Printed headline: Air Superiority?
Pneumatic systems power and provide air for an array of aircraft functions. Best known are cabin pressurization and air conditioning, main engine start and anti-icing, but other functions include fuel tank pressurization, fuel tank inerting, avionics cooling and engine pneumatics.
Distinct from hydraulic systems, which tend to power mechanical devices such as flaps and landing gear, pneumatics also differ in that they use their medium—air—as a power source and a function. This has been the case for most of the modern jet era, but new technology is shaking things up.
Until the Boeing 787, most aircraft performed the pneumatic functions outlined above by drawing air from an aircraft’s ram-air intakes and from the engines using bleed-air systems. This can be inefficient, because a bleed system takes air that could be used for thrust. Furthermore, the ducting and valves needed to pipe that air around the aircraft add weight, and more bleed air is taken than can be used.
With the 787, Boeing replaced the traditional bleed-air control with electric-motor-driven compressors that feed the rest of the aircraft’s environmental control system (ECS), and changed its wing deicers from pneumatic to electric. In comparison with an Airbus A320, which can generate up to 270 kW of electrical power, the 787’s alternators can generate up to 1,500 kW—enough to power the homes of a small town.
UTC Aerospace Systems manufactures the 787’s electrical ECS, and its vice president of engineering confirms that electric systems are increasingly competing with traditional pneumatics. The company has provided electro-pneumatic bleed systems for commercial aircraft since the 1990s.
“Aircraft designers’ preferences are influenced by balancing overall energy optimization at the aircraft level,” says UTC’s Bill Dolan, adding: “Depending on the aircraft and ECS configuration, it could be more efficient to generate pressurized air using an electric compressor.”
Electric systems are no magic bullet, however, and involve several tradeoffs with regard to cost, weight and reliability. More-electric aircraft require bigger and heavier generators, for example, and the 787 was beset by electrical quirks in its first few years of service. Notably, Airbus did not follow the 787’s lead with its later-built A350, which uses an electro-pneumatic bleed-air system that it describes as “more efficient at the aircraft level.”
“Aircraft OEMs have been striving, and still strive, for substantial progress in cost, weight, reliability and maintainability,” says Nicolas Bonleux, managing director and chief commercial officer of Liebherr-Aerospace & Transportation. “All those aspects have been driving them to rely more and more on pneumatic power, at least as long as electrical technologies have not reached the sufficient level of performance.”
Nonetheless, Liebherr is making significant investment in electric systems as well as in its traditional pneumatic specialties. Among its new products are hybrid electro-pneumatic bleed-air systems for the A330neo and Embraer E2, and trials of all-electric, bleedless systems on A320 and ATR test aircraft.
In the future, Bonleux says, the choice between electric and pneumatic will come down to specific aircraft configurations and the technology used by other onboard systems. “There will be some aircraft in the future for which it will make sense to fully replace pneumatic power by electrical power. However, we are convinced that for other aircraft pneumatic power will remain the best-suited technology,” he says.
Pneumatic power is an old technology, dating back to at least the 17th century, albeit one that is constantly being refined as operators seek improvements in weight, cost and reliability. New materials and production processes—such as additive manufacturing—can help, but there is also still plenty of room for design ingenuity.
One example is to combine pneumatics with electrical power to improve their efficiency. “Liebherr has developed very convincing power electronics and electrical drive solutions that supplement the pneumatic power to run the pneumatic systems and that hence substantially enhance the efficiency of our pneumatic systems,” says Bonleux.
Liebherr has also proved the potential of additive manufacturing by 3D printing a titanium hydraulic valve for the A380’s flight controls. The valve offers the same performance as the conventional valve block made from a titanium forging, but is 35% lighter and consists of fewer parts.
Intrinsic specifications aside, pneumatic systems must adapt to the changing designs of the wide array of aircraft components they interact with. Examples of such interfaces are with the engine, pylon, slat, electrical generation and distribution systems, APU, avionics and interiors.
“Design of the pneumatic systems is closely interrelated with the design of other parts and systems, and any change in other interfacing systems is likely to impact the pneumatic system—and the other way around,” says Bonleux.
The operational profile of an aircraft also matters. In general, the pneumatic systems on widebody aircraft are scaled-up versions of those on narrowbodies, but they do involve some specific considerations. A widebody, for example, needs a supplemental cooling system since its galleys need to keep food cold longer. Furthermore, the configuration of widebody pneumatics needs to be adjusted to optimize energy usage for aircraft that spend a greater proportion of each flight in high-altitude cruise than narrowbodies do.
“Changes in maximum cruise altitude, air speed, maximum passenger count and engine power—bleed and electric—availability affects the definition of every major part of the pneumatic system,” says Dolan.
Design is also governed by safety regulations, which cover four main requirements for pneumatic systems: minimum per person fresh air flow rates; maximum bleed-air temperatures in different parts of the aircraft; system redundancies; and component insulation for composite structures.
Some of those regulations force pneumatic systems to be larger than needed from a functional standpoint. For example, redundancies mean some systems are duplicated, while a typical heat exchanger is oversized to allow for safe flight operations in the event of a failure of similar components.
Air conditioning is a key part of any pneumatic system, and aircraft employ either air-cycle- or vapor-cycle-based technology. In the former, air is compressed and expanded through an air cycle machine and passed through a series of heat exchangers. A variant of this is the condensing cycle, for which UTC offers patented technology that Dolan says is light and efficient, and which uses software-based controls to optimize performance at all stages of flight.
He adds that by replacing the oil-lubricated bearings of air cycle machines with frictionless air bearings, the industry has extended the mean time between failure of air cycle systems from 5,000 hr. to more than 100,000 hr. over the past few decades. UTC also produces vapor-cycle conditioners, which use a refrigerant combined with a condenser, evaporator and electrically powered compressor to generate cool air. “Technology such as the UTC Aerospace Systems ‘economized’ cycle enhances the cycle efficiency and allows the use of smaller and lighter components when compared with a standard vapor cycle,” Dolan says.
Although fuel efficiency is important to airlines and OEMs, pneumatic system weight and performance are only a small part of the total fuel-burn equation. At some point, energy optimization takes a backseat to other factors.
“Once the pneumatic system is configured to meet the required functional and safety needs of the specific aircraft, original equipment cost and maintenance cost become the highest priority for the OEMs,” says Dolan.
Liebherr supplies the full range of aircraft pneumatic systems, and Bonleux says the reliability of its equipment has increased many-fold in the past few decades. This is thanks to the introduction of new technologies such as electro-pneumatic bleed-air systems as well as operating and maintenance optimizations. Nonetheless, the aftermarket is still hugely important to the pneumatic systems supplier, accounting for 30% of total sales.
“We expect this share to stay stable in the future because [of] the huge progress we have achieved in bringing down operating costs, and thus our revenues, will be compensated by the rapid extension of the in-service fleet,” he says.
Further life-cycle gains are achievable as pneumatic and electro-pneumatic systems incorporate more sensors to generate data that can be used to enhance reliability through better design, or through predictive and preventive maintenance.
“We’ve made considerable progress in system and component reliability through the use of big data, which enables Liebherr to develop tailored solutions for each airline and each aircraft for the most maintenance-intensive parts,” says Bonleux.
Dolan agrees, pointing out that while electrification adds to system complexity, it also means “opportunities to monitor equipment with more sensors in order to develop better diagnostic and prognostic capabilities and bring a reduced cost of ownership.”