Failure to recognise a cabin pressure problem caused the crew of a Helios Airlines Boeing 737 to pass out and crash into a mountain near Athens in August 2005, prompting a major investigation by Greek aviation authorities.

Of five pressurisation incidents examined as part of the probe into the Helios tragedy, the Air Accident Investigation Unit (AAIU) in Dublin supplied accident reports on four Irish cases, involving three planes owned by Ryanair and one by Aer Lingus.

In one incident in December 2000, on an Aer Lingus flight from Cork to Amsterdam, the cabin altitude warning horn sounded after the Boeing 737 climbed over the critical 10,000 feet mark. After this level without pressure, all on board would suffer from hypoxia . . . lack of oxygen to the brain . . . and pass out.

But the flight crew misread the warning horn and continued the climb. They also failed to fully appreciate information relayed to them by the cabin crew that the passengers' oxygen masks had dropped and that the problem was pressurisation, according to the AAIU.

Though the pilot eventually turned back, the AAIU noted that "the continued persistence of the senior cabin attendant in keeping the flight crew informed of the cabin situation was a major factor in ensuring the safe outcome of this serious incident."

In another incident on a Ryanair flight from Derry to London in 2002, the flight crew misread the cabin altitude warning horn as indicating some other problem. The AAIU also noted that the crew's inability to handle the situation suggested they had started to suffer from hypoxia.

The report on this flight also noted that, in accordance with international safety measures, the door to the cockpit was locked, meaning that, had the crew passed out, nobody would be able to gain access to the controls. While this had no direct bearing on the DerryLondon incident, "the potential for a full-scale accident is self-evident in this type of emergency, " said the AAIU.

In the Helios tragedy, the flight crew lost consciousness just after take-off from Larnaca airport because of the onset of hypoxia but the plane continued on its programmed flight to Athens. Greek F16 fighter planes, which were scrambled after the plane was declared "renegade", observed through the window of the doomed craft that a man . . . later identified as a cabin crew member . . . was desperately trying to fly the plane. By this stage, the Helios plane had run out of fuel and, while the cabin attendant acknowledged the F16s' presence, the plane banked off to the left and plunged to the ground.

The jet fighter pilot also noted that almost all the passengers were sitting motionless . . . some with masks on . . . in their seats, suggesting they were already dead.

In yet another Irish pressurisation incident involving a Ryanair flight from Spain to Dublin in 2005, the AAIU remarked on the "insidious" effects of hypoxia, after a cabin attendant noted some passengers had shown signs of the condition. "Some appeared to be dizzy and laughing and some did not bother to put on their oxygen masks, " the attendant reported.

The plane eventually successfully executed an emergency descent to Biarritz.

"A flight crew operating under a high workload on the flight deck may not fully appreciate or recognise the initial symptoms of hypoxia, " said the AAIU. "It is therefore possible that judgment may be impaired to such an extent that corrective actions associated with dealing with an emergency situation may lead to incorrect or inappropriate response which could endanger the aircraft."

In its submission to the Greek aviation authorities, the AAIU also cited a cabin pressure incident on a Ryanair flight from Stansted to Knock in October 2000.

The direct cause of the Helios tragedy was that the flight crew failed to recognise that the cabin pressurisation switch was in the wrong position at take off. The crew had also failed to recognise that the cabin altitude warning horn meant there was a pressurisation problem, while cabin crew had failed to inform the crew that the passengers' masks had dropped.