Leak in a passenger aircraft ? – Decompression | Hypoxia

aircraft decompression hypoxia

Unfortunate decompression at 24000 feet

A freezing wind of hurricane force is roaring through the cabin. There’s not enough oxygen onboard. The flight declares an emergency yet nobody can hear you. Sounds like a nightmare? Unfortunately, for everyone onboard Aloha Airlines Flight 243, it was a reality.

On April 28, 1988 , Aloha Airlines flight 243, scheduled to fly between Hilu and Honolulu in Hawaii, sustained extensive damage during flight. Though the aircraft was able to divert safely at the nearby Kahului airport in Maui, 65 passengers including the crew were injured, not to mention a flight attendant at the time was also ejected out of the aircraft. This incident consequently, had far-reaching effects on aviation safety policies.

Aloha Airlines Flight 243 decompression
Aloha Airlines Flight 243

So, what exactly caused the emergency that injured many and led to the ejection of a crew member at 24,000 feet ?

What is Aircraft decompression ?

The Aloha Airlines Flight 243 is a classic example of an aircraft depressurisation/decompression. Decompression in simple terms refers to the inability of the aircraft to maintain its designed pressure schedule. Failure to maintain this schedule can be attributed to a malfunction of the system or structural damage to the aircraft itself.

aircraft decompression hypoxia
boldmethod – Time of useful consciousness

Why should aircrafts be depressurised, you may ask? As an aircraft gains altitude, the air becomes thinner and it gets difficult to breathe which is why it becomes imperative to mimic conditions similar to that at sea level. Typically modern aircrafts are designed to maintain a pressure of 14.7 PSI -which is generally found at sea level. Now this implies the air pressure inside the cabin is higher than the pressure outside and this pressure differential is maintained to protect the pilots, cabin crew and the passengers from Hypoxia- which is a state of reduced awareness due to insufficient supply of oxygen to the brain.

Types of decompression

Depending on the type of severity and the loss of pressure, it is predominantly divided into 3 categories:

Explosive Decompression : Any decompression which occurs rapidly, say less than 0.5 sec, is graded as explosive decompression-Exactly the one which occurred aboard the fateful Aloha Flight 243. This is one which is potentially fatal as it occurs faster than the lungs can decompress. The extreme noise in such an event can lead to utter confusion and distraction from flight duties, not to mention the danger of flying debris onboard.

Rapid Decompression : Almost similar to explosive decompression yet different in the fact that here the lungs can decompress faster than the cabin and hence the risk of lung damage is significantly less. One can experience  decrease in temperature, as the cabin temperature equalizes with the outside air temperature, Cloud of fog or mist in the cabin that is due to the drop in temperature, and the change of humidity.

Slow Decompression : Warning systems onboard generally notify the crew about depressurisation but it so happens that they do not always indicate the incidence of a slow decompression which can turn fatal only if undetected at early stages. One of the first physiological indications of a slow decompression may be ear
discomfort or ‘popping’, joint pain, or stomach pain due to gas expansion.

aircraft decompression
Blurred sight

Possible causes

It is to be noted that specific outflow valves maintain cabin altitude at the desired levels. A failure of the system that manages the valves can result in depressurisation. Irregular airflow , unserviceable components in the aircraft air-conditioning system or an engine malfunction can inhibit the free flow of fresh air. Now technically cabin doors are so designed such that the risk of depressurisation is next to nil even if the door is opened while in flight or intentionally. That said, improper maintenance of aircraft parts, fuselage, cracked windows, improper sealing of a door or a window can all lead to structural failure mid-flight , leading to a fatal decompression event.

Hypoxia

The effects of hypoxia (lack of oxygen) cannot be over emphasized. It is important for the cabin crew to realize that even mild hypoxia, though not fatal, can have fatal results. This is because hypoxia can significantly reduce the crew member’s ability to perform, and consequently lead to errors that may be fatal. The insidious nature of hypoxia causes a subtle decrease in individual performance, followed by incapacitation, the symptoms may not be identified until it is too late.

Hypoxia can cause a false sense of well-being. It is possible for a person to be hypoxic and not be aware of their condition. Therefore, it is important that the cabin crew recognizes the signs of hypoxia, and provides oxygen as soon as possible, in order to prevent a loss of consciousness.

fighter pilot oxygen mask use
Oxygen used by Pilots of Fighter aircrafts

Expected steps from the flight crew

The pilots are trained to immediately notify the ATC of the current issue. Normally in the event of depressurisation, pilots descend to a minimum safe altitude of 10,000 feet where the pressure outside and inside becomes more or less similar and it gets easier to breathe. However, the crew may initiate descent with/without obtaining clearance depending on the severity of the situation.

Following a decompression event, the crew is expected to execute the following:

  • Immediately grab the nearest oxygen mask and stay put
  • Sit down on the chair or secure oneself against any tight object.

“The amount of time for which the crew and passengers can perform their duties in an environment devoid of sufficient oxygen” – EFFECTIVE PERFORMANCE TIME

Incapacitated crew would only be a liability in assisting other crew members and passengers. In one such decompression event, a flight attended was heaved out of the cabin head-first but fortunately a passenger clanged onto her ankle that ultimately saved her. Hence personal safety of the cabin crew first cannot be stressed enough.

Aircraft decompression
Cabin oxygen mask

Is this preventable ?

Though such incidents are rare, they do happen and when they do, the aftermath can be catastrophic. Hence to prevent such mishaps from occurring it becomes very important for the pilot and cabin crew to recognize the types of decompression and the immediate course of action that should be followed following such an event and limit the risk of hypoxia and ensure flight safety. The role of operators is equally important as regular recurrent training modes should be implemented emphasising the importance of effective communication (CRM). All it takes is just seconds for an incident to turn into an accident. And that can always be prevented.

At 20,000 Ft, Air India Flight To Frankfurt Suffers Cabin Decompression, returns back safely to Indira Gandhi International  – NDTV (6th March 2019)

IndiGo’s Lucknow-Bengaluru flight on Friday night landed safety at its destination after the aircraft suffered cabin depressurisation when it was about 240 km away from Bengaluru. The Airbus A320 (VT-ITM) has been grounded there for checks and the Directorate General of Civil Aviation is probing the snag. – Times of India (3rd April 2021)

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