What is cabin altitude?
Cabin altitude is the air pressure inside an aircraft, expressed as the altitude at which that pressure occurs naturally. Older aircraft hold around 8,000 feet; the Boeing 787 and Airbus A350 manage roughly 6,000, which is the main reason passengers report arriving less tired on them.
Cabin altitude explained
An aircraft cruising at 38,000 feet cannot be pressurised to sea level, because the pressure difference would demand a heavier and stronger fuselage than the aircraft could carry. Manufacturers therefore choose a compromise, and for decades that compromise sat at 8,000 feet, roughly the pressure at the top of a modest mountain.
Two thousand feet sounds trivial and is not. At 8,000 feet the oxygen saturation in your blood falls measurably, which contributes to headaches, fatigue and the dulled taste that makes airline food seem bland. Bringing the cabin down to 6,000 feet reduces that effect enough that most passengers notice it without being able to name it.
The change was made possible by composite fuselages. Aluminium fatigues under repeated pressurisation cycles, so raising cabin pressure on a metal aircraft shortens its life. The carbon fibre structures of the Boeing 787 and Airbus A350 tolerate the extra stress, which let both manufacturers lower cabin altitude and raise humidity at the same time.
Humidity is the quieter half of the story. Traditional cabins run at around five percent relative humidity, drier than most deserts, which is why long flights leave skin and sinuses uncomfortable. The newer types hold roughly twenty percent. On a fourteen hour sector the combination of lower altitude and higher humidity is a genuine difference rather than a marketing line.
More on cabin altitude
Around 8,000 feet on most aircraft. The Boeing 787 and Airbus A350 hold roughly 6,000 feet, which is low enough that most passengers notice feeling better on arrival without knowing why.
The pressure difference at cruising altitude would require a heavier, stronger fuselage than the aircraft could carry efficiently. Every design is a compromise between structural weight and passenger comfort.
It contributes to the fatigue that gets blamed on jet lag, though the two are separate. Lower oxygen saturation and very dry air both leave passengers tired independently of any time zone change.
The Boeing 787 and Airbus A350. Their composite fuselages tolerate the extra stress of higher pressurisation, which allows both a lower cabin altitude and roughly four times the humidity of an older cabin.
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