The crew of a transatlantic Aer Lingus flight to New York were forced to wear oxygen masks because of a pressurisation problem in the cabin.
The flight, with 225 passengers and 12 crew on board, had to make an emergency descent and return to Shannon airport.
The Department of Transport's air accident investigation unit said the incident had been "very serious" and could have rendered the flight crew unconscious because of the lack of oxygen - a condition known as hypoxia.
However, the crew responded appropriately in the circumstances when they donned their oxygen masks immediately.
The pressurisation problem was traced to an aft cargo hold door seal which had been fitted wrongly three days before the incident. It had been installed both inside-out and upside-down.
Air accident investigation unit inspector John Hughes said neither the crew leader nor crew members understood how the seal functioned in flight. As a result, he recommended the need for a review of the contractor's initial and ongoing training system.
He also acknowledged that the Airbus, the manufacturer of the seal, had made the instructions to install it clearer as a result of the incident.
The pilot first noticed problems when the flight was en route from Dublin to Shannon on the morning of August 18th, 2005. As the aircraft reached 10,000 feet the crew noted that the cabin pressure altitude was indicating 4,900 feet.
The captain decided to continue the flight at a lower altitude and circled Shannon before landing.
The post flight report, carried out automatically by the aircraft maintenance computer, diagnosed the problem as being an engine bleed.
The report concluded that the problem with the cargo door might have been identified had the aircraft climbed to a higher altitude during the initial flight from Dublin to Shannon.
Although the report did not mention a pressurisation problem, the engineers carried out a check of the pressurisation system, but no fault was found and the aircraft was released for service.
The flight was cleared for take-off to JFK airport in New York. The flight was 150 miles into its journey to New York when the crew realised that it could not control the cabin pressure level.
Mr Hughes said the pilot took the appropriate action in the circumstances.The passengers were never in danger, he said.
