Page last updated at 11:53 GMT, Tuesday, 26 August 2008 12:53 UK

What to do when planes lose pressure

By Tom Symonds
BBC transport correspondent

Ryanair plane
Aircraft oxygen systems have about 12 minutes worth of reserves

A Ryanair flight carrying British holidaymakers has been forced to land after losing cabin pressure. But how common is this? And how well was this particular incident handled?

We've all sat through those pre-flight briefings, waiting for the plane to get in the air and the refreshments to start flowing.

Regular fliers can probably recite it by heart: "The oxygen mask will drop down, pull it down and towards you, fit yours before helping an infant..."

The Ryanair depressurisation incident is a dramatic demonstration of why this briefing is needed.

Passenger airliners routinely fly at altitudes approaching 40,000ft. At that height the air is so lacking in oxygen, that if you were to breathe it normally, you would have around 12 seconds before starting to suffer the effects - disorientation, lack of consciousness, and eventually death.

Cabin pressure is constantly changing on board planes as the aircraft climbs and descends. This is why your ears pop when you go up or down

When a Cypriot airliner operated by Helios lost air pressure in 2005, the crew failed to notice and continued climbing.

They passed out, and cabin crew were unable to get into the cockpit. Eventually all on board lost consciousness and the plane flew on autopilot, crashing when it ran out of fuel.

Cabin pressure is constantly changing on board planes as the aircraft climbs and descends. This is why your ears pop when you go up or down.

Passenger complaints

When it fails, the procedure is well-rehearsed but swiftly executed.

The crew need to descend fast to about 8,000ft where the air can be breathed normally.

They need to consult emergency checklists, speak to air traffic control to ensure the airspace below is clear, and try to check whether other systems have failed.

Aircraft oxygen systems have about 12 minutes worth of reserves, so there is no time to lose.

Experienced airline cabin crews say that in depressurisation situations passengers tend to make two complaints - that the oxygen masks were not working, and that they were not told anything by the crew.

Generally the oxygen flow through the masks is very light, which can confuse passengers into thinking there is something wrong.

Indeed, in some incidents, they've become so worried that they have started tugging at the mask, and pulled it from the ceiling.

Crashed Helios Airways plane
Loss of pressure led to the crash of a Helios Airways plane in 2005

In the Ryanair incident passengers were angry they were not told what was going on. This is hardly surprising, given that an emergency descent can be terrifying if you do not why it is happening or how it is going to end.

The problem for the flight deck crew is that they are working hard to resolve the situation, and usually do not have time to explain what's happening.

Of course, they are also wearing oxygen masks - more tightly fitted than those of the passengers, to keep out fumes and smoke.

So what about the cabin crew?

Just as passengers have to get their masks on quickly, so do they. Their emergency procedures dictate that they find the nearest mask, put it on and sit down.

Sometimes this might mean using a passenger mask - aircraft usually have one more per row than there are seats, so that travellers with babies on laps have a spare.

All of which makes it difficult to pass on information to passengers.

For an airline, an emergency pressurisation incident is a customer relations nightmare.

But on this occasion the procedures at least appear to have worked and the aircraft was safely returned to the ground.

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Ryanair's Michael O'Leary defends the crew's actions




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