A descent during air travel is any portion where an aircraft decreases altitude, and is the opposite of an ascent or climb. Descents are an essential component of an approach to landing. Other intentional descents might be to avoid traffic, poor flight conditions (turbulence, icing conditions, or bad weather), clouds (particularly under visual flight rules), to see something lower, to enter warmer air (see adiabatic lapse rate), or to take advantage of wind direction of a different altitude, particularly with balloons. As well what may require an aircraft descent is during emergencies, such as a sudden decompression forcing an emergency descent to below 10,000ft, the maximum safe altitude for an unpressurized aircraft. An example of this is Aloha Flight 243. Involuntarily descent might occur from a decrease in power, lift (wing icing), an increase in drag, or flying in an air mass moving downward, such as a terrain induced downdraft, rotor, near a thunderstorm, in a downburst or microburst. Rapid descents relate to dramatic changes in cabin air pressure—even pressurized aircraft—and can result in discomfort in the middle ear. Relief is achieved by decreasing relative pressure by equalizing the middle ear with ambient pressure ("popping ears") through swallowing, yawning, chewing, or the valsalva maneuver. Normal descents take place at a constant airspeed and constant angle of descent (3 degree final approach at most airports). The pilot controls the angle of descent by varying engine power and pitch angle (lowering the nose) to keep the airspeed constant. Unpowered descents (such as engine failure) are steeper than powered descents but flown in a similar way as a glider. If the nose is too high for the chosen power the airspeed will decrease until eventually the aircraft stalls, or loses lift. Helicopters which lose power don't simply fall out of the sky. In a maneuver called autorotation, the pilot configures the rotors to spin faster driven by the upward moving air, which limits the rate of descent. Very shortly before meeting the ground, the pilot changes the momentum stored in the rotor to increased lift to slow the rate of descent to a normal landing (but without extended hovering).
