Creating a safe, comfortable cabin environment has never been more important and the latest technology means airliner cabins and the air used, are healthier than ever before, even in the face of today’s unprecedented challenges, as Paul Eden discovers

    Cabin environmental control has never been more important to crew and passenger health and safety. Just as passengers need to be reassured by visible cleaning and disinfecting activities at airports, the perception of healthy cabin conditions has now gained
    new significance.

    Evidence suggests that of all the confined public spaces in which large groups of people might choose to spend time, airliner cabins are among the safest in terms of limiting the chances of transmitted infection. But any deviation from the environmental norm – an unpleasant odour, for example – is likely to generate the perception of cabin air that is ‘dirty’ or unsafe.

    A Boeing spokesperson provides a useful overview of how cabin air is managed in a modern airliner at cruising altitude. “In today’s aircraft designs, outside air at 36,000ft or so continuously enters the airplane, either through the engines or, in the case of the 787, through electric compressor inlets. At this altitude, the air is very clean, dry, low in oxygen and practically particle-free. It is compressed, then travels to the air conditioning packs, where it is conditioned to the appropriate pressure and temperature.

    Next, it goes into a mix manifold where it is mixed in approximately a 50:50 ratio with filtered cabin air. Boeing aircraft use high-efficiency particulate air (HEPA) filters with an efficiency greater than 99.9 per cent at removing particulates, including viruses and bacteria, from cabin air before it is returned to the cabin.”

    Mentioned in the specialist and popular press many times in recent months but nonetheless worth repeating here, the HEPA filters installed in cabin air conditioning systems are equal in performance to those used in hospital operating theatres and industrial clean rooms. The fact that they must also meet strict European and US regulations has been less well recognised.

    The Boeing spokesperson continues: “The mix of outside and filtered air is supplied to the cabin through an overhead air distribution network that runs the cabin’s length. This air circulates before leaving the cabin through return air grilles that also run the length of the cabin. Cabin air flows primarily from ceiling to floor, in a circular pattern, and leaves through the floor grilles near the same seat row that it enters. This helps minimise front-to-back air movement, limiting the potential spread of contaminants.

    “About 50 per cent of the air leaving the cabin is exhausted overboard, while the other half passes through the HEPA filter before entering the mix manifold, where it is mixed with outside air and resupplied to the cabin. The volume of cabin air is exchanged every two to three minutes.”

    The quest for cleanliness

    Introduce an odour into the cabin, however, and it’s easy for passengers to believe the air is unhealthy. Among the most common sources of smells, volatile organic compounds (VOCs) may enter the cabin on the ground, through the bleed air system, from a variety of sources, as BASF’s global marketing communications manager Paul Warkentin explains. “These may include exhaust fumes from the engines on neighbouring airplanes and service vehicles, as well as de-icing fluids. Engine oil and hydraulic fluids may also leak into the bleed air system. These odorous VOCs may result in crew and passenger discomfort, or even alarm.”

    The most efficient solution to this unpleasant problem is to catalytically remove VOCs from the cabin. BASF’s Deoxo ozone/VOC dual function converters do exactly that, while also removing ozone at altitude. Warkentin says: “The catalyst converts certain odorous VOCs to water vapour and carbon dioxide – which people also naturally breathe out.” But what are the benefits of reducing the ozone levels in the passenger and crew environment?

    BASF 167 flat

    “Airplanes flying at high altitudes are exposed to significant levels of ambient ozone,” Warkentin notes. “If unabated, this enters the aircraft cabin through the air conditioning ducts. Prolonged exposure to ozone at concentrations typical of these altitudes causes adverse health effects, including headaches, fatigue, shortness of breath, chest pains, coughing and irritation of the eyes, nose or throat.”

    Health & humidity

    As Boeing’s Peter Pedraza explains, ambient air at high altitude is very dry, which inevitably translates into dry cabin air, less than ideal for passengers. Air drier than typical ambient relative humidity (RH) on the ground – around 22 per cent – produces a variety of well-being issues. Installing humidifiers therefore improves the passenger experience, even if only for premium cabin spaces. Peter Landquist, VP of sales & marketing at CTT Systems, a manufacturer of cabin zonal humidifying and zonal drying equipment, elaborates.

    “Every cabin is divided into temperature zones with their own air supply. Passenger numbers and the recirculation rate of the incoming air are the two main factors contributing to cabin humidity level. Thus, passengers in a more densely occupied temperature zone experience higher cabin humidity than those in a less populated zone. The paradox is that first and business class passengers are therefore exposed to extremely low humidity levels of between 5-10 per cent RH, while in economy, cabin humidity is typically between 15-18 per cent RH.

    “If a humidifier is installed in premium class to cover both cabin temperature zones and increase their humidity level to 22 per cent RH, the remaining five zones also see an increase of between 3-4 per cent RH owing to the aircraft recirculation system. Cabin humidity is, therefore, improved in every class.”

    Landquist notes that CTT offers humidifiers and associated driers as options for the Airbus A350 and A380, as well as the Boeing 787 and 777X, with further choices available for the flight deck, crew rest and premium cabin. “Airline single aisles are not yet covering ranges that justify the installation of humidifiers,” he notes, although CTT has installed such equipment in ACJ320 and BBJ737 aircraft.

    A benefit of increased cabin humidity, now more relevant than ever, is that virus transmission can also be hindered, while passengers leave a humidified cabin and proceed through the airport better equipped to combat pathogens.

    “We believe humidity plays a significant role in creating more comfortable, healthier cabins and in minimising virus transmission,” Landquist affirms.

    “Generally, viruses are more readily spread in extremely dry air, since exhaled droplets lose their water content through evaporation. The resulting smaller droplets stay airborne longer, extending the possible infection time. In an airplane cabin, infection risk is mitigated by the advanced, efficient air ventilation systems, but generating significantly higher humidity is also effective against the spread of viruses.

    Cabin Humidifier

    “Higher humidity helps the immune system function more efficiently”, asserts Landquist. The human body is optimised for conditions on the ground. Placed in a significantly different environment, we feel discomfort and our bodies work less well. Cabin air with very low water content weakens the functionality of our mucous membranes, increasing our susceptibility to pathogens, including viruses. Mucus is an important component in our first line of defence; it contains antibodies and physically catches and helps move pathogens away for destruction.

    “Increasing cabin humidity to the 22 per cent RH we’re used to on the ground helps keep mucous membranes hydrated, reducing the risk of viral infection during flight and as passengers navigate their way through the airport.”

    All under control

    Control of the cabin environment – including cabin air quality, temperature and humidity – is effected through multiple systems where failures can lead to costly delays, expensive repairs and even unwell passengers. Honeywell, itself a manufacturer of environmental control system hardware, including catalytic VOC and ozone convertors, offers Fresh Air Check, a Connected ECS software service designed for MROs and airline maintenance customers.

    Fresh Air Check integrates with the company’s Forge Connected Aircraft platform, monitoring ECS health and fresh air flow rate, and predicting air conditioning system failures. The software is available for the Airbus A320 and A330, and Boeing 777 platform parameter monitoring system, with a hardware and software solution for the 737-800 and 900 under development.

    “By monitoring parameters relevant to the aircraft environmental control system’s capability to provide full, as-designed fresh and recirculated/filtered air flow to the cabin, the software provides troubleshooting and proactive maintenance suggestions with actionable recommendations,” points out Honeywell’s VP & GM of air and thermal systems, Tom Hart.

    This reduces return-to-gate events, decreases no-fault-found rates, and shortens the troubleshooting and repair sequences caused by misdiagnoses. “The solution actively monitors ECS system parameters, including air pressure, temperature and flow rate sensor data, valve positions, aircraft flight condition and aircraft configuration. The Connected ECS software monitors changes for each individual aircraft across many flights and various operating conditions and uploads the information to Honeywell Forge, where our systems analytics identifies degradation of performance and alerts maintainers prior to a full ECS failure that could result in reduced air quality/air flow.”

    The cabin environment is likely to leave the most lasting impression on passengers through what they see and smell; to the individual, well-being is less tangible until it’s too late. In terms of cabin air, the industry is already doing well, but CTT’s Peter Landquist sums up the ECS challenge facing airlines in the current climate. “For passengers to be confident enough to travel again, it’s important all measures are considered and acted upon to minimise the risk of virus transmission in the cabin.”

    Read about more of the latest cabin innovations and news in the latest issue of Aircraft Cabin Management.