updated 10/10/2006 9:19:44 AM ET 2006-10-10T13:19:44

Investigators cited human error Tuesday as the main cause of the Helios Airways crash that killed all 121 passengers and crew near Athens on Aug. 14, 2005, the deadliest air disaster in the history of Greece and Cyprus.

The two pilots of the Cypriot 737-300 failed to competently operate controls regulating cabin pressure and misinterpreted a subsequent warning, which eventually led to the crew passing out and the crash of the jetliner north of the Greek capital, according to a report delivered to Greece’s transport minister.

Maintenance officials left pressure controls on an incorrect setting, the report said, and the aircraft’s manufacturer, Boeing, was cited for “ineffectiveness of measures taken in response to previous pressurization incidents in the particular type of aircraft.”

The report was prepared by Akrivos Tsolakis, head of Greece’s National Aviation Safety Board.

It also said the direct cause of the crash was the crew’s failure to recognize that the plane’s pressurization switch was in the “manual position” before takeoff and not set to automatic — which would have allowed the cabin to pressurize by itself.

After takeoff, the plane failed to pressurize and the two pilots did not recognize “the warnings and the reasons for the activation of the warnings,” including one showing that the oxygen masks dropped.

That led to the “incapacitation of the flight crew due to hypoxia,” or lack of oxygen, and resulted in the plane being flown on autopilot for two hours until it ran out of fuel and crashed.

Maintenance officials in Cyprus were also indirectly blamed, along with Cypriot civil aviation authorities.

‘Deficiencies in the organization’
The report said latent causes included Helios’ “deficiencies in the organization, quality management, and safety culture” and the regulatory authority’s “inadequate execution of its safety oversight responsibilities” over time.

The captain contacted Helios’ operations center after takeoff and as the aircraft climbed though 16,000 feet, reporting that there were warnings going off in the cabin, according to the report.

“Several communications between the captain and the Operations Center took place in the next eight minutes concerning the above problems and ended as the aircraft climbed through 28,900 feet. Thereafter, there was no response to radio calls to the aircraft,” the report said.

During the climb, “the passenger oxygen masks deployed.”

The report did not say if any of the passengers had managed to put on the masks, but it found that at least one man without a mask was seen alive in the cockpit 14 minutes before the crash by the pilot of a Greek F-16 fighter jet that had been scrambled to intercept the airliner.

“The F-16 pilot tried to attract his attention without success,” the report found. The plane issued two mayday signals when the first engine flamed out and crashed 13 minutes later after the second engine flamed out, it said

Wrestling with the controls
Earlier this year, Tsolakis said a flight attendant had wrestled with the controls for at least 10 minutes before the crash, in a failed effort to regain control of the airliner.

Helios Airways Flight 522 had been traveling from Larnaca, Cyprus, to Prague, Czech Republic, with a scheduled stop in Athens.

The aircraft was shadowed by Greek fighter jets before it crashed at Grammatiko, some 25 miles north of Athens. All 115 passengers — all Greek Cypriots and Greeks — and six crew were killed in the crash.

Helios Airways, a budget Cypriot airline, was replaced earlier this year by a new carrier called ajet.

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