From: veritasium
Airplane cabins are meticulously engineered to maintain a breathable and safe environment for passengers at high altitudes. This involves managing both air pressure and oxygen levels within the cabin.
Why Planes Fly So High
Planes typically cruise at altitudes around 30,000 to 43,000 feet, or about 10 kilometers [00:01:42]. Reasons for flying at these altitudes include:
- Smoother Ride: Flying high in the troposphere, where most weather occurs, results in less turbulence and fewer storms [00:01:20].
- Cost Efficiency: This is the primary reason [00:01:37]. At 33,000 feet (10 km), air density is only one-third of what it is at sea level [00:01:45]. This reduced air resistance allows planes to fly about 73% faster for the same thrust, reaching destinations quicker and burning less fuel [00:01:56]. Jet engines are also more efficient in the colder air at altitude, where temperatures can be around minus 50 degrees Celsius [00:02:45].
- Jet Stream Advantage: Planes can utilize jet stream tailwinds at high altitudes, further reducing fuel consumption [00:03:07].
The Challenge: Unbreathable Air
Despite the benefits of high-altitude flight, the air at these elevations is unbreathable [00:03:21].
- Low Pressure: Air pressure significantly drops with altitude because it depends on the weight of the air above [00:03:39]. At 10 km, the air pressure is only a quarter of that at sea level [00:03:43].
- Insufficient Oxygen Partial Pressure: While the air still contains 21% oxygen, the partial pressure of oxygen—the pressure exerted by oxygen molecules alone—is too low [00:03:52]. At 10 km, it’s around 5.5 kilopascals, compared to 22 kilopascals on the ground [00:03:57]. Humans need at least 16 kilopascals of oxygen partial pressure to function normally, or they would remain conscious for only about three minutes, as if on Mount Everest [00:03:25].
Cabin Pressurization: The Solution
To overcome the challenges of high altitude, airplane cabins must be pressurized [00:04:16].
- Air Source: A continuous flow of air is brought into the cabin from the compression stage of the jet engines, which maintains breathable air inside [00:04:21]. This process, however, slightly reduces engine efficiency [00:04:34].
- Cabin Pressure: Airplane cabins are typically pressurized to approximately 77 kilopascals at cruising altitude, ensuring an oxygen partial pressure of around 16 kilopascals—the minimum required for passengers to feel normal [00:06:50]. This is less than sea level pressure (101.3 kilopascals) [00:06:30].
Aircraft Design for Pressurization
The requirement for pressurization led to a significant redesign of aircraft [00:04:39].
- Plug-shaped Doors: Before pressurization, plane doors opened outward, as pressure was the same on both sides [00:04:56]. With pressurization, all doors were changed to a plug shape, wider on the inside than the outside [00:05:06]. This design means the higher internal cabin pressure pushes the door into its frame, creating an airtight seal [00:05:12].
- Door Security: This pressure difference is why airplane doors and emergency exits don’t need locks [00:05:34]. The force required to pull a door inwards against this pressure differential at altitude is equivalent to lifting 9,000 kilograms, making it impossible for a person to open it [00:06:20]. Even modern outward-opening doors are designed with an initial inward movement that prevents them from opening at altitude [00:05:55].
- Exception: In May 2023, a passenger managed to open an emergency exit on final approach when the plane was close to the ground, and the pressure differential was minimal [00:09:26].
Effects of Cabin Pressure
The lower-than-sea-level pressure inside airplane cabins has several noticeable effects:
- Chip Bags: A bag of chips taken on a plane will inflate like a balloon as the cabin pressure drops at cruising altitude [00:06:38].
- Human Body: The expansion of gases due to lower pressure can cause physiological changes, such as increased flatulence [00:07:26].
- Taste and Smell: The dry air (as low as 5% relative humidity, compared to 25% in the Sahara Desert) can dry out nasal passages, hindering the sense of smell and, consequently, taste [00:14:20]. Lower cabin pressure can also decrease the intensity of salt and sugar sensations [00:14:39]. Airplane food often tastes different as a result [00:14:03]. Notably, the flavor of umami (savory) in foods like tomatoes appears to be enhanced in flight, possibly due to loud cabin noise stimulating a nerve connected to the eardrum [00:15:33].
Why Minimum Pressurization?
While the International Space Station is pressurized to full sea level pressure (101.3 kilopascals) [00:07:42], planes are pressurized only to the minimum extent necessary for human comfort [00:07:51].
- Fatigue Cycles: Unlike the ISS, planes undergo repeated pressurization and depressurization cycles with every flight, causing the fuselage to stretch and relax [00:08:36].
- Aloha Airlines Flight 243 (1988): This incident highlighted the dangers of fuselage fatigue. The plane, with nearly 90,000 flight cycles (far more than designed), experienced an explosive decompression at 24,000 feet when a crack widened and the roof tore off [00:07:58].
- Extended Aircraft Life: Pressurizing cabins to the least extent possible minimizes these stresses, extending the life of the aircraft [00:09:07].