Modern aircraft are designed to fly at high altitudes of between 28,000 and 35,000 feet. This is because aircraft consume less fuel and fly in relatively smooth air, avoiding bad weather and turbulence. However, the human body is not designed to survive at such high altitudes, so the air pressure inside the cabin has to be controlled. At 40,000 feet, the outside air is so thin and has only a fraction of the oxygen available in comparison to at sea level. An unprotected person would lose useful consciousness in well under a minute.
Aircraft are normally flown close to their cruise ceiling. Aircraft pressurization systems ensure the comfort and safety of crew and passengers by controlling the cabin pressure and the exchange of air from the inside of the aircraft to the outside. Cabin pressurization solves this by pumping the sealed fuselage full of compressed air until the interior feels like a far lower altitude, typically around 8,000 feet on conventional jets.
Cabin Pressure Explained
In most airliners, the air is “bleed air” tapped from the hot compressor stages of the engines, then cooled and filtered through the environmental control system before flowing into the cabin. The trick to holding a steady pressure is the outflow valve: fresh air is fed in faster than it escapes, and the valve continuously modulates how much bleeds out, raising or lowering cabin pressure as needed while capping the difference between inside and outside, so that the airframe isn’t overstressed.
Aircraft pressurization systems operate automatically, but crews must confirm correct operation by monitoring cabin altitude, cabin rate of climb and descent, and differential pressure. In practice, as an aircraft climbs, for the comfort of the passengers, the pressurization system will gradually increase the cabin altitude and the differential pressure at the same time. If the aircraft continues to climb once the maximum differential pressure is reached, the differential pressure will be maintained while the cabin altitude climbs.
The maximum cruise altitude will be limited by the need to keep the cabin altitude at or below 8,000 feet, according to Skybrary. The safety valve acts to prevent cabin pressure from exceeding the maximum, allows air into the cabin when ambient pressure exceeds cabin pressure, and as a dump valve for the flight crew to dump cabin air manually. A cabin altimeter, differential pressure gauge, and cabin rate of climb gauge help the crew to monitor the aircraft pressurization.
Loss Of Cabin Pressure
Sudden loss of normal cabin pressurization at high altitude is often a result of an explosive or rapid decompression or a gradual or slow decompression. This can occur due to a pressurization system malfunction or damage to the aircraft’s structure that enables cabin air to escape outside the aircraft, for example, the loss of a window or a break in the aircraft fuselage due to an explosion. This can happen in seconds and occurs very suddenly.
Signs of a rapid decompression include a loud bang in the cabin, fog or mist in the air, a rush of air, a decrease in temperature, and liquids boiling over. There may also be moving debris in the cabin. Communication may become difficult due to the noise, fog, and debris. Oxygen masks will drop in the cabin automatically when the cabin altitude reaches 14,000 feet, according to information at Airbus. At 10,000 feet, normal breathing is possible, and the flight crew will start descent.
A slow decompression can be caused by a small air leak and a gradual decrease in cabin pressure. This may be because of a faulty door seal, a crack in a window, or a malfunction in the pressurization system. These are much harder to detect, but passengers and crew may feel some discomfort in their ears or stomach. There may be a whistling noise, and cabin crew may not notice until the oxygen masks automatically drop. Any cabin defects that may lead to a slow decompression should be reported to the flight crew immediately.
Real Life Examples
In 2005, a Helios Airways Boeing 737-300 was en route to Prague via Athens when it crashed in Grammatiko near Athens. There were six crew and 115 passengers onboard. They perished due to lack of pressurization. A faulty door seal had been reported on the previous flight, and an engineer checked it with a pressurization leak check and switched the pressurization switch to manual. He forgot to switch it back to automatic, and this also went unnoticed numerous times during flight crew checks. With the crew incapacitated by hypoxia, the aircraft flew on under the flight management computer and autopilot control until it ran out of fuel and crashed.
In October 1999, US golfer Payne Stewart’s aircraft crashed when its fuel supply had been exhausted. The aircraft, a Learjet corporate jet, was destroyed and all six occupants on board, including Stewart, were killed. It is believed that the accident may have been related to depressurization, and the pilots and passengers may have been incapacitated by hypoxia. In 2018, a Southwest Airlines flight suffered a decompression when debris from the engine impacted the fuselage. A passenger died after being sucked into a damaged window, but the flight crew and cabin crew acted immediately, and the aircraft diverted to safety.
The immediate actions of the flight crew in the event of a decompression are to don their oxygen masks, establish crew communication, and start an emergency descent. Cabin crew should grab the nearest mask and secure themselves until the flight crew says that it is safe to move. This will be around the 10,000 feet mark where the air is breathable. They will then move onto portable oxygen to check on the passengers and make sure that they are wearing masks and that no one is injured and report the status of the cabin.
What Is Hypoxia?
Hypoxia is defined as a lack of oxygen in the body tissues and is caused by a shortage of oxygen in the air breathed. The immediate effects are fatigue, confusion, euphoria, impaired performance, and decision-making. Lips and nail beds may turn blue. It is essential to go onto oxygen immediately, as in seconds, there is loss of consciousness and then death, if oxygen is not received. There is no discomfort or pain, so it can easily pass undetected.
Factors that affect the onset of hypoxia include physical fitness, cabin temperature, rate of ascent, altitude and duration at altitude. Onset can be gradual or sudden, depending on the loss of cabin pressure. Gradual hypoxia relies on an individual’s awareness of the symptoms and consequent actions, and why flight crew and cabin crew are trained to recognize it. Sudden onset hypoxia requires a rapid response before incapacitation occurs.
If pressurization fails, physics turns unforgiving fast; the time of useful consciousness at cruise altitude can be as little as 15–60 seconds. The time of useful consciousness may be very short. For example, at 35,000 feet, some individuals may only have as little as 15 seconds of useful consciousness in which to make rational decisions and take actions, following an explosive decompression. This is why the oxygen masks drop in the cabin automatically, and passengers should don them immediately before hypoxia sets in.
The Boeing 787 Difference
The Boeing 787 Dreamliner rethinks how cabin air is delivered, conditioned and distributed. The aircraft draws in fresh air from the atmosphere outside through intake inlets ahead of the wings. Air does not pass through the engines but goes through electric compressors that pressurize it and then feeds it into an environmental control system. This bleedless system is integrated with the aircraft’s electrical generation, which includes four generators and two auxiliary power units that provide electricity to drive the compressors and other onboard systems, according to a previous article at Simple Flying.
The Boeing 787 differs from older jets because its carbon-composite body handles higher pressure differences, allowing a lower cabin altitude of 6,000 feet instead of 8,000 feet, while using electric compressors instead of hot engine bleed air for cleaner, more humid air. The new no-bleed system reduces load on the engines and allows them to provide more thrust and increases efficiency. Indeed, the Dreamliner is 20% more fuel efficient than previous generation aircraft.
In the cabin, the new system results in more humidity, fresher air, and more comfort in the cabin. The air is refreshed every two to three minutes and the cabin altitude in the Boeing 787 Dreamliner is set to 6,000 feet, lower than the usual 8,000 feet, making it much more comfortable for passengers and crew. It allows for more oxygen in the blood and cuts down on tiredness and reduces headaches.
Key Points
Aircraft pressurization systems ensure the comfort and safety of crew and passengers by controlling the cabin pressure and the exchange of air from the inside of the aircraft to the outside. Cabin pressure is an artificial air environment inside an airplane that keeps everyone breathing safely at high altitudes, using compressed engine air and outflow valves. Aircraft maintain cabin pressure by continuously pumping in compressed air and regulating its release through an outflow valve.
Loss of cabin pressure can occur if there is decompression; this can be rapid or slow. In a rapid decompression, there is a loud noise, fog, possible debris, a rush of air, and a lower temperature. This may be caused by something like a window blowing out or damage to the aircraft’s structure. Slow decompression may be signified by a whistling sound or a broken window and is less easy to detect. Hypoxia can occur very quickly and is life-threatening, so it is crucial to use oxygen as quickly as possible.
The Boeing 787 modernized the concept, ditching engine bleed air for electric compressors and using its carbon-fiber fuselage to hold a more comfortable 6,000-foot cabin altitude that reduces fatigue, prevents dry eyes, and reduces jet lag. Traditional jets keep the cabin at an 8,000-foot altitude level.
