On July 3 2023, Aegean Airlines flight AEE560 from Thessaloniki (SKG) to Barcelona (BCN) performed an emergency diversion to Naples (NAP) following a cabin pressurization issue. The aircraft landed safely in Naples at 10:39UTC, with passengers on board citing the professionalism and teamwork of the crew in delivering a safe outcome.

Why do we pressurize aircraft cabins, and what actions do pilots and cabin crew take to safely get you back on the ground? Let’s take a look. 

Why are aircraft pressurized? 

In short, commercial aircraft are pressurized due to the high-altitude, hostile environment in which they operate. The amount of oxygen in the air at a cruising level of 35,000 feet (10,650 meters) is around 26% of that at sea level. Without sufficient oxygen humans encounter a condition called Hypoxia, which reduces our abilities to solve problems and perform basic tasks. Commercial aircraft cabins are pressurized to prevent this. 

Simply put, air is brought in via bleed air from the engines, compressed, and fed into the cabin. The air then leaves the cabin via an outflow valve. This constant cycle of refreshed air helps to regulate the pressure. It also contributes to making commercial aircraft cabins one of the most sterile spaces possible.

Aircraft cabins are set to replicate the air pressure at a given altitude that is both suitable for humans, and places minimum stress on the aircraft – we call this a ‘Cabin Altitude’. Most modern commercial aircraft fly with a cabin altitude of around 8,000 feet but newer generation aircraft such as the Boeing 787 Dreamliner and Airbus A350 use cabin altitudes as low as 6,000 feet. This is often a key selling point for aircraft manufacturers, as the higher oxygen levels and humidity at lower cabin altitudes can reduce fatigue and improve passenger comfort. 

What happens when an aircraft cabin de-pressurizes?

It is important to highlight that pressurization failures can occur rapid or gradually. Whilst many of us can picture the loud noise, flying objects and white mist associated with a rapid decompression, gradual decompressions are equally dangerous and far more insidious. A famous example of a gradual decompression resulting in fatalities occurred on board Helios Flight 522 in August 2005. It is common for aircraft to auto-pressurize as they climb by using a switch on the pressurization panel. In the case of flight 522, the switch had been set to manual during maintenance, and not reset to auto. The crew missed this in their checks. The aircraft never pressurized, and those on board slowly lost consciousness as the aircraft climbed to its cruising altitude. 

A loss in cabin pressure at high altitude immediately places those on board at risk of oxygen deprivation, Hypoxia and unconsciousness. We can calculate how long a person can remain usefully conscious at high altitude using the ‘Time of useful consciousness’ concept. 

What do pilots do in the event of a decompression?

1. Mask up – upon discovering a loss in cabin pressure, the first action for pilots is to don their oxygen masks. Due to the time of useful consciousness at typical cruising levels being under one minute, it’s vital that both pilots remain conscious, in control, and able to take the follow up actions. Pilot’s masks have an oxygen supply of up to 2 hours. Cabin crew and passenger oxygen lasts around 14 minutes, which is considered an ample amount of time for the aircraft to descend to a safer altitude.
   
   
2. Descend – the next step is for the pilots to initiate an emergency descent to a lower altitude where there is more ambient oxygen. This is typically done at a high but structurally safe vertical speed with autopilot, idle thrust and speed brakes deployed. If the aircraft is not damaged, the crew will likely choose the maximum safe operating speed (‘VMO/MMO’) for the descent. In the case of Aegean flight 560, the aircraft descended from 34,300 feet to 10,000 feet in 5 minutes – a rate of descent of 4,860 feet per minute.
   

   

    

3. Level off – The next step is to level the aircraft at a safe and appropriate altitude, at around 10,000 feet or below, that allows passengers to breathe unaided. Pilots will have to carefully consider terrain during the descent, and make course changes as appropriate if flying in the vicinity of high ground and mountains. If an operator regularly flies for extended periods of time over mountains where the minimum safe altitude (MSA) is very high, extra oxygen for the passengers and crew may be mandated. The crew will also have to consider the local temperature and air pressure to ensure the altitude is measured accurately.
   
   
4. Assess the situation – once the aircraft is stable and level, the crew will work together to assess any damage to the aircraft or injuries to the passengers and crew. The transponder code will also be set to 7700 to indicate a general emergency if this hasn’t already been done.
   
   
5. Re-plan – the pilots will assess whether it is appropriate for the flight to continue to its original destination or to divert to the nearest suitable airport. A number of factors will contribute to this decision. As the aircraft is now limited to flying lower and slower, fuel burn will be higher and range reduced. They also need to consider whether any high terrain will block their intended track. Aegean flight 560 was 42 miles (68km) south east of Naples at the time it commenced its emergency descent. NAP was an effective choice of diversion as it allowed the crew time to stabilize the aircraft and prepare for arrival. Furthermore, Naples is a major airport with sufficient emergency equipment and facilities for handling stranded passengers.