An environmental control system (ECS) of an aircraft provides air supply, thermal control, and cabin pressurization for the crew and passengers. It is designed to maintain a healthy and comfortable setting from boarding until deplaning by controlling heating, cooling, and ventilation of the flight deck and cabin. The ECS, one of the largest power-consuming and non-propulsive systems in civil aircraft, is built with redundancy so that pressurization and comfort are preserved even after the failure of one air-conditioning pack.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is aircraft ECS?
An aircraft ECS is an environmental control system that provides air supply, thermal control, and cabin pressurization for crew and passengers. It includes cooling of avionics, and in some aircraft, incorporates cabin humidity control. A modern transport aircraft's ECS controls heating, cooling, and ventilation of the flight deck and cabin.
Environmental control system (ECS) is the set of equipment that provides a healthy and comfortable setting for occupants throughout the flight. It brings cabin altitude down to a comfortable, breathable level, protects occupants from extreme conditions, and ensures that passengers can breathe more easily, resulting in a safer flight experience.
ECS controls ventilation, heating and cooling of both flight deck and cabin. It first supplies air by ducting bleed air that has been bled off from the airplane's turbine engines or from the APU. The bleed air management system performs bleed air source selection, pressure regulation and precooling before the air enters the environmental control unit (ECU). ECU, alternatively called the pack, usually consists of an air-cycle machine and one or more heat exchangers. It establishes the temperature necessary for heating or cooling the airplane and removes excess moisture. In cruise, ambient temperatures are low enough for bleed air to be cooled by heat exchangers alone, so the air-cycle machine is bypassed. Conditioned air is then pumped into the cabin and flight deck to meet ventilation, pressurization, cooling and heating requirements. Each pack consumes about 200 kW. Operation cost of the nominal ECS is the work spent in compressing the bled air. Most transport aircraft also have a duct through which conditioned air is supplied to the cabin from an external ground unit when the engines are not running.
ECS provides fire protection and includes smoke detection and fire suppression. Moisture and food odors are managed by exhausting air from galleys overboard through a port in the skin of the aircraft, while cabin humidity control keeps relative humidity from extremely low levels and prevents condensation. Commercial aircraft operated by airlines therefore rely on ECS to maintain cabin temperature within tolerable limits, regulate air quality and pressurize the cabin whether the airplane is on the ground or at 30,000 ft (9,144 m) where outside temperatures reach as low as 80°C (112°F).
What is aircraft ECS design?
Aircraft Environmental Control System (ECS) design is the multidisciplinary task of creating the network that makes flight in a hostile setting habitable. At its core the system consists of an air distribution system that supplies temperature-controlled air to crew and passengers. It is arranged as an open-loop configuration that facilitates overpressure and collective protection, or as a partial closed-loop configuration that still ensures the establishment of collective protection within the aircraft. Packs are typically located close to the wing box section, in the belly and in the vicinity of the centre fuel tank, so that conditioned air is fed through ducting whose design includes variable cross-sections, bifurcations, mounting features, overhang angles and minimum wall thickness, composite lay-ups are often chosen for the ducts to save weight.
Today's conventional architecture is reliable but expensive to operate, consuming roughly 1 kW per passenger, and designers must balance the total flow rate of air required to provide adequate circulation, the flow rate of conditioned air required to remove heat, the flow rate of outside air required to pressurize the aircraft, and the flow rate needed to remove contaminants. The air conditioning pack is split per engine, with two equal ACM nominal systems implemented so that one failure is tolerated, and the unit is designed so that air exits at a speed not higher than 2 m/s (6.56 ft/s) in each nozzle. Additional functions include cooling of avionics and fire suppression, while a cabin humidity control system is added to keep relative humidity from extremely low levels and to prevent condensation.
Next Generation Electrical ECS (eECS) aims to overturn the traditional penalty by developing high power motorised turbomachines, power electronics, control optimisation and thermal management technologies. Performance improvements will be achieved by combining components for processing cycled air with an innovative and refined vapour cycle system, and the demonstration will be optimised with respect to system weight, power consumption, reliability, aerodynamic efficiency and boosted engine power efficiency. The resulting high-performance system solution corresponds to the future needs of single-aisle aircraft, offering an effective candidate for reducing power consumption while still meeting all ventilation, pressurisation and protection requirements.
What are the components of an environmental control system?
The components of an environmental control system are listed below.
- Air Conditioning Packs (Pack Valve and Unit)
- Air Cycle Machine (ACM)
- Heat Exchangers
- Water Separators
- Air Distribution System
- Pressure Control System
- Sensors and Controllers
- Filters/Odor Absorbers
- Emergency/Back-up Equipment
Bleed air is supplied by a compressor stage of each gas-turbine engine and enters the BAS through a flow-control valve. Bleed temperature is controlled via a fan air valve that modulates fan-cold-air flow through a precooler, an air-to-air heat exchanger that gives bleed air its first stage of cooling. Non-return, motor-operated and solenoid-operated bleed air valves regulate pressure and temperature while preventing reverse flow. The PACK is the primary system for conditioning airflow. Each PACK contains heat exchangers, valves, a High-Pressure Water Separator, and an Air-Cycle Machine that is completely bypassed during cruise when ambient temperatures are low enough for adequate cooling by heat exchangers alone. Most aircraft install at least two Passenger Air Conditioners to guarantee redundancy. These units remove excess moisture, establish temperature, and deliver conditioned air to the cabin and flight deck.
The cabin subsystem is built around the Cabin Pressure Control System. A cabin air inlet valve admits external air, a cabin fan circulates air through a cabin heat exchanger, and a cabin air outflow valve ejects air to maintain target pressure. A cabin pressure control valve and a cabin pressure relief valve release excess pressure, while cabin pressure sensors and cabin air composition sensors continually monitor pressure and composition of cabin air and alert the crew to problems. Modern jetliners use high-efficiency particulate-arresting HEPA filters that trap more than 99% of bacteria and clustered viruses, and a cabin humidity control system keeps relative humidity from extremely low levels. Water and sanitation are part of the environmental control system: water discharge handles sanitary waste and condensates, while emergency oxygen system, emergency ram air ventilation, avionics cooling fans, and fire suppression and smoke detection subsystems complete the protection of passengers and equipment.
