Armstrong Limit - Hypoxia Below The Armstrong Limit

Hypoxia Below The Armstrong Limit

The Armstrong limit does not delineate the altitude at which it first becomes necessary to wear a pressure suit. A pressure suit is customarily required at around 15,000 meters for a well conditioned and experienced pilot to safely operate an aircraft in unpressurized cabins. The prompt physiological reaction when breathing pure oxygen through a face mask in an unpressurized cockpit at altitudes greater than 15,000 meters above sea level is hypoxia—inadequate oxygen causing confusion and eventual loss of consciousness. Air is 20.95% oxygen. At 15,000 meters breathing pure oxygen through a face mask, one is breathing the same partial pressure of oxygen as one would experience with regular air at around 4,700 meters above sea level.

Commercial jetliners are required to pressurize their cabins to an equivalent altitude not greater than 8,000 feet (2,438 m). U.S. regulations on general aviation aircraft (private pilots in small planes) require that the pilot—but not the passengers—be on supplemental oxygen if the plane spends more than a half hour at an altitude above 12,500 feet (3,810 meters). General aviation pilots must be on supplemental oxygen if the plane spends any time above 14,000 feet (4,270 meters), and even the passengers must be provided with supplemental oxygen at 15,000 feet (4,570 meters). Sky divers, who are at altitude only briefly before jumping, do not normally exceed 4,500 meters. Since 15,000 meters is the point at which breathing pure oxygen through an oxygen mask delivers the same oxygen partial pressure as is found with regular air at a hypoxia-inducing 4,700 meters, an altitude of 15,000 meters or higher requires increasing the pressure delivered into the lungs—as well as outside the lungs to avoid pulmonary barotrauma (lungs expanding like a balloon and tearing); thus, the requirement for a pressure suit.

For modern military aircraft such as the United States’ F‑22 and F‑35, both of which have operational altitudes of 18,000 meters or more, the pilot wears a “counter-pressure garment,” which is a G‑suit with high-altitude capabilities. In the event the cockpit loses pressure, the oxygen system switches to a positive-pressure mode to deliver above-ambient-pressure oxygen to a specially sealing mask as well as to proportionally inflate the counter-pressure garment. The garment counters the outward expansion of the pilot’s chest to prevent pulmonary barotrauma until the pilot can descend to a safe altitude.