Cabin climate

4 Jun 2023 by Tamsin Cocks
Paseenger on a flight (istock.com/tongpatong)

Thermal comfort and humidity are set to become the next step change in cabin improvements.

 The Atacama Desert in Chile is generally considered to be the driest place on Earth. The South American plateau receives little precipitation (about 15mm annually) and is virtually devoid of vegetation or animal life as a result, with humidity reaching just 10 per cent.

In a business class cabin, the average humidity is 7 per cent. In other words, it’s drier than the driest place on Earth.

The arid conditions arise because the outside air that’s drawn into the cabin is very cold – around -15˚C – so it can’t hold any moisture. As cabin air is also frequently refreshed (roughly every two minutes) the small amount of moisture that’s generated is also filtered out very quickly.

Most humidity in the cabin actually comes from passengers via exhalation. The most humid part of a plane is therefore in economy (around 12 per cent), with premium cabins suffering more. First class passengers wither in around 5 per cent humidity, while the cockpit and crew rest areas suffer from the lowest levels, with just 2 per cent humidity.

“Your body dehydrates quickly when you’re exposed to such an environment,” says Ola Haggfeldt, chief commercial officer at CTT Systems AB, a Swedish company that specialises in aircraft humidity. “You’re basically losing 70 grams of water every hour, so even if you’re drinking water, you’re going to dry out.” We’re all aware of feeling parched and dehydrated on a long-haul flight, disembarking with flaky skin and bloodshot eyes more suited to a Halloween costume, but the lack of humidity has more serious effects than personal aesthetics and comfort.

“From a health perspective, what happens is you dehydrate your mucus and respiratory system,” Haggfeldt explains. “Typically, how the immune system works is if you catch a virus or bacteria, your mucus basically traps it and your stomach acid kills it. When you’re dried out, this doesn’t happen. So, while you don’t immediately get sick because of a lack of humidity, your chance of getting sick increases quite significantly.”

It’s the same reason why more people get sick in the winter – the cold, dry air suppresses your immune system, making you more susceptible to airborne bugs. Aircraft these days are equipped with hospital-grade HEPA filters, which kill 99.9 per cent of any airborne pathogens, so you’re unlikely to contract anything mid-flight. But the second you step into a busy airport or a crowded bus, your wrecked immune system leaves you vulnerable.

Aside from your health, quality of sleep is also affected, says Haggfeldt: “Studies have shown you sleep better when you breathe better, and you breathe better when there is natural moisture in the air. Having better sleep on an aircraft also means less negative effects from jet lag.”

Humidity also plays a role when it comes to inflight dining. Around 80 per cent of your taste perception comes from scent and odour, yet your sense of smell is greatly reduced in dry conditions. As anyone who’s ever had a cold can attest: if you can’t smell something, you can’t really enjoy it. If humidity was improved in cabins, it would also improve the enjoyment of food and wine, which airlines invest a lot of time and effort into.

Liquid assets

It’s a no-brainer that more humid cabins would be more humane, so why aren’t humidifiers a staple on modern aircraft? Common issues with cabin humidifiers relate to the increased onboard weight as well as the unwanted byproduct of increased condensation, which can cause corrosion or mould and can also get into the aircraft’s intermediate spaces and potentially cause issues with things like the electrical systems. However, newer technology, such as that produced by CTT, offers a tandem dehumidifying effect to reduce these unwanted side effects, while increasing humidity for the cabin.

Another issue, as with everything, is cost and demand. One barrier to airlines investing in more humid cabins is that it’s hard to market something you can’t see, or necessarily even sense. “When airlines configure a new cabin interior, they start with a bag of money and a wish list – we want those seats, that galley, that fabric. Of course, they’re going to exceed that bag of money, and then have to start taking away. What do they take away? The things you cannot see,” says Haggfeldt.

“When you walk into a premium cabin, in a glance you can take it in and think ‘Wow this looks really nice’, but how do you present humidity? It’s a feeling. The strange thing is everyone knows the cabin air is dry. If you fly a lot, you really do feel the difference – you don’t need to take out your contact lenses, you don’t get sick, you feel more comfortable, but not much has been done about it.

“It’s like pressurisation in aircraft. Now on the 787, A350 and 777X, there is a higher pressure in the aircraft to get closer to the pressure on Earth, which is more comfortable. But getting to that step has taken many years, with manufacturers having to build a fuselage that could withstand the increased pressure and so on. So, it’s going to take time.”

However, things are starting to change. On private jets, where humidity is more of a problem due to the low passenger load, and passenger expectations are higher, onboard humidifiers have started to become the norm. Commercial airlines have also started adding humidifiers to the cockpit and crew rest areas, which are now prevalent on aircraft including the A380, A350 and Boeing 787.

But what about in the passenger cabin? China Southern was the first airline to select an inflight humidification system for its A350 business class cabins, delivered in 2019, while  Emirates and ANA plan to fit humidifiers on the Boeing 777X, and Lufthansa has requested humidifiers for its first class cabin on the A380.

“There is also a big Australian carrier (we can’t officially say who) that has selected it to debut on a coming project. Once these early adopters get it out there, I think it’s going to influence a lot of others,” says Haggfeldt.

Premium cabins are most likely to be the beneficiaries of new humidifying technology (not least because they are the most in need), but the positive is that even if just the front of the plane was given more moisture it would affect the entire aircraft, meaning all passengers would reap the benefits.

Lufthansa Allegris

Hot and bothered

Another invisible factor that has a major impact on overall passenger comfort is temperature. It’s perhaps not something that we consciously think about, given that we don’t have much control over it, save from asking for another blanket. But it’s an area that’s increasingly being looked at, as it can have a major impact on passenger satisfaction.

Dr Peter Vink, a professor of industrial design engineering at TU Delft university, says: “If you look at studies of aircraft, there are three main factors influencing comfort or discomfort. Number one is physical comfort: dimensions of the seat and so on. Number two is humidity, and number three is temperature. In some cases temperature is even more important, and it can greatly influence a passenger’s choice of airline.”

In a study conducted by New York’s Tapis Corporation, which develops materials for aircraft interiors, sales director Matthew Nicholls revealed the wrong temperature significantly contributes to a poor flight experience, and in more than 50 per cent of cases, passengers were either “too hot” or “too cold”, proving it’s a significant issue for overall comfort.

Getting the formula “just right” is difficult, however. Temperature controls on aircraft have traditionally been very old-fashioned, with basically a hot or cold switch in the cockpit. On more modern aircraft, cabin crew have more flexibility to adjust the temperature by five degrees in different sections of the plane – packed economy cabins can be cooled to negate all the body heat, while first class cabins may need more ambient temperatures – but there are certainly still limits to what can be achieved.

In some cases, airlines may also deliberately adjust temperatures without passenger comfort in mind. It’s an unspoken practice on some carriers to keep the cabin temperature lower on day flights in order to upsell blankets and hot drinks, while on night flights the preference is warm cabins to encourage passengers to relax and nod off.

Brenna Wynhof, regional director of cabin marketing for Boeing says: “If you’re talking about if improvements are possible to aircraft existing today, that’s a really significant challenge. The best opportunity for improvement is a clean sheet design. We saw that in the 787. That aeroplane was a step change for thermal comfort and environmental control systems as that’s the only aircraft that doesn’t use bleed air to bring it into the cabin. We have cabin air compressors (CACs) that bring in more humid air. When you couple that with a composite fuselage, it just naturally holds average temperature better than aluminium and it can also withstand more moisture better. When you have more humidity in the cabin it’s more comfortable, and you’re maintaining temperature better as well.

“We’re going to see more improvements on the 777X as well. We have doubled the number of nozzles in the air distribution system and we’re also reducing thermal gradients so it will be a better cabin experience and a better comfort story than what we’re seeing today.”

In February, Lufthansa made a splash with the reveal of its new Allegris products (due to launch this autumn). One function in particular stood out: a “microclimate” for first and business class seats. The personal seat heating and cooling technology in its premium cabins is a world first.

Similar to the technology found in cars, the new Allegris seat can be heated according to personal preference via a heat mat under the seat and resistive wires. Likewise, the seat can be cooled, with a device that pulls air through the seat cushion to cool it, along with a ventilated trim and a three-mesh-layer for unobstructed airflow. All can be easily controlled by an individual passenger, independently from aircraft cabin climate, at the touch of a button.

Lufthansa Group’s head of customer experience design, Kai Peters says: “Thermal comfort is a key contributor to passenger well-being. We’re certain that seat-heating and cooling will be a great new feature and add comfort for our customers. We know that this feature has interested other airlines and expect others to follow.”

While encouraging developments are being made in the world of thermal comfort and cabin humidity, it’s going to be a while before we walk off a plane feeling like we’ve had a spa treatment. But next time you’re on a crowded flight, at least you can be thankful for the additional heat and humidity your fellow passengers provide!

Relative humidity

Average cabin humidity without active humidification on long-haul flights

  • Pilots 2.3 per cent
  • Crew rest area 5 per cent
  • First class 5 per cent
  • Business class 7 per cent
  • Economy class 12 per cent
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