Why do my ears pop when I’m flying, even though the cabin is pressurized?
Breathing well, up in the rare air.
Air pressure at sea level is approximately 15 pounds per square inch, or PSI. This means that if you could weigh a square vertical column of air 1 inch by 1 inch wide at sea level, it would weigh 15 pounds!
Air gets thinner with altitude. If you are in Denver, Colorado, the “Mile-High City”, at an elevation of approximately 5,280 feet, that same column of air- your local air pressure- will weigh only about 12 pounds or 12 PSI.
Drive 90 minutes to Breckenridge, CO at 9,600 feet, and the pressure will be about 10 PSI, two-thirds that at sea level. Fifteen minutes south of Breckenridge, if you climb to the summit of Quandary Peak at 14,265 feet, your local air pressure will be about 8.5 PSI. The summit of Mount Everest at 29,035 ft? About 4.5 PSI.
The human body needs oxygen to survive. As the air gets thinner, so does its oxygen content. If you are at sea level and somehow manage to teleport yourself to sightsee at Everest's summit, you would not survive due to lack of oxygen.
Climbers ascending Everest typically spend weeks acclimatizing at increasing altitudes, including an extended period at Base Camp (~17,600 feet), plus several partial climbs up and down Everest before ever attempting a final assault- which is usually done with supplemental oxygen above a certain level. Some have done it without oxygen.
Acclimatization involves numerous bodily adaptations, including changes to your body’s oxygen-carrying red blood cells.
At a typical cruising altitude of 38,000 feet, the air pressure outside a jetliner is only 3 PSI, so we need pressurized cabins to survive. But just surviving isn’t enough, especially for the pilots, because we need lots of oxygen to be mentally alert and functional.
Mental alertness is definitely an issue above 12,500 feet.
When flying between 12,500 and 14,000 feet in unpressurized airplanes, flight crews must use supplemental oxygen masks if flying at those altitudes for more than 30 minutes.
At altitudes above 14,000 feet, a flight crew must use supplemental oxygen during the entire flight time at those altitudes.
Above 15,000 feet, each occupant of the aircraft must be provided with supplemental oxygen.
To enable us to breathe without oxygen masks, an airliner operates as a pressurized metal cylinder… just like a can of soda, or an aerosol can of deodorant or spray paint.
Pressuring that flying cylinder creates a lot of stress on the aircraft. In an airplane pressurized to sea level while flying at 38,000 feet, the pressure differential between the inside and outside of the plane would be 12 PSI (15 PSI inside vs 3 PSI outside).
That’s 12 pounds pushing out on every square inch of the aircraft, and means that the outward pressure on just a 12-inch by 12-inch window would be 1,728 pounds!
Due to this pressure differential, every time an aircraft ascends and descends, it also expands and contracts, leading to fatigue and stress fractures.
Thus, an aircraft's fuselage is limited to a specific lifetime, a maximum number of pressure cycles (flights) before being retired- a number typically between 35,000 and 85,000 cycles.
Let’s now come full circle to our original question. We’ve shown that pressurization stress can be tremendous across an entire aircraft. If a window just the size of a large iPad can have almost a TON of air pressure pushing against it… How do you mitigate this?
Airplane designers have long reached a happy medium by only partially pressurizing aircraft to a level that will be comfortable enough for almost all people.
Generally, when flying at 38,000 feet, the “altitude inside the cabin” - the pressure altitude- is set to 8,000 feet, instead of sea level.
This creates a pressure differential of only about 8 PSI, meaning that the outward pressure on that 12x12 inch window is now “only” 1,152 pounds- quite a difference, and quite a reduction in stress on the aircraft!
So, when you fly, you *do* feel the physical effects of climbing up to 8,000 feet above sea level. Hence, the popping ears!
PS- Newer designs, such as Boeing’s 787, are built mainly with lighter composite materials instead of aluminum alloys. This means they can accommodate higher pressures within the fuselage, typically cruising at cabin altitudes of just 6,000 feet.
Boeing states that “Altitude chamber tests show that because the body absorbs 8% more oxygen into the blood at this altitude, passengers experience fewer headaches and less dizziness and fatigue.”
And yes, they presumably pass gas and burp less, too- as these emissions are another side effect of being at altitude. Ever notice how quickly your soda goes flat when flying?