14

ASIAN AIRLINES & AEROSPACE AUGUST 2007

FEATURE

The seasoned traveler knows all too wellhow daunting air travel can be. Dry,cramped conditions in an airplane makethe flying experience far from comfortableand oftentimes, passengers couldexperience flu-like symptoms shortly afterthey’ve boarded the plane.

The slight headaches, light-headedness,sore throats, coughing, dry lips and dry orwatery eyes are in fact due to the lack offresh air that only an aircraft cabin onmodern jets can so effectively create –since BOAC inaugurated the world’s firstcommercial jet service on 2 May 1952.

Such environmental conditions are dueto the airplane’s cabin being pressurized atcurrent international requirements set for

an altitude of 8,000 feet – resulting inpassengers breathing in roughly 25% lessoxygen compared to sea levels.

The culprit here is an increased level ofcarbon dioxide in the bloodstream,exacerbated by conditions of limited airsupplies shared by too many people. Suchan artificially-created altitude pressure inaircraft cabins also results in passengersfeeling dried out at the end of a longairplane flight due to the lower humiditylevels.

The new passenger jets coming on-stream in the next few months promise todrastically reduce this discomfort by keepingcabin pressures at altitudes ranging between5,000 and 6,000 feet. These altitudes are

more reminiscent of an elegant resortrather than a lofty mountain spire. But isthere a real difference the passenger canlook forward to in terms of comfort levels?

Comfort issuesThe complex interplay between three mainfactors – oxygen, carbon dioxide andhumidity levels – that affect passengercomfort is the guideline used in the settingof cabin altitudes.

Starting with oxygen, its air compositionration remains at roughly 21% regardlessof altitude. However, as overall air pressuredrops with increasing altitudes, the numberof oxygen molecules per breath is reduced.Hence, to properly oxygenate the body,

Comfort above the cloudsBy Rosemarie John

Despite various innovations to improve passengercomfort in 55 years of commercial jet aviation,very few individuals would describe any flight astruly relaxing and resulting in one feeling fresh asa daisy when disembarking at the destinationairport. So, what gives with the new generation ofjetliners – the Airbus A380 and Boeing Dreamliner787 – which aim to bring passenger comforteven closer to this subjective standardby increasing cabinpressures to ahitherto-unheardof 5,000 feetaltitudes?

15

ASIAN AIRLINES & AEROSPACE AUGUST 2007

FEATURE

one’s breathing rate (even while at rest)has to increase. Since the amount of oxygenrequired for activity is the same, the bodymust adjust to having less oxygen.

The usual side effect, which typicallystarts once you pass the 3,000 feet altitude,is reduced alertness and a growing sense oflethargy unless one’s body becomes usedto a more rapid breathing rate. Once the8,000 feet altitude is passed, these sideeffects become more pronounced.

The new lower cabin altitude pressurizedat 5,000 to 6,000 feet aims to reduce thissense of lethargy and increase alertnesswith higher oxygen pressures – whichwould theoretically result in passengersfeeling more refreshed as the pressure

differential would not be too far removedfrom what one would usually beacclimatized to.

Even so, there is the second factor ofcarbon dioxide levels which also have tobe considered.

At higher altitudes, oxygen saturationin the bloodstream drops from about 98%(at sea levels) to about 90% once onepasses 6,000 feet. The difference results ina higher saturation of carbon dioxide inthe bloodstream.

These changes in gas saturation levelsrepresent a virtually imperceptible changefor most travelers as it creeps up on apassenger like a mild hangover with themost common signs being anything fromshortness of breath, nausea, loss of appetite,mild headache, and fatigue.

But those who are worse affected mayexperience increased shortness of breath,a dry cough, wheezing, increased weaknessand rapid pulse.

By keeping cabin altitude pressures atbelow 6,000 feet, new aircraft aim toreduce or completely eradicate thesesymptoms – thus increasing overallpassenger comfort.

The third factor of humidity is actuallyless important than one might perceive forpassenger comfort as it is actually moredependent on the relative cabintemperature than on altitude pressures.

By lowering cabin temperatures,passenger activity is lowered (since thereisn’t really all that much space available forany vigorous exercising). This results in

overall reduction in the need for higheroxygen saturation in passengers’bloodstreams – which ties in well with theset cabin altitudes.

Since the cabin air is on the dry sidewith lower temperatures, the passengerstarts to feel parched as the flight wears onand begins to feel more lethargic unlessone gets hydrated regularly. But, withchanges in the new planes reducing thisneed, it is possible to raise cabintemperatures and overall humidity levelsas well. The more moist air results inpassengers feeling less dried out and hencefeeling fresher at the end of the trip.

In theory, the lower cabin altitude willhelp to moderate almost every ill-effect oflong-haul flying, from dehydration to jetlag. However, it should be noted that somepeople might still experience symptoms ofaltitude sickness despite lower cabinpressure.

“It is extremely difficult to quantify theeffect on passenger comfort level in realconditions as your overall feeling of well-being depends on so many inter-relatedfactors such as temperature, air flow, airfiltration and humidity as well as cabinpressure and its rate of change,” saidAirbus aircraft interiors marketing headBob Lange.

“Although cabin altitude in the A380can be up to 1,000 feet lower than in someolder wide-bodies, we pay attention to allof the parameters that affect cabin airquality in order to improve passenger wellbeing in flight.”

Airbus A380with compositefuselage

16

ASIAN AIRLINES & AEROSPACE AUGUST 2007

FEATURE

“The A380 at a typical cruising altitudeof 35,000 feet has the cabin air pressurizedto an equivalent of 5,000 feet altitude.When the aircraft climbs to 39,000 feettowards the end of the flight the cabinaltitude will gently rise to around 6,000feet. However, great attention has beenpaid to limit the rate of change of cabinpressure because many of the symptomsdescribed below are linked to the body’sspeed of adjustment to the changes,” saidLange.

Fuselage issuesWhy has it taken so long for aircraftmanufacturers to offer these small butsignificant improvements in passengercomfort levels?

The biggest issue just happens to berelative air humidity. As can often be seenin cabins in-flight, condensation forms oncold surfaces – and this can cause mold,corrosion and moisture relateddeterioration in an aircraft’s fuselage.

There is also the matter of the fuselagecomposition itself, which has to withstandhigh pressure differentials within andwithout the aircraft.

The fuselage of older planes are made of

aluminum and as it is pressurized anddepressurized, the metal skin of the airplaneexpands and contracts, resulting in metalfatigue and also corrosion if humidity isincreased.

In aluminum airplanes, cabin humidityis kept low intentionally to preventcondensation on the airplane’s interiorsurfaces. Condensed moisture can buildup in insulation materials and eventuallycontribute to corrosion of the airplane’smetal structure.

“Changing cabin pressures in existingaircraft is not that simple because of theiraluminum structures. Airplanes aredesigned to withstand the continuouspressurization/depressurization cycles thatoccurs in takeoffs and landings throughouttheir service life,” said Boeing environmentalperformance director Jeanne Yu.

“To lower the maximum cruise cabinaltitude, the pressure difference must beincreased between the interior and exteriorof the airplane. This change would fatigue

The air conditioning system aboardconventional airplanes gradually replacescabin air with the much drier air from thehigh-altitude atmosphere outside. The cabinatmosphere, which starts out with thesame humidity as the city where youboarded, becomes increasingly dry as theflight progresses.

Exposure to low humidity environmentwithout sufficient fluid intake will dehydratethe body through perspiration andrespiratory water loss. Dehydration canlead to headaches, tiredness and fatiguewhile low humidity can cause drying of thenose, throat and eyes; plus it can irritatecontact lens wearers.

Altitude sickness is a reaction to thelower amounts of oxygen available at highaltitudes. As an airplane pressurizes anddecompresses, some passengers willexperience discomfort as trapped gasseswithin their bodies expand or contract in

response to the changing cabin pressure.The most common problems occur withgas trapped in the gastrointestinal tract,the middle ear and the paranasal sinusesalong with headaches and nausea.• Rapid changes in air pressure causethe air pocket inside the ear to expandduring takeoff and contract during descent,stretching the eardrum. To equalizepressure, air must enter or escape throughthe Eustachian tube. Air trapped in thesinuses due to a cold or allergy may causeunbearable ain during rapid descents.• Anyone with intestinal gas or gastrapped in an infected tooth may alsoexperience Barodontalgia, a toothacheprovoked as pressure decreases and theair expands.

While the chances of extremeunplanned cabin pressurization failuremight be relatively small on any givenflight, it is still wise to take the necessary

Our body’s reaction

Golmud

5,000 ft

10,000

15,000

20,000

25,000

30,000

35,000

29,028

16,640

12,000

Sea level

Cabin pressurised toa maximum of 8,000 ft.

8,000 ftAltitude sickness commonly occurs aboveappoximately 8,000 ft

17

ASIAN AIRLINES & AEROSPACE AUGUST 2007

FEATURE

(wear out) the structure more rapidly andimpact the airplane’s service life in anuncertain way, perhaps even compromisingthe design integrity of the airplane,”

“Differential pressure is the differencebetween the pressure of the air inside theaircraft cabin and that of the air outside atflight altitude,” said Lange. “It is expressedin psi (pounds per square inch), which canmost easily be imagined as a force exertedacross a surface area. The greater thatforce, the stronger the surface (in this casethe aircraft fuselage) needs to be towithstand it.”

Aircrafts have a positive pressure reliefvalve in the event of excessive pressure inthe cabin to protect the aircraft structurefrom excessive loading. The maximumpressure differential between the cabinand the outside ambient air is between 7.5and 9psi.

“On a B747-300/200/100, the flightengineer would readjust the cabin altitudeto the en-route clearance which includesthe initial cruising altitude provided by theATC,” said retired flight engineer JohnPaul who has been flying for 39 years withvarious airlines. “Having adjusted thecruising altitude, the automatic

pressurization controller will automaticallycontrol the cabin altitude to maintain acabin differential pressure of 8.5psi.”

Pressurization failures rarely take place.Should they occur, it is usually due to leaksthrough door seals, a pneumatic leak or anelectronic pressurization controllerproblem – easily correctable by cockpitcrew when a rapid depressurizationchecklist is carried out.

“Oxygen helps avert the effects ofdepressurization at altitude. The oxygenfrom these masks usually last for about 10minutes, giving sufficient time for the pilotto descend the aircraft from the cruisingaltitude to about 10,000 to 15,000 feet,”said Malaysia Airlines medical servicessenior manager Dr Daljit S Parmar.

“Cabin crew are trained on the usageand applications of various oxygen systemsused onboard the aircraft and are alsotrained to handle passengers who mayreact differently to the effect of hypoxiaoccurring from rapid depressurization,”he added. However, should an aircraftsuffer a pressurization failure above 10,000feet due to worse causes, the pilot wouldimmediately place the plane in anemergency descent and activate oxygen

masks for everyone on board. Passengeroxygen masks are automatically deployedif the cabin altitude is above 14,000 feet.

With the advancement of technologyand having the fuselage made out ofcomposites rather than aluminum, it’spossible to increase the humidity withoutcorroding the airplane over time.Composites used in the new aircraft arenot subject to the same fatigue conditionsthat limit the amount of pressure cyclesthat can be applied to an aluminumairplane.

Looking at the Boeing 787 Dreamliner’scomposite structure – which does notcorrode when exposed to moisture plusinsulation designed to resist the buildup ofmoisture – the engineers could thenincorporate an air conditioning systemthat retains a comfortable level of humiditythroughout the flight.

Cabin pressuresThe first aircraft with cabin pressurization(though restricted to crew areas) was theB-29 Superfortress. Post-war pistonairliners such as the Lockheed Costellationbrought the technology to civilian serviceand as jet airliners were always designed

TanggulaMountain Pass

Lhasa, Tibet

Highest point above sealevel – peak of Mount

Everest

precautions before traveling. Some solutionsto prevent unsavory effects:• Do not fly when having a cold; if yourears hurt when you fly, try taking adecongestant before you get on the plane.You can also swallow often and chew gumduring the flight. Babies can suck on bottlesor a pacifier during the flight.• Drink plenty of fluids (preferably water)before, during and after your flight. Stavingoff dehydration will also decrease your riskof getting jet lag.• Apply some moisturizer and lip balm tocombat the dry air in the flight cabin.• Don’t fly with contacts - don yourspecs instead. Contacts will only dry outand further irritate itchy or burning eyes.• Don’t leave home without your inhalerif you have asthma.• Symptoms such as breathlessness, chestpain or confusion may signal that a personhas dropping oxygen levels. In such cases,passengers with heart or lung disease canask the crew for oxygen. Healthy people

may need only to drink some water, asdehydration compounds the effects ofoxygen loss. Avoiding alcohol and sleepingpills may also help.• It is suggested that before taking aflight, people with heart or lung diseasehave their doctors measure their oxygensaturation. If it is already low, then patientswill know it could drop to problematiclevels and they could tell the airline theywill need oxygen during the flight.• If you’re on a connecting flight andhave sufficient time, try to get as muchfresh air as you can between connections.• Avoid smoky bars when you reachyour destination or while you wait for aconnecting flight• Clear your head with a hot, steamyshower after you land.• It’s dangerous to fly immediately afterscuba diving. You’ll need to wait 12 to 24hours after diving. Ask your doctor ordiving authorities for guidelines on flyingafter scuba diving.

18

ASIAN AIRLINES & AEROSPACE AUGUST 2007

FEATURE

for high-altitude operation, every jetlinerfeatures the technology.

Cabin pressurization is the activepumping of air into an aircraft cabin toincrease the air pressure within the cabin.The cabins of most civil transport arepressurized with air supplied by the engines.

Bleed air extracted from the engines iscompressively heated and extracted atapproximately 200°C and then cooled bypassing it through a heat exchanger andair cycle machine. An environmental controlsystem (ECS) using air provided bycompressors or bleed air pressurizescommercial aircrafts.

Today’s aircraft have a dual channelelectronic controller for maintainingpressurization along with a manual back-up system. These systems maintain cabinair pressure equivalent to 8,000 feet orless, even during flight at altitudes above43,000 feet. In passenger-carrying aircraft,the degree of pressurization of the cabin isdetermined by the requirements to preventsignificant hypoxia (shortage of oxygen in

one’s blood, and hence the body) at altitudeand damage to the middle ear on descent.

The flow of air through the cabin isdetermined principally by the requirementsfor ventilation and thermal comfort. Thecabin air outlet valves control the differentialpressure between the pressure cabin andthe environment.

Cabin air on older planes has very lowhumidity levels (15-20% relative humidity),due to very dry air being brought in fromoutside at high altitude. The air outside theplane is very cold and thus has a very lowabsolute humidity, which translates into avery low humidity level when warmed.

Low humidity in the cabin is caused bythe frequent renewal of cabin air withoutside air. Since the outside temperatureat typical cruising altitudes is very low (-5oC to -25oC), it contains little moisture. Itis this very dry air that is supplied to thecabin. During flight, the relative humidityin the cabin ranges from approximately 5%to 35%, with an average of 15% to 20%.This is similar to the dry summer climate of

the southwestern United States or typicalwintertime indoor levels.

In addition, for older generationairplanes flying currently, cabin air comesfrom the inside of the jet engine while innewer airplanes, the cabin air system willbe vented directly from the outsidethrough dedicated inlets on each side ofthe plane’s belly and won’t pass throughthe engines.

These airplanes are powered by high-bypass-ratio fan engines which are muchquieter, much cleaner burning, morepowerful and much more efficient.

At the front end of this engine type isa large-diameter fan, which is powered bythe core. The fan moves a large volume ofair past the core rather than through it,and actually generates most of the thrust.

By providing the cabin with a mixtureof about 50% outside air taken from thecompressor and 50% recirculated air, abalance has been achieved that maintainsa high level of cabin air quality in turnenhancing passenger comfort.

502 adult volunteers participated in a 20-hour simulated flight to determine theeffect of barometric pressures equivalentto altitudes of 650, 4,000, 6,000, 7,000 and8,000 feet above sea level on arterial oxygensaturation and the occurrence of acutemountain sickness and discomfort.Discomfort rose with increasing altitudeand was greater at 7,000 to 8,000 feet thanat all the other altitudes combined.

hours the volunteers reported symptomslike backaches, headaches, shortness ofbreath and impaired coordination. Therewere no health effects, but the discomfortwas there.• Researches found that an ascent fromground level to 8,000ft by otherwise healthyunacclimatised adults lowered the level ofoxygen in their arteries by as much as 4%,which is attributed to the increased level ofdiscomfort.• Mild altitude sickness is common. In theUnited States, more than 20% of peoplevisiting the western mountains have it.Experts do not know who will get it andwho will not. Your age, sex, and howphysically fit you are play no role in whetheryou get altitude sickness.• Altitude sickness is more likely to occurin people who have a previous history ofaltitude sickness.• On high performance airplanes and interms of symptoms of hypoxia, spendingthree hours at 10,000 feet is often enoughto cause discomfort.• On average, women need oxygen 2,000feet lower than men and an oxygen systemcan help you arrive comfortably.

Discomfort became apparent three tonine hours into the flight.• When an aircraft is pressurised to analtitude of about 8,000ft while flying attheir maximum height, people above theage of 60 are less likely to report symptomsof minor altitude sickness than youngerones and men seemed more affected thanwomen.• Researchers found that after three

Some interesting findings