Impact of Recent Altitude Physiology Research on Design of Cockpit Pressurization Systems

Design of cockpit pressurization systems impacts operational exploitation of the maximal altitude capabilities of some aircraft. The U-2 exposes the pilot to a cockpit altitude of approximately 30,000 ft with a 3.8 psid pressurization system. Approximately 70% of active/retired U-2 pilots reported that decompression sickness (DCS) occurred during their career of flying 9-hour missions at this altitude. Design of the F-22 pressurization system was based on previous fighter aircraft systems which maintain 5.0 psi differential pressure above 23,000 ft. Unfortunately, the 60,000 ft planned cruise capability of the F-22 places the pilot at 22,500 ft. The recently-reported threshold for DCS while breathing 100% oxygen is approximately 21,000 ft. The threshold altitude, which is lower than USAF and FAA regulations currently allow for unpressurized flight, implies a need for greater cockpit differential pressure for the F-22 and reinforces the need for breathing 100% oxygen prior to and including the high altitude portions of the flight profile. Increasing the cockpit differential pressure to keep the pilot at less than 21,000 ft would eliminate the vast majority of DCS cases. Breathing 100% oxygen during this period would provide additional protection from hypoxia in the event of unexpected rapid decompression. Another recent finding indicates that denitrogenation by breathing 100% oxygen at altitudes up to 16,000 ft is as effective as ground-level prebreathing. In addition, gas emboli formation at 16,000 ft is lower than at higher altitudes. Continuing research in altitude physiology would help define optimal design parameters.