Despite operating at significant altitude, passenger aircraft offer one of the most statistically safe ways of traveling. It is an extreme environment though, and one that puts hardware and crew under a great deal of literal and metaphorical pressure.
The aviation industry has learned a great deal about uncontrolled decompression over the last 75 years. In this short article, we explain why aircraft are pressurized, discuss what happens in the event of depressurization and look back at a handful of losses that have been the result of uncontrolled decompression and discuss how they helped the aviation industry move forward.
Jet aircraft cruise between 30,000 feet and 40,000 feet (9,000-12,000 meters) above sea level, but at those altitudes the air contains considerably less oxygen than it does on the ground. If humans breathe this air, the brain doesn’t get enough oxygen, which results in a condition called hypoxia which can be fatal.
To get around this, aircraft pump air diverted from the engines into their hulls, providing a comfortable level of oxygen for the passengers and flight crew. It’s a neat solution that enables aircraft to fly at efficiently at high altitudes, but containing the oxygen puts an aircraft’s hull under pressure. This pressure can expose structural weaknesses and in extreme cases cause uncontrolled decompression.
When an aircraft depressurizes, oxygen masks drop automatically from the overhead panels. These can allow the passenger to breathe air with more oxygen and are mandated to last for a minimum of 10 minutes.[1] This should be more than enough time for the pilot to descend the aircraft to an altitude where the air is more oxygen rich. In the cockpit, the crew are repeatedly trained for what procedures to follow if an event occurs, to the point where they operate almost from “muscle memory” rather than solely from disciplined adherence of procedure.
The most recent uncontrolled depressurization in commercial aviation occurred on January 5, 2024, when Alaska Airlines flight 1282 departed Portland in Oregan, headed for Ontario in California. The aircraft had only recently entered service and according to initial reports, the take-off went without hitch, but as it reached 16,000 feet (4,900m) and began pressurizing for its journey, a door plug blew out. This caused uncontrolled decompression and forced the aircraft into an immediate landing, which occurred safely back in Portland.
Aircraft are built to be adaptable to fit different economic requirements, changing market cycles and the demands of the passengers in different regions. Aircraft are also built to last for a few decades, during which time they may be used in several different roles. This means that most aircraft have a flexible basic model that can be modified in several ways.
If you take the example of seating, this can be adapted from high density for a full economy class aircraft, to four cabin classes at the other end of the scale (economy, premium economy, business and first class). Different seating density affects the number of passengers that can be sat on an aeroplane, but the rules are quite clear: it must be possible to fully evacuate an aircraft within 90 seconds.[2] This means the number of exits required varies according to how the aircraft is being used.
The aircraft used for flight 1282, a B737 Max 9, can accommodate up to 220 passengers, but in the configuration deployed by Alaska Airlines, it only had 178. This means that it did not require additional doors[3] and there was seating where the door would otherwise have been fitted. The unused exit is covered by a door plug, which slots over fittings in the fuselage frame and is held in place with bolts. Without these bolts, or if these bolts are not correctly in place, the door plug can become dislodged as the aircraft is pressurized.
In the immediate aftermath of the incident, the US Federal Aviation Authority issued a grounding notice for B737 Max 9 fleets operating within its airspace. This action was followed by aviation authorities around the world, which may have considerable implications from an insurance perspective, as we discussed in this recent article about Grounding Liability Insurance.
Enhanced engineering and better understanding of aerodynamics means that inflight fuselage failures and uncontrolled depressurization are very rare these days. It was a different story at the dawn of commercial jet age, most notably with the de Havilland DH.106 Comet, the world’s first commercial jet aircraft. First flown commercially in 1952, by 1954 21 Comets had been built but seven of them had crashed under unknown circumstances.
After one of these losses both the wreckage and the deceased passengers were retrieved for examination to determine what had gone wrong. The coroner at the time noted that most of the victims had both skull fractures and ruptured lungs but it was believed that the ruptured lungs which caused death. To support this theory a model fuselage was created, with dummies seated within, and then pressurized until the fuselage exploded. This validated decompression as the cause of death, with the skull fractures occurring as the dummies were thrown from their seats and struck the ceiling. But why did the decompression occur in the first place?
Repeated pressurization tests conducted on a model fuselage in a water tank[4] discovered that metal fatigue caused cracking near a rivet hole in the region of the window. This caused inflight depressurization and aircraft break-up. As a result, the industry learned a lot about metal fatigue that occurred with higher altitude flights and repeated fuselage pressurizations.
As technology progressed, aircraft life became extended, but there were still knowledge gaps. A B737 manufactured in 1969 and operated in 1988 by Aloha Airlines as flight 243, had part of its roof torn off as the aircraft reached 24,000 feet (7,300 meters) on a flight to Maui.[5] The aircraft had flown nearly 90,000 flight cycles and over 35,000 flight hours when this occurred, an incredible amount of employment for an aircraft at that time. Fatigue cracks were again found to be the culprit. In a testament to both the skill of the pilots and the strength of the aircraft, this flight continued to a safe landing.
Decompressions remain rare but do still occur. A Qantas Airbus A330-202 suffered inflight decompression at 40,000 in 2021[6], and an Airbus A319-100 operated by Sichuan Airlines suffered a decompression in 2019 when a piece of the windshield separated from the aircraft[7].
These days, such events are more likely to be caused by external forces rather than aircraft design or knowledge gaps. There have been incidents of depressurization occurring after unreported ground vehicle damage causing a small hole in an aircraft fuselage that blows out to a larger hole when the aircraft is subsequently pressurized. Procedures with ground handlers and other airside vehicle operators are constantly tightened, and focused recruitment and improved training procedures constantly refined. Ground damage still occurs, but it no longer goes unreported.
Major incidents can also be caused when parts from the engine escape the cowling and penetrate the fuselage. The engine cowls are lined with armour plating and other materials to absorb fragments that are released if a turbine disk releases a fan blade, but the extreme forces involved can damage a fuselage.
This happened most recently in 2018 on a Southwest Airlines flight between New York and Dallas. The aircraft suffered an uncontained engine failure and fragments of the engine broke through the engine cowl. These fragments were ejected from the engine, damaged the aircraft’s fuselage and led to the aircraft suffering uncontrolled decompression. Despite the extent of the damage to the aircraft, it was landed safely with only a single fatality and eight minor injuries among the passengers and crew.
As a result of the incident, engine cowls are being strengthened.
It is worth pointing out that while safety figures for 2023 haven’t yet been released by the International Air Travel Association (IATA), based on the 2022 numbers,[8] the average passenger would have had to catch one flight every day for 25,214 years to be certain to be involved in a fatal aviation accident. In the US, between 2002 and 2020 there were 614 serious injuries related to air travel, compared to 44 million in cars and trucks on US highways.[9] The average injury rate for air travel was 0.01 injuries per 100 million passenger miles travelled, which compares to 48 injuries for road journeys by car or truck and 367 for motorcycles.[10]
From a risk management and insurance perspective, the aviation industry and regulators learn from every incident, and the increasing prevalence of data across aviation adds depth to every lesson. The risk management and insurance sector also works in partnership with the aviation industry and is an important link in the information-sharing chain between regulators, airlines, airframe manufacturers and the myriad of organizations that that are involved in flying and maintaining an aircraft. The safety of the aviation industry is constantly being enhanced.
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