Environmental control systems are crucial to maintaining the comfort and safety of passengers and crew, but despite the wide variety of aircraft in commercial use across the world, very similar systems are in place on most passenger jets. While fume events are a worry for researchers, and passengers remain concerned about getting sick from germs in cabin air, the systems arguably are in need of more scrutiny.
Generically, the environmental control system (ECS) uses bleed air from the engines. This goes through a number of coolers, pre-coolers and various systems, and is managed via a series of ducts and filtration systems before going into the cabin. In simplified terms, air is first compressed to high pressure and temperature, and then conditioned in an environmental control unit. Excess moisture is removed, and the temperature necessary for heating or cooling the aircraft is established. The conditioned air is then delivered to the cabin and cockpit to maintain a comfortable environment.
“Bleed air is what I would call essentially a first-generation system; it has been around since pressurized aircraft first came along. The technology has changed—we have seen some changes in terms of the components—but most of it is pretty traditional,” says Mark Scully, head of technology, advanced systems and propulsion at the UK Aerospace Technology Institute (ATI).
On the ground, compressed air to supply the environmental control system is sometimes obtained from an auxiliary power unit. It also can be obtained via ground carts at the airport, or high-pressure systems such as hydrants or the aircraft’s engines.
During flight, compressed air is almost exclusively drawn from the compressor stages of the aircraft engines. This has led to concerns being raised about the safety of obtaining air from the engines to feed the cabin. In theory, because the bleed air comes from the engines but is bled upstream of the combustor, it should be free of contaminants from the combustion cycle. But sometimes seals leak oil, which contains potentially hazardous chemicals. This is known in the industry as a “fume event.” Fume events must be dealt with quickly, since failed oil seals will reduce the engine’s life.
Although ECSs tend to be generic across most aircraft, that is not the case for the Boeing 787 Dreamliner. The increased utilization of electrical systems on the aircraft means it uses more electricity instead of pneumatics to power systems such as hydraulics, engine-starting and wing ice protection. The benefits of this include more efficient power generation, distribution and use of electric power, and improved fuel economy. A greater level of electric architecture also translates to lower maintenance costs and fewer maintenance tasks.
The Boeing 787’s environmental control system could also take advantage of this change. “When the 787 came along, the idea was that by electrifying the aircraft and taking more advantage of electrical power, they were able to better manage the offtake of air from the engine, and hence improve fuel burn. On the 787, rather than just bleeding air continuously from the engine, electrical power that is being generated by the engine is used to drive the environmental control system,” notes Scully.
On the Dreamliner, a series of electrical motors drive the ECS. It features heat exchangers to transfer heat, as well as filtration systems to clean up air. Because the system is electrified, the speed of the pumps can be varied in terms of extracting ambient air from outside the aircraft and pressurizing it for the aircraft cabin.
That has a lower energy cost than continuously bleeding air from an engine. An electrical system such as that on the 787 also can manage the health of the ECS system so it can predict ECS motor failure, for example, for more effective maintenance.
“In the future, the technology means we could introduce even more efficient electrical drives for the compressors within the ECS, to manage the amount of power used. It will also be possible to vary the amount of air being introduced to the cabin and the amount of treatment applied to it. In terms of managing impurities within the air, there’s a host of different things,” says Scully. Environmental control systems are one of a number of key technologies that the ATI identifies in its recent technology strategy for smart and electric aircraft.
There may be future lessons for aerospace from the automotive industry, not just about comfort but environmental control efficiency as well, says Scully. “One of things you need to bear in mind is that when you are extracting air from the outside of the aircraft, is it is pretty chilly—as in -50C [-58F]—so when you compress that air, heat it and look at how best to manage the overall environment, you can manage the quantity of air that is fresh,” says Scully. Could aircraft use more recirculated air? “If you look at any of the cars that we drive today, a lot of what our automatic air conditioning systems are doing is actively managing the amount of fresh air that is coming in.
“For all sorts of different reasons, they are using fresh air only when they need to use fresh air. They do a lot of recirculation,” says Scully.
To reduce fuel burn, from a design engineering perspective, there are also some potential weight-saving opportunities with ECS ducting. Composites or hybrid materials could be used here, subject to cost, says Scully.
Cabin air quality concerns are likely to remain important. The ATI says it expects to become involved in more cabin air quality projects. Some of these are currently not in the public domain. Other research organizations in Britain already have carried out extensive studies on cabin air, with the safety of passengers in mind.
Professor Chris Walton, lecturer in analytical technology at Cranfield University, was one of the lead researchers and authors of a report on aircraft cabin air quality undertaken by Cranfield and two analytical laboratories on behalf of the UK Department for Transport, back in 2011. “One of the concerns remains that lubricated products could get into the bleed air system and cause contaminated air events. Of course, if you are not using bleed air for the environmental control system, then whether contaminated air will result becomes irrelevant,” he says.
The Cranfield study reassured those who were concerned about cabin air quality from bleed air, says Walton. Cabin air was generally found to be of good quality. “What you do find is that when an aircraft is on the ground, taxiing, or on the stand, there is a greater level of contaminants, such as ultra-fine particles. Once you shut the door and turn the air conditioning on, they tend to go away until you get to the other end and you open the door again,” he explains. Data from the Cranfield study sampled air from 100 flights but did not detect an adverse air quality event. “We steered away from making any statements on toxicology or health effects,” he says. “We did not want to speculate on that beyond the data that we had.”
In terms of being subjected to potential contaminants, passengers are likely to spend relatively little time in an aircraft. Their exposure is much less than for the flight crew, which is possibly more of a cause for concern. But Walton says: “Over the long term, in terms of public health, I would worry more about exposure to [emissions from] traffic on the ground and in industrial environments. I spend a very small proportion of my life sitting in an airplane.”
Alison McAfee, representative for Airlines for America (A4A), says that airlines are taking pains to protect passengers via the aircraft ECS. “The safety, security and well-being of our passengers and crew is always our highest priority, and airlines know that the cleanliness of the aircraft and cabin components is important to customers when they make their travel decisions,” she says.
“Each carrier follows their own individual protocols, but airlines work continuously—both between flights and overnight—to ensure a clean and comfortable cabin area for their passengers.”
She adds that most commercial aircraft also use hospital-grade HEPA air filters to remove viruses and bacteria from the cabin. To the extent that a communicable disease threat arises, A4A works closely with the Centers for Disease Control (CDC) and other government agencies such as U.S. Customs and Border Protection, to ensure that member airlines are kept informed regarding any specific government guidelines put in place. “As always, airlines and our airport partners routinely remind customers to follow CDC guidelines on healthy traveling habits,” McAfee says.
Engineers will continue to assess the potential of new environmental control systems, with an eye to the smarter, electric aircraft of the future. There are already signs that the development of a novel environmental control system on the Boeing 787 is having an impact on the passenger experience: Anecdotal evidence from 787 passengers suggests that they don’t feel jet lag to the same degree as on other aircraft.
Researchers say it is not yet known whether that is because the pressurization level is different in a 787 cabin, or because of the novel electrical management of the ECS. c