Printed headline: Breathe In
A cabin air system is more than a standard air conditioner fitted into an aircraft cabin. Its role is to heat and pressurize the cabin while outside air temperature is dozens of degrees Celsius below freezing and air pressure is less than one-fourth of what it is at sea level.
Most of these systems use engine bleed air, which introduces major technical challenges. The three main system providers in commercial aviation—Collins Aerospace, Honeywell and Liebherr Aerospace—are endeavoring to improve their products. They also have to cope with aircraft and engine design evolution, which translates into additional constraints.
Meanwhile, cabin air system maintenance has begun benefiting from big data analytics, which makes it more predictive.
The cabin air system consists of the bleed air system and the environmental control system (ECS), itself made of a refrigeration pack, a cabin pressurization system and a ventilation system. Key components include pneumatic control valves and heat exchangers as well as temperature, pressure and flow sensors, electronic controllers and rotating machinery such as air-cycle machines (cooling turbines and compressors) and fans.
A turbine in the air management system spins at 50,000-70,000 rpm and uses air-bearing technology, thus avoiding the need for grease, says Tom Hart, vice president and general manager for air and thermal systems at Honeywell. Adding to complexity are air quality devices—ozone converters, humidification system and filters.
What are the design drivers of a cabin air system? “It is an integral part of aircraft design,” says Paul D’Orlando, Collins’ business development director for air management systems. Changes in maximum cruise altitude, airspeed, maximum passenger load and engine power availability affect the definition of every major part of the pneumatic system.
Because the air that passengers and crew members breathe is not only a matter of comfort, safety regulations apply. They impose a minimum fresh air flow rate based on the number of occupants. This rate directly impacts the size of major components like air cycle machines, heat exchangers, pneumatic control valves and ducts, says D’Orlando.
In other rules, a maximum is set for the temperature of the bleed air as it travels through different zones in the aircraft. Component insulation is required, to avoid damaging composite structures.
Mechanical and control redundancies are mandatory, in the event of a component failure. This can affect the design through the need for duplicate parts but also requires sizing parts larger than needed. For example, the size of a typical heat exchanger is determined based on an assumption that other components have failed. That means the heat exchanger is oversized during normal operation, D’Orlando notes.
Additional design drivers include the number of passenger areas “where you want to be able to control cabin comfort independently—first class, business class and economy class,” a Liebherr representative points out. At the same time, other areas may have to be provided with air conditioning such as a crew rest compartment or a cargo compartment carrying animals.
All these technologies are evolving with aircraft design. “The latest technologies are mostly linked to the electrification of the aircraft—electrically powered air conditioning, with variable-speed fans and high-power electronics becoming commonplace,” says Liebherr. The Boeing 787 inaugurated a so-called more electric architecture, in which electric systems replaced numerous pneumatic and hydraulic systems. In Collins’ more electric ECS for the 787, the traditional pneumatic bleed air control system has been replaced by electric motor-driven compressors that feed the rest of the ECS.
“Electric power generation and conversion technology is now a key enabler for future improvements in pneumatic systems,” says D’Orlando. The 787 has yet to be followed by another commercial aircraft design in terms of more electric architecture—the Airbus A350 is conventional in that regard. Nevertheless, bleed air usage is likely to be increasingly considered a fuel penalty because it takes away some energy generated by the engine for a purpose other than propulsion. It can be more efficient to generate pressurized air using an electric compressor.
For those aircraft systems that rely on bleed air, higher pressure and temperature must be managed. Increased engine efficiency translates into harsher working conditions for the ECS inlet. New valves and heat exchangers, featuring suitable materials, need to be continuously developed. The air coming out of the ECS is put through additional heat exchangers, says Hart.
But further improving cabin air system reliability is extremely challenging. This is partly due to the number of mechanical components involved. Software developments are key to ensure all those systems work together in an integrated manner that is efficient. The sequence of valves and air movements is complex. “One of the largest issues for the airlines is the air management system; a failure can ground the aircraft,” Honeywell’s Hart emphasizes.
The equipment manufacturer has developed algorithms that factor in parameters such as outside air temperature as well as recent weather-related temperature and humidity, the presence of sand and previous maintenance data. A model is thus built to predict failures and measure system efficiency.
Trials have taken place with “between seven and 10” airlines since 2017, Hart says. “We have seen a 35% cut in delays and cancellations associated with the air management system and no-fault-found events reduced to 2% of the removals.” The main enabler is the emerging power of big data analytics—the improvement is made without any additional sensors. “Just by using available data, we can predict failures and [enable] smoother operations for the airline,” he adds.
Such models are deemed mature for the Boeing 777 and Airbus A320 and A330. Honeywell is developing more algorithms for the A350 and Boeing 737 MAX. Using such a model, a carrier may be confident, prior to aircraft dispatch, that the system will operate faultlessly. “It also works for aircraft with non-Honeywell systems,” says Hart.
Liebherr is betting on predictive maintenance as well. When a system uses outside air, it naturally draws in everything else contained in that air—including water, hail, sand, dust and salt, to name just a few, says the company representative. The more thickly the dirt builds up on the heat exchanger, the less efficient it becomes.
To address this issue, Liebherr put together a team of designers, data scientists and specialists from its technical customer service, and cooperated with other companies in aviation and data management. After analyzing the causes of interruptions in operations for 18 months, the team created a “health manager app” for the A380’s central refrigeration unit, an air system that keeps food cool. Using predictive calculations, the specialists managed to reduce wear and the number of key components to be replaced.
Physical maintainability is critical as well. “At Collins, we focus on designing a system that can be easily diagnosed and dispatched by a simple maintenance action (i.e., locking a valve), and we prioritize component arrangement and accessibility based on historic reliability,” says D’Orlando. The actual ECS arrangement is often constrained by the interfaces established by the airframer, however. c