When an aircraft is flying at high altitudes, the atmospheric pressure is lower than at sea level, and the air is thinner. Without pressurizing the cabin, passengers are prone to discomfort and even life-threatening conditions, especially at an altitude of 40,000 feet (12,192 meters). So, how does the pressurization system in an aircraft maintain the comfort and safety of passengers?
According to the aviation news website “Simple Flying,” modern aircraft are designed to fly at altitudes ranging from 28,000 to 35,000 feet (8,534 to 10,668 meters). This altitude range allows aircraft to save fuel and avoid severe weather conditions and turbulence by flying in relatively stable air.
However, the human body is not adapted to survive at such high altitudes, necessitating the adjustment of cabin pressure. At 40,000 feet in high altitude, the external air is extremely thin, with oxygen levels only a fraction of that at sea level. Without protective measures, individuals would lose consciousness within a minute.
The pressurization system of an aircraft operates by controlling the cabin pressure and exchanging air between the cabin and the outside environment to ensure the comfort and safety of crew members and passengers. The system pressurizes the cabin by injecting compressed air into the sealed fuselage, maintaining the internal pressure environment at a lower altitude.
For most jet airliners, this simulated lower altitude is typically around 8,000 feet (2,438 meters). This means that even at higher altitudes, passengers in the cabin feel as if they are at approximately 8,000 feet.
In most aircraft, cabin air is sourced from bleed air, which is compressed air extracted from the engine’s compression stage. Before entering the cabin, this air undergoes cooling and filtration through the environmental control system.
The key to maintaining stable pressure lies in the outflow valve – the rate of fresh air input is faster than the outflow, and the valve continually adjusts the outflow rate based on the needs to regulate cabin pressure while limiting the pressure difference inside and outside the cabin to avoid excessive stress on the aircraft structure.
Although the pressurization system of an aircraft operates automatically, crew members must monitor cabin altitude, rate of climb and pressure differential to ensure the system functions correctly. During operation, as the aircraft climbs, the pressurization system gradually increases the cabin altitude and pressure differential synchronously to ensure passenger comfort. If the aircraft continues to climb after reaching maximum pressure differential, the system will maintain the pressure differential while gradually raising the cabin altitude.
During high-altitude flight, a sudden loss of cabin pressurization may occur, resulting in cabin depressurization. This could be due to a pressurization system failure or structural damage to the aircraft (such as a window detachment or fuselage rupture), causing cabin air to escape outside. These events often happen suddenly, and the entire process can take only a few seconds.
Signs of rapid cabin depressurization include loud noises inside the cabin, appearance of mist or vapor in the air, sudden airflow, temperature drop, boiling liquid overflow, and scattered debris in the cabin. Noise, mist, and debris can make communication difficult.
According to Airbus, when cabin depressurization occurs at an altitude of 14,000 feet (4,267 meters), oxygen masks in the cabin will automatically drop. At 10,000 feet (3,048 meters), people can breathe normally, and the pilot will begin an emergency descent.
Another type of cabin depressurization is gradual, where cabin pressure decreases slowly. This may be caused by minor air leaks due to sealant issues in the cabin door, cracked windows, or pressurization system malfunctions. This situation is harder to detect, but passengers and crew members may experience discomfort in the ears or stomach. There may also be whistling noise on the scene, and crew members may only notice when oxygen masks drop automatically.
In the event of cabin depressurization, pilots should take immediate action – put on oxygen masks, establish communication among crew members, and start an emergency descent. Cabin crew should access oxygen masks nearby, secure themselves, and not move until the pilot confirms it is safe to do so (usually when the aircraft descends to 10,000 feet where oxygen levels are sufficient for normal breathing).
Following this, the cabin crew will use portable oxygen equipment to check on passengers, ensure they have their masks on and are unharmed, and report on the condition of the cabin.
In real-life scenarios, cabin depressurization can occur and has caused significant casualties. For instance, in 2005, a Boeing 737 aircraft of Helios Airways experienced depressurization on a flight from Athens, Greece, to Prague, Czech Republic.
Due to crew members’ loss of consciousness from lack of oxygen, the aircraft continued flying under the control of the flight management system and autopilot until it ran out of fuel and crashed. All six crew members and 115 passengers on board perished.
In another incident in July of this year, a Ryanair Boeing 737 aircraft experienced a sudden window explosion and displacement while flying to Germany, causing rapid cabin depressurization and nearly sucking a man in his sixties out of the window, hanging half of his body outside the cabin.
Fortunately, surrounding passengers and his wife managed to pull his legs back into the cabin, saving his life. The aircraft then made an emergency landing at Thessaloniki Airport in Greece.
Cabin depressurization is a critical situation that requires swift and coordinated actions from the crew to ensure the safety and well-being of everyone on board.