NYT: 121 are Killed as Jet Crashes Outside Athens - The investigation into causes begins

...Aviation experts were perplexed, saying it was rare for a plane to crash because of depressurization.

"Although there are precedents for both pilots losing consciousness at the controls of the aircraft in the past, for it to happen on a large airliner like a Boeing 737, with all the backup systems they have there, does seem to be really quite extraordinary," said Kieran Daly, editor of Air Transport Intelligence....

Airliners are pressurized by a system that draws air from the engines, which compress air for internal use. A valve at the back of the plane determines how fast air is let out of the fuselage.

Aviation experts say that depressurization is rare, and that when it occurs, a warning horn sounds in the cockpit, and crew members are supposed to put on oxygen masks and descend to 10,000 feet, where there is enough oxygen for humans to function even in a depressurized aircraft.

In October 1999, a Learjet carrying the golfer Payne Stewart and five others crashed in South Dakota after flying hundreds of miles on autopilot. Air Force pilots who shadowed the jet reported that the cockpit windows were iced over, a sign of depressurization.

After the investigation of that crash, a doctor with the National Transportation Safety Board said pilots' cognitive ability could fade away quickly when oxygen was limited, often faster than they realized.

Another source of incapacitation can be smoke or fumes in the cockpit. If the problem is not obvious from the wreckage, it may be discerned from tests on the blood of the victims, which will show what they were breathing before they died.

The cockpit voice recorder is likely to capture the sound of the warning horn, if it sounded, and conversation between the pilots about the problem....

The problem with oxygen masks at very high altitudes is the result of the so-called "partial pressure" of oxygen. Your body needs a certain number of molecules (mass) of O2 to function, but your lungs hold a given volume, so the amount of O2 in your lungs is based on the pressure times the fraction of oxygen in the air. At sea level, regular air with about 21% O2 has a high enough partial pressure of oxygen for normal breathing. Above about 10,000 feet, there's not enough O2 pressure in the air to supply your body adequately, and so that's about the altitude above which people in unpressurized aircraft tend to wear oxygen masks or cannulas (though it's not required until 12,500 feet).

But once you get above about 30,000 feet, the partial pressure of oxygen even in 100% O2 isn't sufficient - in other words, there aren't enough O2 molecules in a lung volume of 100% O2 to supply your body. Above that altitude, you need to breathe pressurized oxygen to force more O2 molecules into a given volume, which isn't available from the cabin oxygen masks (don't know about the pilots' system), or be in a pressurized aircraft.

In any event, the standard procedure for decompression would be to expedite a descent to an altitude at which oxygen masks wouldn't be needed. I'll be curious to find out exactly why that didn't happen before everyone on board lost consciousness.

Leased to Helios by Deutsche BA in April 2004.

Other info about the airline, from a Cypriot newspaper.

Helios beset by a history of problem
By Jean Christou

YESTERDAY’S tragic crash was not the first time Helios had problems with flights. In the past two years alone there were several incidents involving troubled flights and urgent landings.

In April 2003, a damaged Helios Boeing 737 carrying 180 passengers landed safely at Stansted Airport following an incident involving bird strike, which damaged the aircraft’s tail. The flight took off from Luton Airport when the tail sustained minor damage after take-off and the pilot had to decide whether to return to Luton for safety reasons or go on to land at Stansted, where it landed safely.

In September of the same year, a Helios flight from Larnaca to Luton carrying over 100 passengers was forced to divert to Rhodes due to engine failure. The pilots of the Boeing 737 noticed a problem with the engine around 40 minutes into the 9am flight out of Larnaca and immediately diverted to Rhodes where it landed safely. The Boeing was later given a new engine.

Three months later, three passengers from a Helios flight from Warsaw were taken to hospital after the aircraft lost cabin pressure and was forced to make an urgent landing at Larnaca airport.

Several more of the 35 passengers and six crew members were treated at First Aid in the airport after the emergency services were mobilised.

The reports said that the Helios captain had contacted the Control Tower at Larnaca saying they had lost a significant amount of cabin pressure and that passengers had reported feeling dizzy as the plane was coming in to land.

In February this year, A Helios Airways plane was struck by lighting on its way from Larnaca Airport to pick up a number of passengers for a flight to Luton.

The Boeing 737was carrying 62 passengers and seven crew members when it left Larnaca for Paphos at around 9am. As the plane was coming in to land, the lightning struck but the landing was not affected.

It is not clear whether the Boeing that crashed yesterday was involved with any of the four incidents but reports were rife that the particular plane had had problems in the past, including problems in the aircraft’s cockpit.

There was also speculation that the Boeing that crashed was the same one that was known to have had problems with its air conditioning system and that the company knew about the problem.

Several callers to Greek and Cypriot television channels yesterday said they had traveled with Helios and there had been problems with the temperature. One man said that on a flight back from Prague only this week, temperatures fell dramatically some 20 minutes out of Larnaca and all passengers had to be given blankets.

Another caller said that on a flight last Sunday, water dripped on his head and when he complained to cabin crew he was told there was a problem with the air conditioning.

However, Viki Xitas, the commercial manager for Helios told Greek television yesterday said the company had no information about problems with the plane’s air conditioning. “This plane received maintenance as usual and left Cyprus without any problem,” she said.

Helios general manager Demetris Pantazis also said he could not confirm that the plane had the same problems previously since they do not know what the problem was that caused yesterday’s crash.

Communications and Works Minister Haris Thrasou told a news conference later in the day that the plane in question had been given the all clear in examinations.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.