When taking off or landing, changes in air pressure are more noticeable because the variation in air pressure is happening quicker than at ground level. It is more obvious during descent rather than take-off. This is because during descent the plane is coming from an area of low atmospheric pressure down to ground level where the air pressure is significantly higher, this causes the eardrum to retract and stretch and in some cases it can be painful. During take-off and ascent, the eardrum bulges outwards. This is because the pressure around you is dropping but it is easier for the middle ear to relieve the increased pressure via the Eustachian Tube (acting like an escape valve say on a pressure cooker) so most people do not experience any great difficulties as a plane climbs.
The Eustachian Tube is normally ‘patent’ (closed) and opens when we swallow, yawn or chew. However, common causes of a blocked Eustachian Tube arise from mucus and inflammation that can occur with colds, throat infections, hay fever, etc. The mucous membrane lining in the Eustachian Tube can become swollen and inhibit the flow of air through it. This in turns makes it more difficult for the air pressure in the middle ear to equalise to the environmental outside air pressure.