Decompression and Emergency Descent

Decompression, also called depressurisation, is an uncontrolled loss of pressure in the cabin of a pressurised aircraft. As air escapes, the cabin altitude climbs towards the altitude at which the aircraft is actually flying. At airliner cruising levels the outside air cannot keep a person conscious for long, so the response is immediate and practised: oxygen masks on, then an emergency descent to an altitude where everyone on board can breathe without supplemental oxygen.

Decompressions are rare but heavily examined, because the time available is so short. Certification standards set how the crew is warned and how the masks are presented; operating rules set how much oxygen must be carried. The physiology is covered in hypoxia and hyperventilation, and normal operation of the system in pressurisation.

Types of depressurisation

The rate of pressure loss depends mainly on the size of the opening relative to the cabin volume, and on the differential at the time. Three categories are recognised:

A slow decompression is the most insidious, because hypoxia can erode the crew's judgement before anyone realises that the cabin is climbing. In a leak, the pressurisation controller first closes the outflow valve further to compensate. The cabin altitude starts to rise only when the leak exceeds what the packs can supply.

In a rapid or explosive decompression the air in the cabin expands and cools suddenly, so its moisture condenses into a mist. Gas trapped in the middle ear, the sinuses, the gut and even the teeth expands at the same time, and the breath must never be held. Many large aircraft have vent or blow-out panels between the cabin and the cargo holds, so that a sudden loss of pressure in one compartment does not collapse the floor between them. After any significant exposure, occupants may also suffer decompression sickness, caused by nitrogen coming out of solution in the body as the pressure falls; joint pain is the classic symptom, and the treatment is oxygen, landing and medical assessment.

Southwest Airlines Flight 1380 shows a rapid decompression in a large airliner. On 17 April 2018 the Boeing 737-700 was climbing through FL320 after leaving New York LaGuardia when a fan blade in the left engine broke off at a fatigue crack. A fragment of the fan cowl struck the fuselage near a cabin window, the window departed and the cabin depressurised rapidly. The crew made an emergency descent and diverted to Philadelphia. One passenger was fatally injured and eight suffered minor injuries.

Recognising depressurisation

Transport aeroplane certification rules, 14 CFR 25.841 and the equivalent EASA CS-25 provision, require a warning to the flight crew when the cabin pressure altitude exceeds 10,000 ft. An aural or a visual signal satisfies the rule, alongside the normal indications of cabin altitude, cabin rate of climb and differential pressure. On aeroplanes certificated for high-altitude operation, passenger masks must be presented automatically before the cabin altitude exceeds 15,000 ft, and many types deploy them at about 14,000 ft. Masks dropping in the cabin is therefore a late cue, never the first one.

The Boeing 737's cabin altitude warning is an intermittent horn, and its checklist makes one rule absolute: if that horn sounds in flight, the oxygen masks go on first and diagnosis comes afterwards. The loss of Helios Airways Flight 522 on 14 August 2005 shows why. The Boeing 737-300's pressurisation mode selector had been left in MAN after unscheduled maintenance, and the crew did not notice it during the preflight procedure, the before start checklist or the after take-off checklist. As the aeroplane climbed through about 12,040 ft the cabin altitude warning sounded, but the crew took it for the take-off configuration warning and did not put on their masks. Hypoxia incapacitated the pilots, the aeroplane flew on until its fuel was exhausted, and it struck hilly terrain near Grammatiko, about 33 km north-west of Athens airport, killing all 121 people on board.

Emergency descent procedure

The memory items and their order belong to the aircraft manufacturer. The quick reference handbook, the flight crew operating manual and the operator's SOPs take precedence over any general description, including this one (see A320 memory items). In outline, most procedures work through the same stages:

1. Protect the crew. Oxygen masks on, then establish communication between the pilots through the masks. Nothing takes priority over this. Many procedures then set the masks to 100 per cent; the A320 procedure sets the diluter to N once the masks are on.
2. Assess. If the cabin altitude can still be controlled, for example by selecting the standby or manual pressurisation mode, the descent may not be needed. If it cannot, continue.
3. Protect the passengers. Seat belt signs on, and passenger oxygen released manually if the cabin altitude exceeds, or is about to exceed, about 14,000 ft. Boeing 737 crews use the passenger oxygen switch; A320 crews press MASK MAN ON.
4. Descend. Select a lower altitude and a heading, close the thrust levers and extend the speedbrakes. Unless structural damage is suspected, descend at a high speed, at or near MMO/VMO in many procedures (the A320 procedure calls for maximum or appropriate speed). If it is suspected, fly at or near the speed at which the failure occurred, avoid high load factors and use the speedbrakes with care.
5. Communicate. Declare a MAYDAY, squawk 7700 unless ATC has already assigned a code (see transponder and SSR), and tell the cabin crew. Where regional contingency procedures apply, as in oceanic airspace, the crew turn away from the assigned route or track before descending. ATC may broadcast an emergency descent warning to other traffic.
6. Level off at 10,000 ft or the minimum safe altitude, whichever is higher (see minimum safe altitudes).

The pilot flying flies the descent while the pilot monitoring runs the checklist and handles ATC and the cabin. Speed matters because the oxygen supplies are sized around a quick descent. The FAA's two-hour minimum supply for each pilot, for example, is defined as enough for a constant-rate descent from the aeroplane's maximum certificated altitude to 10,000 ft in 10 minutes, followed by 110 minutes at 10,000 ft. Over high terrain the minimum safe altitude may be well above 10,000 ft, and operators plan escape routes accordingly.

Supplemental oxygen requirements

Supplemental oxygen is oxygen carried to keep crew and passengers adequately oxygenated when the cabin altitude is too high. In a pressurised aeroplane it is needed only if pressurisation fails, so the rules size the supply around the descent. EASA sets the requirements for commercial air transport in CAT.IDE.A.235; the FAA does so in 14 CFR 121.329 and 121.333.

Above 25,000 ft EASA also requires spare outlets or portable oxygen units so that each required cabin crew member has oxygen immediately to hand. Aeroplanes not certified above 25,000 ft that can descend safely within 4 minutes at every point on the route may carry a reduced passenger supply.

The FAA adds rules on wearing masks. Above FL250 one pilot at the controls must wear a mask, unless both pilots have quick-donning masks that can be put on with one hand within five seconds. That exception applies up to FL410 for aeroplanes with more than 30 passenger seats or a payload over 7,500 lb, and up to FL350 for smaller ones. Since a 2020 amendment, the pilot left alone at the controls when the other leaves the flight deck must put on a mask only above FL410; the earlier threshold was FL250.

Passenger masks and chemical oxygen generators

Passenger masks are continuous-flow masks with a reservoir bag, stowed in the overhead passenger service units, lavatories and cabin crew stations. On many types they are fed by a chemical oxygen generator: a canister containing a core based on sodium chlorate. Pulling a mask towards the face pulls a lanyard that fires a small charge, and the heat starts a reaction that releases oxygen. Once started it cannot be stopped, the canister becomes very hot, and oxygen flows until the core is exhausted, typically after somewhere between about 12 and 22 minutes depending on the generator fitted.

Some aircraft, such as the Boeing 747-400 below, feed the passenger masks from banks of high-pressure gaseous cylinders instead. The flight crew normally have their own gaseous supply, with quick-donning diluter-demand masks at each flight crew station that can deliver diluted oxygen, 100 per cent oxygen or oxygen under positive pressure (see oxygen systems).

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Qantas Flight 30 involved such a gaseous system. On 25 July 2008 the Boeing 747-438 VH-OJK had been airborne about 55 minutes from Hong Kong to Melbourne when one of the passenger oxygen cylinders in the cargo hold failed suddenly. Its discharge tore a hole about 2 m long and 1.5 m high in the fuselage, and the cylinder was propelled up through the cabin floor beside the second main cabin door. The captain made an emergency descent to 10,000 ft and diverted to Manila, and nobody was injured. The ATSB found no record of any comparable rupture of an aviation oxygen cylinder.

First-aid oxygen

First-aid oxygen is a separate supply of undiluted oxygen for passengers who, for physiological reasons, still need oxygen after a depressurisation, once the aeroplane has descended. It is not supplemental oxygen: it treats individuals rather than protecting everyone during the descent.

Under EASA CAT.IDE.A.230 it is required on pressurised aeroplanes operated above 25,000 ft when a cabin crew member is required. The supply must last for the remainder of the flight after a depressurisation, while the cabin altitude is between 8,000 ft and 15,000 ft, for at least 2 per cent of the passengers and never fewer than one person, at an average flow of at least 3 litres per minute per person measured at standard temperature and pressure, dry (STPD). The FAA's 121.333 is similar: a supply meeting the certification standard of 25.1443(d) for 2 per cent of the occupants for the entire flight after depressurisation, following descent from cabin altitudes above FL250. It is usually administered by the cabin crew from portable equipment.

After the descent

Once level, the crew check each other and the cabin. The cabin crew report injuries and damage and give first-aid oxygen to anyone who needs it. The passenger generators will soon be exhausted, so the aeroplane stays low.

A diversion usually follows. Fuel burn at 10,000 ft is far higher than at cruise, so range shrinks sharply; EASA fuel planning therefore considers a depressurisation at the most critical point of the route, and ETOPS critical fuel considers the same scenario (see fuel planning and fuel reserves).

Anyone with joint pain, skin symptoms or neurological signs after the event should be assessed for decompression sickness on landing, and every fired generator must be replaced before the aircraft flies again.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.