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Saturday, 7 April 2012

Aircraft Loses Height Shock Horror

There have been many column inches devoted to an emergency descent by a flight out of Bergamo airport headed for East Midlands on Wednesday last. What should be routine procedure following a pressurisation incident has been inflated into a scare story of the first magnitude, partly because the aircraft was operated by Ryanair, the Millwall of air carriers (everybody hates us and we don’t care).

So there had to be suitably dramatic coverage, including a front page lead in today’s Mirror, with all the usual simplifications and over-dramatisations. Bergamo airport – also known as Orio Al Serio – is not a principal Milan airport, unlike Linate or Malpensa, but is promoted as such by Ryanair. Ditto Hahn, which is not near Frankfurt am Main, but some 120km distant. Hacks take note.

And the action by the flight deck on Boeing 737-800 registration EI-DAH (the flight number being FR1703) was almost exactly in accordance with the usual procedures, and especially those laid down by the carrier. The “loud bang and ... rush of cold air” was a loss of cabin pressure, and the immediate concern of the crew should be to reduce height as a priority.

This is because too much of that “cold air” and everyone would rapidly freeze to death – literally. Also, the emergency oxygen supply – which feeds through those masks that drop down from the overhead panel – is just that, meaning it too is not intended to last long. So in the rare event of cabin depressurisation, the aircraft’s altitude must be reduced to a height at which the air is breathable.

And that is around 10,000 feet – which is the cabin altitude when cruising at 30,000 feet plus (pressurisation cannot compensate completely). As the procedure for emergency descent of the 737-800 shows (see it HERE [.pdf]), this was followed correctly, with a controlled descent from the 31,000 feet at which the incident occurred. It looked fast to the passengers, but note the word controlled.

The only thing the flight deck got wrong was to announce a MAYDAY call over the PA, rather than to tell Air Traffic Control (ATC), which I suspect was swiftly rectified. The reason for the MAYDAY call, rather than the lesser PAN (Possible Assistance Needed) is to alert ATC that the aircraft is making an unscheduled descent, and for ATC in turn to alert other aircraft that may be below it.

The possibility of meeting “traffic” while making the descent is also reduced by the presence of the Traffic Collision Avoidance System (TCAS) on board all modern airliners. All that remained was to ensure the flight stayed above the Alps, and the diversion to Hahn was mainly because it is a Ryanair base, and a replacement aircraft was most likely to be available.

But routine does not fill newspapers, so it gets sexed up. No change there, then.

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