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OBOGS (On Board Oxygen Generation Systems) technology allows an aircraft to generate its own oxygen from a conditioned air supply during flight, giving significant advantages in reliability, safety and performance over older gaseous and liquid oxygen systems. OBOGS takes advantage of a molecular sieve material, Zeolite, which traps nitrogen molecules when air is passed through it, allowing it to act as a molecular sieve.

At ground level humans breathe air with a 21% oxygen concentration in order to oxygenate the bloodstream and, hence, sustain life. The pressure exerted by the oxygen component of air is termed the Partial Pressure of Oxygen (PPO2). It becomes progressively more difficult for humans to take in oxygen as PPO2 decreases in direct proportion to air pressure with increasing altitude. At higher altitudes this will lead to insufficient oxygen being present in the bloodstream, a condition known as hypoxia, and eventually death. The OBOGS is controlled by a solid state monitor/controller that monitors the PPO2 level of the OBOGS product gas, and adjusts the cycling of the beds to produce the desired level of oxygen concentration. To prevent hypoxia in military aircrew it is necessary, as a rule of thumb, to maintain the minimum of PPO2 to the equivalent of altitudes in the range 8,000 feet to 10,000 feet in order to perform normal levels of work."

OBOGS (On Board Oxygen Generation Systems) technology allows an aircraft to generate its own oxygen from a conditioned air supply during flight, giving significant advantages in reliability, safety and performance over older gaseous and liquid oxygen systems.

And what do they use for the "plan B" redundant/back-up system for the pilot?