Most aviators learn early in their career that flying with a simple upper respiratory infection or seasonal allergies can be painful. Gases in the sinuses and middle ear expand in accordance with Boyles Law as altitude increases and pressure decreases. [For a full discussion on the Gas Laws that apply to aviation, see this post.] Usually on ascent, the Eustachian Tubes open spontaneously allowing expanding gases to travel down into the posterior pharynx and equilibrate pressures. During descent, however, the Eustachian Tube will often not open spontaneously and the middle ear, sinuses, or other closed cavities will experience a vacuum like phenomenon as the gas contracts. Taken to the most extreme case, tissue damage can occur causing hemotympanum (blood behind the ear drum) or even a ruptured tympani membrane (ear drum). This series of medical conditions known by aviators as 'Trapped Gas' is discussed further in another post.In these cases, it is important for the aviator (much like the SCUBA diver) to be well acquainted with several techniques to equilibrate the external pressure with the middle ears. This is often called 'Clearing the Ears'.
It is also critical that aircrew and medical providers involved in aeromedical evacuations are able to educate patients and passengers on these techniques.The congestion, increased mucous production, and inflammation of the upper airways that accompanies colds or allergies will significantly obstruct one's ability to correctly perform any of these maneuvers. This is why pilots and other aircrew with symptomatic colds and allergic rhinosinusitis (nose or sinus inflammation) are at least temporarily not fit to fly (DNIF!).
