At 18,300 metres (60,000 ft), Concorde’s cabin altitude pressure was maintained at an equivalent to 1,700 metres (5,500ft), considerably below that of a subsonic aircraft.
Airliner cabins are usually pressurised to 6-8,000 ft (1,800-2,400 m) elevation while the aircraft flies much higher. Concorde’s pressurisation was set to an altitude at the lower end of this range, 6000 feet. Some passengers can have difficulty even with that pressurisation. A sudden reduction in cabin pressure is hazardous to all passengers and crew. Concorde’s maximum cruising altitude was 60,000 ft (18,000 m) (though the typical altitude reached between London and New York was about 56,000 ft (17,000 m)); subsonic airliners typically cruise below 40,000 ft (12,000 m). Above 50,000 ft (15,000 m), the lack of oxygen would limit consciousness in even a conditioned athlete to no more than 10-15 seconds. A cabin breach could even reduce air pressure to below the ambient pressure outside the aircraft due to the Venturi effect, as the air is sucked out through an opening. At Concorde’s altitude, the air density is very low; a breach of cabin integrity would result in a loss of pressure severe enough so that the plastic emergency oxygen masks installed on other passenger jets would not be effective, and passengers would quickly suffer from hypoxia despite quickly donning them. Concorde, therefore, was equipped with smaller windows to reduce the rate of loss in the event of a breach, a reserve air supply system to augment cabin air pressure, and a rapid descent procedure to bring the aircraft to a safe altitude. The FAA enforces minimum emergency descent rates for aircraft and made note of Concorde’s higher operating altitude, concluding that the best response to a loss of pressure would be a rapid descent. Pilots had access to Continuous Positive Airway Pressure which used masks that forced oxygen at higher pressure into the crew’s lungs. Had access to CPAP (Continuous Positive Airway Pressure) which used masks that forced oxygen at higher pressure into the crew’s lungs
So the purpose of pressurization system is to maintain a pressure which is compatible with the comfort of the Concorde passenger inside the cabin. The cabin pressure control system consists of two systems (System 1, and System 2). These two systems are identical and yet independent.
Only one system is selected by means of a two position switch: SYS 1, SYS 2. Both of these are systems are automatic and each system consists of the following:
- A pressure regulating selector.
- An amplifier.
- Two electro pneumatic regulating and safety valves.
- Two vacuum pumps.
Pressure Regulating Selector
The function of the pressure regulating selector is to produce an error signal depending on the selected cabin altitude and the true cabin altitude. This signal is amplified and controls one forward regulating and safety valve and one aft regulating and safety valve. The required cabin altitude is selected by means of Knob A in upper window.
The aircraft altitude up to which the selected altitude can be maintained appears in the upper window
A knob with a B letter on it makes it possible to perform the barometric corrections.
A knob with an R letter on it makes it possible to select the cabin pressure variation rate.
Warnings and Safety Systems
The regulating and safety valve closes if cabin altitude reaches 11,000 feet. The regulating and safety valve opens if positive cabin differential pressure reaches 11.2 psi. The regulating and safety valve opens if negative cabin differential pressure reaches 0.5 psi. A venturi located at the regulating and safety valve outlet limits the cabin altitude to 15,000 feet.
There is an altitude switch which operates the associated safety devices if cabin altitude reaches 10,000 feet. One differential pressure switch is associated with each regulating and safety valve. These pressure switches are connected to the forward valves only.
The pressure switches will operate the associated safety devices if the cabin differential pressure reaches 11 psi. A ditching system is controlled by a DITCHING VALVE switch.
There is a SYS 1, SYS 2 DISCHARGE VALVE switch with three positions FWD CLOSE, NORM and AFT CLOSE which enables closure of the regulating and safety valve via a solenoid.
The vacuum pump makes it possible to obtain a differential pressure inside the valve chamber so that the valve opens. Above a predetermined value, a pressure switch will cut out the power supply to the vacuum pump.
Cabin Pressurization Operation
While on the ground with throttle control levers in the idle position, the four valves are open. Both amplifiers are supplied via the EMERGY DEPRESS NORM TEST switch in the NORM position and via the SYS 1, SYS 2 switches. Selectors are not used for pressurization.
When the throttle control levers are at the take off setting, if system 1 or 2 is selected, throttle control lever micro-switches feed a signal to the amplifier of each system in order to close the two valves they control.
In flight the selected system amplifier receives the signals selected on the corresponding pressure regulating selector and controls the cabin pressure. The non-selected system remains in standby with both valves closed.
Ground Pressure Relief Valve
There is a ground pressure relief valve which is located in the nose gear bay, and this reduces the differential pressure on the ground.
At take off the throttle control lever micro-switches control the valve closing.
After landing, the landing gear relays control the valve opening. The valve has two motors and can be closed by the GROUND PRESSURE RELIEF VALVE switch on CABIN PRESSURE CONTROL panel.
Thrust Recovery Nozzle
There is a thrust recovery nozzle which is installed downstream of the system 1 forward valve. It recovers a thrust of 300 kg approximately during flight.
As long as the differential pressure is lower than 2.9 psi the flaps are completely open. When the pressure reaches 5.8 psi the flaps are completely closed.
The thrust recovery nuzzle is monitored by a magnetic indicator which is located on CABIN PRESSURE CONTROL panel.
The Nose gear is ventilated by air bleed forward pressure regulating and safety valve discharge orifice.
The Main gear is ventilated by air bleed from the cabin, flowing through a restrictor and two check valves.