Advances in aircraft cabin noise reduction and air quality have progressed significantly since the Douglas DC-1 “set a new standard for passenger comfort”, according to Boeing’s historical account of the aircraft model that debuted during the 1930s as the predecessor for the DC-2 and DC-3 models. “Great efforts were made to insulate the passenger compartment from the noise of the plane’s engines,” as passenger seats were mounted on rubber supports, the cabin was lined with noise absorbing fabric and engines were mounted on rubber insulators.
Passenger comfort further evolved when Boeing introduced the pressurised 307 Stratoliner with a cabin altitude of 2,438m around 1940. Fast forward to 2013, and Boeing operates the 787 with a cabin altitude of 1,828m while featuring new technologies to further improve cabin humidity and filtration to decrease passenger fatigue and other common maladies associated with air travel. The surprisingly quiet cabin on the Airbus A380 has actually produced some complaints amongst passengers and cabin crews that the aircraft is too quiet, as noise once drowned out by the engines becomes more pronounced to the naked ear.
CLEARING THE AIR
Boeing’s long-awaited debut of the 787 in late 2011 on a maiden voyage operated by All Nippon Airways not only closed the chapter on a difficult developmental campaign for the aircraft, but also marked the debut of new cabin innovations that include 20% larger dimmable windows, technology to ease the effects of turbulence and improvements not only in cabin humidity levels but also the introduction of a new filtration technology based on research the airframer carried out with the Technical University of Denmark (DTU).
According to Boeing’s air purification system supplier Donaldson Aerospace and Defense, during two years of laboratory tests Boeing and DTU simulated long-haul flights with participants spending up to 11 hours in an aircraft with and without air purification systems that featured varied airflow and humidity levels.
As a result of its studies carried out in conjunction with DTU, Boeing concluded that while improving cabin humidity levels did increase passenger comfort, using a gaseous filtration system was most effective in reducing the most common symptoms associated with the dryness of aircraft cabins – eye and throat irritation, headaches and occasional dizziness.
Boeing determined the high-efficiency particulate (HEPA) filters that first emerged nearly 20 years ago could rid aircraft cabin air of particulates “down to the size of viruses”. However, “they are not equipped to filter gaseous molecules”, the airframer states.
“If it weren’t for the research, we might have just decided to increase the humidity,” said Boeing once the testing was complete. “But the results clearly showed that we could do even more to improve passenger and crew comfort by also applying new air purification technology.”
In a product description of its air purification system (APS) Donaldson explains that even as an HEPA filter traps more than 99.97% of airborne particulates of a 0.3-micron size, they cannot remove gaseous contaminants from cabin air. The chemical filter Donaldson uses in its air purification system utilises a gas-phase adsorption process that captures gas molecules on its surface and removes contaminants.
Boeing’s rival Airbus states that high-efficiency HEPA filters “remove 99.99% of organic material and carbon odours from re-circulated air”. For hydrocarbon odours originating outside a jet, particularly in the airport environment, Airbus says a volatile organic compound converter (VOC) to rid the cabin of kerosene odours will be standard on the A350 XWB. The manufacturer also highlights its use of gaseous filters installed in cabin re-circulation systems to remove odours emitted during meal service.
Donaldson says its APS is easy to install on both on new aircraft and retrofit for in-service jets, but for now Boeing is keeping any plans to feature the improved cabin filtration technology on next-generation aircraft close to its chest. “Any retrofit on existing models (other than the 787) or inclusion of additional filtering technology on future models would be made on a case-by-case basis,” the airframer states. Based on that information it remains uncertain if the airframer will opt to include gaseous filtration technology on its forthcoming 737 Max narrowbody or the planned upgrade of its 777 widebody currently dubbed the 777X.
Boeing’s advances in cabin filtration on the 787 also rely on the aircraft’s no-bleed architecture system. Typically, cabin air is a mix of outside air compressed by the jet’s engines – bleed air – and re-circulated air. But as Boeing has explained in the past, electrically driven compressors provide the 787’s cabin pressurisation function, with fresh air being brought onboard through cabin air inlets.
The airframer acknowledges on rare occasions contaminants from bleed air used for air circulation on other aircraft models may enter the cabin. “The contaminant levels are expected to be lower than occupational health thresholds established by toxicologists who have studied these contaminants extensively,” says Boeing. However, the airframer also notes that has supported research conducted by the UK and US government examining cabin air quality.
During 2011, Boeing settled a lawsuit filed by former flight attendant Terry Williams, who claimed that exposure to toxic cabin air on a McDonnell Douglas MD-82 resulted in a spate of conditions including migraine headaches, neurological impairment, cognitive deficiencies and uncontrollable tremors, according to court documents. The suit was filed against Boeing after it purchased rival McDonnell Douglas in 1997.
Since Williams filed her lawsuit, reported incidents of odd cabin odours or fumes have continued to build. The Association of Flight Attendants-CWA published a paper in 2011 in which an industrial hygienist built a dataset of air supply contamination reports from a major US carrier spanning a two-year period. In short, 87 fume events were identified on 47 aircraft fleet-wide. Crews reported unusual odours on 83 of 87 flights “most commonly described as dirty socks”, the reported states. Of those 87 fume events, one or more crewmembers had symptoms serious enough to require emergency medical care after 27 flights.
Asked by the APEX Editor’s Blog if Boeing is conducting its own research with respect to contaminants in bleed air a company spokesman declined to comment.
Airbus addressed growing concern over cabin air quality in an article published in its Fast 50 technical publication during August 2012, which read: “A top priority is clean air supply, starting with the sources of pressurised air for the aircraft ventilation, engines, APUs [auxiliary power units] and air conditioning systems. Since the 1990s, Airbus has defined additional requirements for bleed air cleanness on the top of the requirements used for the certification of engines and APUs by the authorities.”
The airframer remarks it is supporting and promoting international research projects “to acquire independent external expertise and scientific knowledge by promoting best standards for an ideal cabin environment and its verification”.
Airbus explained in its technical publication that a measurement system was installed on several A330/A340 aircraft featuring a mass spectrometer and a multifunctional sampling system “to elucidate the origin of a particular smell that occurred during production flights only”.
The system consists of two trolleys that can be installed in aircraft galleys, and contain up to 36 sampling tubes tailored for different compound classes and several sensors. “With this analytical system, a marker substance and finally the root cause for the smell could be identified unambiguously, based on the results or air quality measurements on the aircraft during flight,” Airbus explained. “Knowing the precise root cause and having clear evidence at hand, corrective actions could be implemented at the supplier which is manufacturing the component responsible for the smell, and will be further followed up.”
But as Airbus supports independent research and conducts its own studies to improve air quality, the company concluded in its technical publication it is tempting to “combine sophisticated measurement technologies in order to revolutionise troubleshooting and aircraft system monitoring. Airbus continually reviews whether any air filtration or cleaning technology could add any value to the passengers, crews and aircraft operations…the introduction of such technologies needs to be thoroughly justified.”
Outside the work done by manufacturers and governments, the American Society of Heating, Refrigeration and Air Conditioning (ASHRAE) has a committee dedicated to developing a cabin air quality research plan. “We look at the environmental control system (ECS) as a complete package, which consists of outside air, re-circulated air and exhaust air conditioned by heat exchangers, compressors and air purification equipment,” says Paul Lebbin, vice-chair of the ASHRAE project committee 161 “Air Quality within Commercial Aircraft.”
“We feel the design and operation of the ECS can be optimised to improve cabin air quality while reducing energy consumption,” Lebbin concludes. “The challenge is to comply with or receive special exemption the [US] FAA part 25 prescriptive regulations on outside air supply.”
Throughout the A380’s development campaign and during its nearly six years in commercial service, Airbus repeatedly stated its jumbo jet would feature a cabin 50% quieter than the 747-400. A key element in Airbus delivering that target was the development of a new material lining for the A380’s engine nacelles that wiped out acoustic scattering – a phenomenon experienced in previous engine designs caused by gaps between sections of an individual linings, which essentially caused the engines to rattle, says the European Patent Office.
As the A380 built its service life, some passengers and crew began to complain the aircraft was too quiet, as travellers could hear flight attendant call bells, or conversations transpiring across the cabin. Some flight crews complained of disruptive rest on long-haul flights as cabin noise previously drowned out by the roar of the engines was heard more acutely.
Queried about the remedies it has introduced to create some ambient noise in the A380’s cabin, Airbus says, “We didn’t over-deliver. Passenger and crew feedback on the A380 cabin is overwhelmingly positive and low cabin noise is one of the most frequently mentioned items. It is well-known that noisy environments create stress, so the quiet cabin is inherently better to relax or work in.”
But research conducted by Canada’s National Research Council (NRC) on human perception of noise shows that “in an office environment, a certain background noise level is desired”, explains Anant Grewal, programme lead for a NRC research programme dubbed “Working and Travelling on Aircraft” an initiative that is partly dedicated to looking at new technologies to increase passenger comfort. “As ironic as it seems” the NRC aims to examine ways to add ambient noise in aircraft to preserve speech privacy, perhaps through an aircraft’s PA system, he says.
Boeing also dedicated a significant amount of effort to reduce cabin noise on the 787. Its engine innovation is built on serrated chevrons in the nacelle that drown out engines sounds inside the aircraft. United Airlines, which has increased its order for the widebody jet to 65 despite the aircraft’s multiple service-entry issues, also touts the twin-aisle jetliner’s advanced vibration isolation in the sidewalls and ceilings and greater control over engine vibration.
DRAW THE SHADES
Given the advances in engine design to reduce cabin noise, Grewal of the NRC concludes airframe noise is the next opportunity for improvement, particularly components including aircraft flaps and landing gear. The NRC has done some work in evaluating landing gear design for potential improvements to cut back on cabin noise, he says.
Other immediate options are available to cut down on cabin noise. InspecTech Aero Service cites conclusions from NASA that aircraft windows are “a main path for noise transmission”. The company has developed a product branded iShade iQ, an electronically dimmable window comprised of a seven-layer stack of lightweight and shatterproof substrates, interlayer films, and transparent viscoelastic damping materials that work to provide cabin soundproofing and a reduction in noise vibration.
Company CEO Jim Lang explains that ultimately the iShade iQ’s noise attenuation benefits from the use of a configuration common in architecture (uses an insulated glass unit) adapted for the aircraft environment. He also explains operators would not incur a weight penalty as the iShade iQ – actually there is a weight savings – noting that retrofitting a 757 with the iShade iQ would save about 28 pounds.
Lab tests show the iShade iQ offers a noise reduction of 5-9 decibels, says Lang. There is a price differential, as it is more expensive than conventional knife-shade window shades. But Lang believes given the weight savings, and the longer product life cycle, an operator could recoup the investment within a couple of years. Also, Lang notes that OEMs and operators invest substantially more in thermal insulation than the cost of a shipset of iShade iQ, and in addition to its primary purpose – light and glare control – it provides the “missing link” in combating cabin noise – the windows. The iShade iQ is currently featured on private jets, and Lang says InspecTech is in talks with OEMs and high-end airlines, and is “close to nailing down a launch customer”.
“Trying to put a price on passenger comfort is difficult,” concludes Lang. The push and pull in attempting to improve the passenger experience at a reasonable cost is something that is constantly felt amongst airlines and manufacturers. It remains a universal conundrum as the industry works to evolve passenger comfort and safety at a reasonable cost.
Perhaps Airbus has captured the sentiment that prevails in the industry most accurately. “Rational decisions in design and operation of aircraft are key to safe air travel…considering that civil aviation is still the safest mode of public transportation, this basic principle seems to work well so far.”
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Image credit: Donaldson.com