High Altitude Parachutists generally and typically exit aircraft in the Physiologically Deficient Zone which extends from 10,000 to 50,000 feet. Exit from aircraft at such high altitudes at and above 13,000 feet requires use of portable life support equipment (supplied oxygen source carried by the parachutists). Exit from an aircraft at such high altitudes exposes the parachutist to more extreme temperature and inclement weather and climate conditions than directly confronted by aircrews.
Although current Air Force Instructions (AFIs) allow flight of unrestricted duration in unpressurized aircraft up to 25,000 feet most military aircraft sustain an internal ambient air environment comparable to that found at 6,000 to 8,000 feet (1,830 to 2,440 m)m until the aircraft is depressurized to open an exit so that the parachutists can exit the aircraft.
In general, most pressurized cargo and passenger transport aircraft maintain an internal pressure comparable to that found at 6,000 to 8,000 feet. The pressure will obviously increase at lower altitudes to equalize with the external pressure encountered at takeoff and landing.
Fighter and attack aircraft may be unpressurized to about 8,000 feet (10.9 psi); then cabin pressure is held at this altitude until a preset pressure differential of 5.0 psid is reached at 23,000 feet (5.9 psi). This 5.0 psid is then maintained as aircraft altitude increases.
High-altitude reconnaissance aircraft (U-2, SR-71, ER-2) have historically kept cockpit pressurization equal to 9,600 meters, or 30,000 feet, but it should be noted pressurized suits (not a G-suit) and helmets are also worn by the pilot.
In regards to pressurized suits it should be noted the extravehicular activity (EVA) spacesuits worn by astronauts to do work outside of the space craft have pressures lower than the ambient cabin pressure of a spacecraft.
The context of pressurized aircraft cabins and cockpits is although current Air Force Instructions (AFIs) allow flight of unrestricted duration in unpressurized aircraft up to 25,000 feet most military aircraft pressurize to sustain an internal pressurized air environment pressurized to lower risk from altitude maladies or loss of consciousness caused by hypoxia.
However on any sortie or mission scheduled for high altitude parachutist drop at or above 18,000 feet MSL all aircrew and parachutist are required to prebreathe 100% oxygen at or below 10,000 feet MSL or at cabin altitude below 10,000 feet MSL. This required prebreathing will be completed prior to the 20-minute warning and before cabin altitude ascends through 10,000 feet (unpressurized). The duration of the prebreathing of 100% oxygen depends on jump exit altitude and for the parachutists whether the jump is HALO or HAHO.
The process of becoming a military free-fall parachutist involves extensive physical and physiological screening. A screening physical (medical) exam must be accomplished within one year before entering initial MFF qualification training. A perquisite Initial Physiological Training, to include oxygen procedural training and a high altitude chamber flight must also be accomplished.