The maximum
altitude to which an airplane is able to fly is 11.000 m; this
altitude is worth completely the aircrafts to exception of the
smallest ones (with helix engines) and of the Cocorde (16.600
altitude m). See as it is possible reaching this altitudes:
The pressurization system has the purpose to maintain the box pressure the more near as possible to the existing value at the level of the sea, independently from the flight altitude of the aircraft, in the way to guarantee the safety and the comfort of the passengers and the crew; that happens valves (outflow valves) regulating the flow of air conditioning produced from the Packs (air conditioning generators), that's discharged out of the airplane.
The pressurization system is planed so that on various phases of the flight the difference between the cabin pressure and the external pressure (differential pressure) doesn't overcome a pre-established value, that's included normally between 8 and 9PSI (Pound Square Inch). Such a value is naturally less than the maximum one that the fuselage is able to support without suffering structural damages.
After the take-off, cabin rates begin to climb (i.e. the intern pressure diminishes) with an accrual lower than the one the aircraft climbs . When that one reaches the cruise altitude, cabin rates reaches the consented maximum value (value lower than the internal pressure), so that the difference of pressure between the cabin and the external environment corresponds to the established value, previously suitable.
The graph up here shows the existing relationship between the environment pressure and the flight altitude of the aircraft. Note than an airplane to an altitude of 45.000 feet (15.000m) and environment pressure protect to 2, 14 PSI, the cabin pressure results of 2, 14+8,9=11,04PSI, equivalent to an altitude of 7.300 feet (2.433m).
The pressurization system installed on the modern aircrafts foresees three principals ways operation: automatic, semiautomatic and manual.
In the automatic way cabin altitude is set (for the whole duration of the flight) by calculator in according to a program that takes note of the airplane altitude and the profile of the flight. Before the take-off the pilot needs to select the altitude value of the field of landing on a special panel .
In the semiautomatic way, it's necessary to set up the cruise altitude manually (to whom it corresponds a fixed cabin altitude), and the variation accrual cabin pressure. The computer drives the outflow valves in the way to maintain cabin altitude on the value (constant) selected.
The manual control may be carried out through a special connected switch to the outflow valves across cables and rods, by direct control on electric actuator of the valves through switches . This system is obviously used only in case of damage of the control pressure automatic systems.