Environmental Control Systems, Chapter ATA 100-21, are systems that control temperature, pressure and flow in areas with pilots and passengers, but also pressure and temperature in compartments that have installed electronics.
The main functions of the environmental control system, according to subchapters of ATA 21, are: 21-00 Air Conditioning - General, 21-10 Compression, 21-20 Distribution, 21-30 Pressurization Control, 21-40 Heating , 21-50 Cooling, 21-60 Temperature control, 21-70 Humidity / air contamination control.
The prominent ATA 100-21 subsystems are: air conditioning (21-00), avionics cooling (21-50), and cabin pressurization (21-30). Being extremely important for flight safety, for the accomplishment of the mission and for the occupants of the aircraft.
The air conditioning system can be steam cycle or air cycle, or both, depending on aircraft mission and design and certification requirements. The air cycle is quite common in large aircraft, FAR 25, and usually has a turbine that drives two compressors (two stages) and a ground fan. Alternatively the air cycle air conditioning machine can be electric and have a motor in place of the turbine to drive the compressors and ground fan. The steam cycle, most common in FAR 23 small aircraft and defense applications, is similar to a residential air conditioning system and works with an evaporation, condensation cycle and has an expansion valve for the refrigerant fluid.
Avionics or electronics cooling systems can use a bypass from the aircraft's central air conditioning system or use dedicated fan systems, outdoor air intakes or steam cycle machines. The main objective is to keep the temperature within a range for the proper operation of the electronics, which can have their own cooling system from the ambient air. For every 10˚C of temperature rise, the electronics can cut its operating life by half. Therefore, high temperatures can cause malfunctions or shorten life, causing unscheduled maintenance replacements for components.
The pressurization systems can operate with pressurized air from the air cycle machine or from a compressor driven by a dedicated electric motor, when the air conditioning is steam cycle or when the aircraft is all electric (all electric aircraft).
The analyzes required are generally pressure drop, distribution, gasper valve flow, thermal comfort, pulldown, pullup, thermal performance and temperature. Other analyzes involve the control of pressure, flow or temperature, in other words, the dynamics of the plant and its control. CFD analysis may also be required for thermal comfort, manifolds, valves, sensor placement, head loss determination, air or Ram Air inlets, and header inlets for example.
Humidity also plays a fundamental role in air conditioning systems as it needs to be controlled because of thermal comfort conditions for passengers and crew, operational conditions of electronics not resistant to water and condensation, and structural corrosion primary aircraft. Ice melt and condensation can occur on the inner surface of the fuselage and cause dripping on people or electronics.
Another very important aspect is the dispersion of chemical contaminants and agents such as viruses, bacteria and fungi. The aircraft cabin must be safe for passengers, crew and animals in the cargo hold. There are specific standards and certification rules that need to be met.
According to Airbus studies, 80% of passengers are concerned about cabin comfort. Part of this perceived comfort is due to the temperatures and speeds of the in-cab air conditioning system. To ensure a suitable design, it is necessary to perform thermal comfort analyzes based on test data or numerical simulation results by CFD. Executive aircraft have the highest cabin pressure during flight (approximately 6,000 feet). The commercial ones, on the other hand, operate at 8,000 feet, reaching 10,000 feet in case of failure. Remember that La Paz, Bolivia, is 12,000 feet high, but few places on Earth are that high. Above 10,000 feet, in cases of failure or aircraft that do not have pressurization, oxygen systems must be used.
The main environmental control systems certification reports are: system description, safety analysis, test proposal, test results as per FAR 25 or 23 requirements, but also component qualification according to RTCA-DO-160.
A very important test for avionics cooling is the temperature qualification which is carried out in large climate chambers. Another very important test that verifies the correct installation of the electronics is the aircraft temperature test, which is carried out in places with extreme (maximum and minimum) temperature around the globe.
According to the certification requirements applied to the design of the aircraft, through the Functional Hazard Analysis, Failure Mode and Effect Analysis (FMEA), the necessary level of redundancy for the systems is obtained.
The system and its components can be certified by analysis, testing or similarity. This is determined in a means of compliance analysis (MOC).
The numerical tools that support the design and certification of environmental control systems are CFD++, FEA (NASTRAN or Adina), Matlab/SciLab or Siemmens AMESim/Modelica (for control or transient regime) and Siemens FloMaster (for permanent regime) . ATS also has tools specifically developed for Python and Excel applications.
ATS has experience in environmental control system design, certification and testing, including simulation, analysis and installation.
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