Environmental Control System (ECS)

The Environmental Control System of an airliner provides air supply, thermal control and cabin pressurization for the passengers and crew. Avionics cooling, smoke detection, and fire suppressionare also commonly considered part of the Environmental Control System.

On most jetliners, air is supplied to the ECS by being "bled" from a compressor stage of each gas turbine engine, upstream of the combustor. The temperature and pressure of this "bleed air" varies widely depending upon which compressor stage and the RPM of the engine.

A "Manifold Pressure Regulating Shut-Off Valve" (MPRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems. This flow restriction results in efficiency losses. To reduce the amount of restriction required, and thereby increase efficiency, air is commonly drawn from two bleed ports (3 on the Boeing 777).

When the engine is at low thrust, the air is drawn from the "High Pressure Bleed Port." As thrust is increased, the pressure from this port rises until "crossover," where the "High Pressure Shut-Off Valve" (HPSOV) closes and air is thereafter drawn from the "Low Pressure Bleed Port."

To achieve the desired temperature, the bleed-air is passed through a heat exchanger called a "pre-cooler." Air from the jet engine fan is blown across the pre-cooler, which is located in the engine strut. A "Fan Air Modulating Valve" (FAMV) varies the cooling airflow, and thereby controls the final air temperature of the bleed air.

The Cold Air Unit, or "Airconditioning pack" is usually an air cycle machine (ACM) cooling device. Some aircraft, including early 707 jetliners, used vapor-compression refrigeration like that used in home air conditioners.

On most jetliners, the A/C packs are located in the "Wing to Body Fairing" between the two wings beneath the fuselage. On some jetliners (Douglas Aircraft DC-9 Series) the A/C Packs are located in the tail. The A/C Packs on the McDonnell Douglas DC-10/MD-11 and LockheedL-1011 are located in the front of the aircraft beneath the flight deck. Nearly all jetliners have two packs, although larger aircraft such as the Boeing 747, Lockheed L-1011, and McDonnell-Douglas DC-10/MD-11 have three.

The quantity of bleed air flowing to the A/C Pack is regulated by the "Flow Control Valve" (FCV). One FCV is installed for each pack. A normally closed "isolation valve" prevents air from the left bleed system from reaching the right pack (and v.v.), although this valve may be opened in the event of loss of one bleed system.

The A/C Pack exhaust air is ducted into the pressurized fuselage, where it is mixed with filtered air from the recirculation fans, and fed into the "mix manifold". On nearly all modern jetliners, the airflow is approximately 50% "outside air" and 50% "filtered air."

Modern jetliners use "High Efficiency Particulate Arresting" HEPA filters, which trap >99% of all bacteria and clustered viruses.

Airflow into the fuselage is approximately constant, and pressure is maintained by varying the opening of the "Out Flow Valve" (OFV). Most modern jetliners have a single OFV located near the bottom aft end of the fuselage, although some larger aircraft like the 747 and 777 have two.

In the event the OFV should fail closed, at least two Positive Pressure Relief Valves (PPRV) and at least one Negative Pressure Relief Valve (NPRV) are provided to protect the fuselage from over- and under- pressurization.

The atmosphere at typical jetliner cruising altitudes is generally very dry and cold, and the outside air pumped into the cabin on a long flight typically has a relative humidity around 10%. The fact that cabin pressure is generally lower than the pressure at ground level does not of itself contribute to the dryness.