According to the regulations, a jet airplane must be pressurized, if it is flying about 15000 ft. Also, oxygen is required to breathe above this altitude for all the passengers and crew. When the aircraft is climbing, the pneumatic air is sucked from the suction of the jet engine, which is the turbine section and the air is filtered, cooled and moisturized.
It goes through many cycle compressor and goes through the trim valves to get into appropriate temperature so that passengers can have oxygen inside the airplane, most of the planes have this automatic pressurization system, which is necessary for all the jet airplanes.
Once the plane is pressured, the pressure is built inside the airplane. At high altitudes, (like some airplanes at 39000 ft.) different pressure will be there inside and outside, the delta pressure could reach eight psi which is quite reasonable. Physics behind this phenomenon is that, if the pressure inside the airplane is higher inside than the outside pressure, then it will not allow any window or door of the aircraft to get opened unless there is some damage or fault, or the cabin is decompressed. So if anybody tries to open the door or window while the aircraft is pressurized, and pressure is more than one psi, it will not be possible
Once the airplane starts to descend, the pressurization system by losing the altitude begins to depressurize the cabin slowly and gradually, until we reach the cabin altitude equal to the elevation of the destination or the place where we are going to land. Although there is an outflow vault that will be fully open once the airplane has fully landed and that will allow the pressure to be out l, i.e., 0 differential pressure inside and outside. At this state, doors can be easily opened. Due to system malfunction, cabin crew will have pressure alert, and they will not be allowed to open the doors.
Saudia Flight 163 was a scheduled Saudia passenger flight which caught fire after takeoff from Riyadh International Airport (now the Riyadh Air Base) en route to Jeddah, Saudi Arabia on 19 August 1980. All 287 passengers and 14 crew on board the Lockheed L-1011-200 TriStar (registration HZ-AHK) died from smoke inhalation after the aircraft made a successful emergency landing at Riyadh. It is assumed that most passengers and flight attendants were damaged during the landing roll, or they would not have attempted to open a door on a moving aircraft. It is known that the plane remained pressurized during the landing roll as the cabin pressurization system was on standby, and the plane was found with both pressurization hatches almost completely closed. The pressurization hatches should have entirely opened on a touchdown to depressurize the aircraft.
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