Some people call it barotrauma, barotitis media, aerotitis medi, or airplane ear—the pressure you feel in your ears while on a plane. You might feel it in an elevator or underwater too.
Airplanes and air pressure
Air pressure is the weight of the atmosphere pushing down on us. Usually we don't notice it because it's balanced out. When you get into an airplane and move away from the surface of the Earth, the effect of gravity and the amount of atmosphere decrease, as does air pressure.
An airplane cabin is built to maintain an artificial air pressure to deal with these changes, but it is not perfect. Making airplanes more pressure-resistant would probably require too much material to allow them to fly. By international agreement, cabin pressure is about 75% of what it would be at sea level, which is comparable to the air pressure at the altitude of Mexico City. This way, everyone is equally uncomfortable.
The human ear
Our ears are sensitive to slight changes in air pressure, which is, after all, what sound is. This explains the popping.
The human ear consists of three imaginatively named parts: the outer, middle and inner ear. The middle ear is separated from the outer ear by the ear drum. The Eustachian tube connects the middle ear to the back of the nose. It sounds like part of a transit system ("Next stop, Yew Station"), but is named after an Italian anatomist, Bartolomeo Eustachi (1524–1574).
This tube is narrow and usually closed until you swallow or yawn, which engages muscles outside the Eustachian tube that pull it open, allowing air into your middle ear and equalizing the pressure. If your ears do not pop, your eardrum would have different pressures and you might have trouble hearing. If your Eustachian tube is blocked, so you can't equalize the pressure, it can hurt a lot. This can happen if you are sick and your tube gets clogged up with fluid. You should either not fly or try a decongestant, in this case.
Take-off and landing
When you take off in a plane, you are moving into an area of lower pressure, and the air in your middle ear pushes out through the tube and makes a pop when the tube opens suddenly. Usually this part is easier.
When you are coming in for a landing, the higher pressure is pushing in. You need to push air into your middle ear to even things out. This tends to be more difficult. You can try swallowing, laughing or yawning to help.
Kids have narrower tubes and are more prone to have them plug up. My daughter likes having an excuse to chew gum and does that to help balance out the pressure. The mayo clinic points out using sugarless gum is preferable.
Infants don't understand what's going on, so feeding them or using a pacifier can help them swallow more to equalize the air pressure.
Another method is the Valsalva manoeuvre of pinching your nose with your mouth closed and gently blowing your nose, which opens up the tube from the inside. I learned this when I took scuba diving lessons, but you should not do it if you have fluid in your Eustachian tube or it could lead to an infection.
I looking forward to hearing your comments about airplane ear.