heart of an ECS system is the air conditioning packs. In most aircraft, at least two are installed. Compressed bleed air tapped from the engines supplies the packs through flow control valves. Air entering the system at this stage is extremely hot. The air is cooled to more comfortable temperatures through the use of heat exchangers and air cycle machines (ACMs).

In typical ECS designs, bleed air entering through the flow control valve receives the first stage of cooling at a heat exchanger. Ram air enters through an inlet valve and flows over the heat exchanger, thus somewhat cooling the hot bleed air. The bleed air, still fairly hot, then flows through an air cycle machine. The ACM does not use a chemical refrigerant. Instead, it cools the air adiabatically, through rapid expansion. Many ACM systems include a compressor that further compresses the bleed air before it reaches a turbine, which causes the rapid expansion.

Air leaves the turbine section of the ACM at a very cold temperature. Meanwhile, some of the hot bleed air from the engines is diverted around the ACM. The diverted hot air joins with the cold air in a mixing chamber, which delivers conditioned air at the desired temperature for crew and passenger comfort. The air passes through filters to remove contaminants, and water separators to remove excess moisture.

Ground Operation

On the ground when engines are not running, most ECS systems can use bleed air tapped from the aircraft's auxiliary power unit (APU). The system conditions APU bleed air in the same way it conditions engine bleed air. Most transport aircraft also have a duct through which conditioned air may be supplied to the cabin from an external ground unit.

Abnormal Operation

ECS systems are usually designed so that the aircraft can remained pressurised and comfortable even after the failure of one air conditioning pack. For example, the Embraer 170 can maintain adequate pressurisation and temperature control on one pack at altitudes up to 31,000 feet.

In case of smoke in the cabin or complete pack failure, many designs include an emergency ram air valve. The valve can open to let in outside air for ventilation and smoke evacuation.

Protections and Failure Modes

Modern, highly automated ECS systems normally include protections that prevent the system from extracting engine bleed air (and thereby reducing engine power) during certain engine failure scenarios. For example, control system logic might shut off air conditioning packs on takeoff if an engine fails or if the thrust levers are set to maximum power. The system re-opens the packs when the aircraft climbs above a set altitude. Typical systems will also shut off the packs during other types of emergencies, such as a bleed air leak. Some systems will also prioritise bleed air use in certain situations. For example, if wing icing is detected during takeoff or go-around, the system might temporarily close the packs to direct more bleed air to the anti-icing system.

Additionally, typical engine bleed air systems will shut off bleed air to the ECS if an engine fire or overheat is detected.

Integration with the Pressurisation System

The air supplied to the cabin by the ECS system also pressurises the aircraft. ECS air is pumped into the cabin to bring the cabin altitude down to a comfortable, breathable level. To maintain a given cabin altitude, the ECS air is expelled at a controlled rate through an outflow valve. However, air enters the cabin at a higher rate than it exits through the outflow valve. This keeps the air pressure inside the cabin higher than the ambient pressure outside the aircraft. (Higher differential pressure corresponds to a lower cabin altitude. Think of the cabin as a balloon continuously supplied with air that is allowed to flow out through a small hole at a metered rate, keeping pressure constant inside the balloon.)

For more detail on pressurisation systems, please see the related article.