The aircraft cabin is actually a pressurized cabin. At altitudes where atmospheric pressure does not allow human life, breathing within the aircraft must be guaranteed. This is achieved by maintaining the pressure inside the cabin at a certain level so that fresh air is supplied to the human respiratory tract. The air pressure in the cabin exceeds the ambient air pressure many times over, making the cabin a room of relative overpressure.
Figure 6.1 Exaggerated illustration of the extent of a pressurized cabin with increasing height (Stefan 1966)
In order to prevent the expansion of material and structure due to the high pressure, the hull structure is reinforced appropriately. In order to reduce weight, the structure is not reinforced at will, but only to a certain extent. In addition, the pressure difference (differential pressure) is reduced during the flight. The latter is achieved by adjusting the cabin pressure to the pre-vailing air pressure equivalent to an altitude up to 8000 ft (2400 m). Consequently, the cabin pressure can be printed out as the equivalent of a height, which is why it is often referred to as the cabin height. Figure 6.1 shows that the pressure conditions change considerably during a flight. Already at an altitude of 18000 ft (approx. 5450 m) the atmospheric pressure has halved from normal pressure at sea level (1013 hPa), at 34000 ft (approx. 10300 m) it is only one quarter.
The cabin pressure in modern aircraft is controlled by outflow valves and protected by emer-gency valves. Weak points of such a pressurized cabin are especially the aircraft doors, the aircraft windows and the rear bulkhead.
Gaps, e.g. in form of holes, lead to decompression of the cabin during flight at lower pres-sures, i.e. the relative overpressure in the cabin is eliminated by pressurization with the at-mosphere. At altitudes above 10 000 ft, the human respiratory tract is not sufficiently supplied with oxygen and there is a risk of unconsciousness and, in the worst case, death. In addition, the pressurization creates a kind of "suction" that "pulls" any objects and persons in the im-mediate vicinity out of the aircraft. The smaller problem is that the temperature inside the cab-in drops to a mcab-inimum. Consequently, opencab-ing the cabcab-in door durcab-ing the flight has predicta-bly serious consequences for crew and passengers, but can be compensated in an emergency by a rapid descent.
So, how likely is such an incident? Not at all. Because the aircraft door cannot be opened at all during the flight. In general, a frame on the outer frame is provided for the door. When opening the door, the cabin crew must lift it slightly so that it has a certain angle to the longi-tudinal axis of the aircraft and engages in the frame provided for it. When the door is lifted, it moves inwards before finally opening in a swivelling movement by turning the lever out-wards. During the flight there is a high-pressure area in the cabin as shown above compared to the surroundings of the aircraft, thus an immense pressure on the door, so that it cannot be lifted and opened by human hand in the first place. Aside from the multiple safety locks. It is estimated that there is a force of several tons on the door during cruising flight, making it im-possible to open it. The Flug Revue (2007) provides the following simplified calculation ex-ample in the same wording:
„Die vordere Kabinentür einer Boeing B767 ist 1,07 Meter breit und 1,88 [Meter] hoch, das ergibt eine Fläche von fast genau zwei Quadratmetern. Auf Reiseflughöhe von 10 000 Metern hat die Außenluft nur noch einen sehr geringen Druck von ungefähr 0,3 bar. Das entspricht in etwa einem Drittel des Drucks auf Meereshöhe. Der Druck in der Kabine wird dagegen ... auf 0,8 bar erhöht, das entspricht dem Luftdruck auf ... 2000 [Metern] ... [Höhe]. Der Druckunterschied beträgt also gut 0,5 bar. Generell gilt: ... Druck entspricht ... Kraft pro Fläche. Die 0,5 bar lassen sich dann auch als 50 000 Newton pro einen Quadratmeter darstellen. Da die Tür der Boeing B767 aber eine Fläche von zwei Quadratmetern besitzt, wirkt also zweimal die Kraft von 50 000 Newton auf die Tür, das macht 100 000 Newton ... Demnach müsste man also mit einer Kraft von zehn Tonnen gegen die Tür drücken, um sie im Flug zu öffnen ...“5
5 Translation by the author: "The front cabin door of a Boeing B767 is 1.07 meters wide and 1.88 [meters]
high, which gives an area of almost exactly two square meters. At cruising altitude of 10,000 metres, the outside air pressure is only about 0.3 bar. This corresponds to about a third of the pressure at sea level.
The pressure in the cabin, on the other hand, is... 0.8 bar, which corresponds to the air pressure at ...
2000 [meters] ... [altitude]. The pressure difference is therefore ... 0.5 bar. Generally, it is valid: ... pres-sure is equal to ... force per area. The 0.5 bar can then also be represented as 50,000 Newton per one square meter. However, since the door of the Boeing B767 has an area of two square metres, the force of 50,000 Newton acts twice on the door, which makes 100,000 Newton ... So, you'd have to press against the door with a force of ten tons to open it in flight..."
In the absolutely unlikely event of an explosion that tears out an airplane window, for exam-ple, a suction to the outside is indeed created that is strong enough to transport objects and persons in the surrounding area into the stratosphere. The seatbelts are designed for these load cases, among other things. The force generated by the lost window is never large enough to release the passenger sitting at the window from the seat belt. In this case, too, aircraft are once again adequately equipped, and passengers are secured. The myth of the crazy passenger who opens the cabin door during the flight contradicts all basic physics and is thus refuted.