Pressurization System

 The pressurization system permits normal operation at high altitudes. Modern aircraft usually fly at high altitudes, where the oxygen partial pressure is not enough for the human body. (Federal Aviation Administration, 2022) Almost every pressurized aircraft has a sealed cabin capable of containing air at a higher pressure than outside. In order to pressurize the cabin, the pressurization system takes air from the compressor stage of the engine; this air is known as bleed air. Pressurization systems are made to maintain a cabin altitude of no more than 8000 feet at the cruise phase and normally are automatically controlled. Failure of this system could entail a loss of cabin pressure which may be catastrophic depending on the circumstances.

Giving an example, a malfunction of the outflow valve may produce a slow depressurization. This type of depressurization is the most dangerous because it is difficult to identify. Therefore, modern aircraft have cabin altitude gauges, cabin rate of climb or descend gauges, and visual and aural alarms when the cabin altitude is too high. Also, these systems generally have redundancies, for example, in the A320, when a Cabin Pressure Controller fails, the other takes control of the pressurization, and if both CPCs fail, the pilot can manually override the system and preclude a depressurization. However, let’s suppose that these barriers do not work at all.

On August 14, 2005, a Boeing 737 from Helios Airways crashed near Grammatiko, Greece. The reason for the crash was the slow loss of cabin pressure because the maintenance team left the pressurization mode knob in the manual position and the flight crew did not realize this. Cabin altitude started to increase as the plane was gaining altitude and then an aural alarm sounded. Unfortunately, this alarm had exactly the same sound as the takeoff configuration alarm, and the flight crew got confused. Then they suffered from hypoxia and eventually, unconsciousness. Finally, the plane ran out of fuel while everybody was unconscious and crashed killing 121 people.

If the flight crew notices a loss of cabin pressure on time, they must act as soon as possible and perform an emergency descent, and assuring that the passenger’s oxygen masks are deployed. When the airplane reaches a lower altitude, the masks are not necessary to breathe normally. (Federal Aviation Administration, 2022) When you experience a rapid decompression or explosive decompression, your period of useful consciousness is reduced because exhale the air from your lungs rapidly, and this reduces the blood pressure. For these reasons, the crews must be trained regularly to deal with this kind of emergencies. Also, it is a good idea to test pilots in hypobaric chambers in order to let them recognize the hypoxia symptoms on the ground.

Reference:

U.S. Department of Transportation, Federal Aviation Administration, Flight Standards Service. (2022). Pilot's Handbook of Aeronautical Knowledge. Federal Aviation Administration . Retrieved 2022, from https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak.