Can an aircraft implode in flight?

Recently, as the world waited with bated breath, news broke confirming the catastrophic implosion of the Titan submersible vessel.

Subsequently, numerous people contacted me to inquire whether an aircraft can implode in flight.

On June 18, the Titan imploded during its descent in the North Atlantic ocean, approximately 400 nautical miles off the coast of Newfoundland, Canada.

The submersible, carrying five people, was part of a tourist expedition to observe the ill-fated wreck of the Titanic.

The Titanic, a British ocean liner dubbed “unsinkable,” was four days into its maiden voyage from Southampton, England, to New York when it collided with an iceberg in the North Atlantic in the early hours of April 15, 1912 and sank, resulting in the deaths of 1,517 of the 2,240 passengers and crew.

Communication with the Titan was lost 105 minutes into its dive to the wreck site. Authorities were alerted when it failed to resurface at the scheduled time later that day.

The implosion of a vessel is caused by excessive fluid differential pressures causing the force of the external pressure, significantly higher than the internal pressure, to overcome the strength of the vessel structure, causing its rapid inward collapse.

Aircraft operate in either a non-pressurised or pressurised mode.

In a non-pressurised mode, the pressure inside the aircraft is equal to the outside atmospheric pressure. At sea level, the atmospheric pressure is 14.7 psi (pounds per square inch). It decreases with an increase in altitude.

Non-pressurised aircraft normally have a ceiling of 10,000 feet altitude, as above that, the air pressure would be less than 10.11 psi and too thin to provide adequate oxygen for normal human breathing and might cause hypoxia.

Hypoxia is a condition caused by low levels of oxygen in human body tissues. There are four main types of hypoxia, of which hypoxic hypoxia, also known as “altitude hypoxia,” is the most common form encountered in aviation. Hypoxic hypoxia occurs when there is insufficient oxygen available in the air to breathe, causing the lungs to struggle to transfer enough oxygen to the blood. This form of hypoxia can occur in unpressurised aircraft flying at altitudes above 10,000 feet.

On the other hand, pressurised aircraft operate at much higher altitudes to reduce fuel burn and avoid adverse weather such as thunderstorms. In order for passengers to breathe normally, the cabin is pressurised to maintain a pressure of approximately 11.34 psi. This is the equivalent atmospheric pressure at an altitude of 7,000 feet and is called the cabin altitude.

At a cruising altitude of 41,000 feet, the outside air pressure is 2.59 psi, giving rise to a difference in pressure of 8.75 psi, known as the differential pressure.

All aircraft fuselage structures are designed and certified to safely withstand a predetermined maximum differential pressure. Any exceedance of the maximum differential pressure can cause a structural failure, resulting in the rapid decompression of the aircraft cabin, with possible catastrophic consequences.

On commercial jet aircraft, compressed high-pressure air is bled off the engines. After being conditioned by mixing valves to maintain the cabin at the desired temperatures for passenger comfort, the air is pumped into the passenger cabin. The cabin pressure controller signals the outflow valves to release air from the cabin at a controlled rate to maintain the selected cabin pressure. The ventilation system uses differential pressure to remove stale cabin air by suction into the atmosphere.

There are several safety features on pressurised aircraft to mitigate the exceedance of the certified maximum differential pressures and any rapid decompression of the cabin.

Positive pressure relief valves prevent the over-pressurisation of the aircraft cabin, which could cause the differential pressure to increase above the design limits. The valves open at a preset differential pressure and allow air to flow out of the cabin, preventing overpressure damage to the aeroplane structure. The positive pressure relief valves are fail-safe devices that bleed fuselage pressure overboard if the outflow valve fails in the closed position.

The negative pressure relief valves prevent negative differential pressure (vacuum pressure) damage to the aeroplane structure that can occur during a rapid descent during an aircraft emergency. The spring-loaded relief valve opens inward to allow atmospheric air to enter the cabin.

Most pressurised aircraft have automated systems with manual override features for regulating the pressures and temperatures of the air in the passenger cabin. An indicator in the cockpit allows the pilots to monitor the differential pressure to ensure it is within the safety margins. If the pressure in the cabin drops to 10.11 psi (10,000 feet cabin altitude), a warning will sound in the cockpit requiring immediate corrective action by the pilots.

During an in-flight decompression, when the cabin pressure drops to 8.63 psi (14,000 feet cabin altitude), oxygen masks automatically fall from the overhead compartments above passengers. During the safety briefing prior to take-off, passengers are advised to take the mask and firmly place it over their nose and mouths and continue to breathe normally.

There are no recorded instances of an aircraft imploding during flight, as it is highly improbable that the outside atmospheric pressure will exceed an aircraft's cabin pressure.

However, there have been instances of rapid decompression caused by structural failures or damage during flight. These will be discussed in a subsequent article.