Refer to figure.

Eustachian tubes are small tubes that run between the middle ears and the upper throat. They are responsible for equalizing ear pressure and draining fluid from the middle ear, the part of the ear behind the eardrum. The end of the eustachian tube acts as a flap valve which allows air to escape with relative ease (required in the ascent) but can restrict air entering the middle ear (required in the descent). During descent the danger is higher because the air in the middle ear is at a lower pressure than the cabin, closing the Eustachian tube and preventing pressure equalisation, the valsalva manoeuvre might have to be performed.

If the eustachian tube is blocked, air cannot reach the middle ear, so the pressure there decreases. When air pressure is lower in the middle ear than in the ear canal, the eardrum bulges inward. The pressure difference can cause pain and can bruise or rupture the eardrum. This is called Barotrauma. The problem is increased if the person has a cold or any other condition which has caused the mucous membrane lining the eustachian tube to become inflamed and swell.  Someone with a cold will probably not be able to equalise the pressures and is at increased risk of Barotrauma. Pilots learn at an early stage of their flying training not to fly with a cold.

Symptoms of Barotrauma include ear discomfort or pain, hearing deterioration and, potentially, hearing loss and feeling of pressure in the ears.