If an aircraft is to operate at 25,000 feet or above, the airplane must be able to maintain a cabin pressure altitude of not more than 15,000 feet in the event of any failure or malfunction in the pressurization system. A pressurized cabin requires the appropriate valves, controls and indicators for control of the cabin pressure. Two positive-pressure release valves are required to prevent greater internal pressure than external pressure; two are required in the case that one fails. Two negative-pressure release valves are required for the same reasons.
If an aircraft is approved for operation at or above altitudes where oxygen is required to be used or a certificate with supplemental oxygen equipment is requested, oxygen equipment that meets the regulations of the FAA is required. The oxygen equipment must not be hazardous in any way--in its operation, on its own or in its effects on other components. Crew must have a means to readily determine the quantity of oxygen available in each source of supply during the flight.
The minimum amount of supplemental airflow required for passengers in the cabin must be the same that flows through the oxygen masks. At cabin pressure altitudes above 10,000 feet and up to 18,500 feet, 0.8 liters per minute of oxygen must flow to the masks. Altitudes above 18,500 feet but lower than 40,000 feet require 3.5 liters per minute of oxygen flow.
