So What Really Happens in an Airliner Depressurisation?
Recent events have highlighted aircraft loss of cabin pressure as an issue.
Causes include:
Structural failure – such as loss of windscreen (BAC1-11 1990), fuselage plug door (737-9 MAX, 2024), forward fuselage (737, 1988), cargo doors (DC10, 1974 and 747, 1989)
Deliberate acts – such as door opening (A321 2023), bomb (747, 1994)
Failure of (or incorrect) operation of cabin pressurisation system(s) – Helios 2005
There are, of course, many other instances of involuntary aircraft depressurisation.
Background:
Without pressurisation, the time of useful human consciousness at 35,000’ is around 30-60 seconds.
Yet, for jet aircraft to operate effectively, they need to be flying at those sorts of altitudes. As a result, the cabin is pressurised by either clean air from the engines or from an electrical system. Air flows into the cabin under pressure to give a “cabin” altitude of around 8,000’ to allow normal breathing. The air is then vented out of the aircraft via outflow valves. Note, the cabin is not “sealed”.
However, if an event causes the pressurisation to fail, the cabin altitude starts to rapidly rise to reach the aircraft’s real altitude, and at normal jet cruising altitudes this is not survivable.
In this case, detectors in the cabin cause the oxygen masks to drop from the overheard panels.
Pilot actions in the event of an uncontrollable depressurisation:
1) Immediately don emergency oxygen masks and commence the emergency descent procedure that involves:
2) Turn away from other air traffic below.
3) Select and descend to an altitude that keeps the aircraft above terrain but below the height at which people can breathe.
4) Control the aircraft’s speed (if there is a structural failure, high speed in the descent may exacerbate it). Use of speed-brake can significantly increase the descent rate to the breathable altitude.
5) Advise air traffic control (ATC) both verbally and via a transponder code.
6) Let their passengers know that the aircraft IS under the control of the pilots. This allays fears caused by the steep cabin angle, the appearance of “smoke” (usually condensing water vapour) and the significant rise in wind noise (particularly in the case of structural failures).
Once at breathable altitude, the pilots will assess the cause, liaise with their cabin crew, operations, and engineering departments, and then plan their diversion accordingly.
Barry Eustance CMgr MCMI
Kotter Change Leader Certified
Principal Change & Transformation Consultant
The Sixsess Consultancy
Barry is a former long-haul airline captain having operated globally in wide-body passenger jets. His aviation career spanned 48 years and he now also works with news broadcast media providing expert commentary on aviation and airline matters.
https://sixsess.org
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