In 2005, the Helios Airways Flight 522 crashed near Grammatiko, Greece, killing all 121 passengers and crew on board, making it the deadliest aviation accident in Greek history.
The investigation into the crash found that the most likely cause of this tragic incident was a maintenance error. In order to perform a pressurization leak check, the pressurization system was set to "manual." However, the engineer failed to switch it back to "auto" upon completion of the test. Before takeoff, no one on the crew noticed this incorrect setting.
As Helios Flight 522 climbed, the cabin pressure dropped and a warning horn sounded, but the crew mistakenly identified it as a takeoff configuration warning. Despite several other warning lights illuminating, including the passenger oxygen light, the crew did not realize the seriousness of the situation.
Within minutes, the crew and passengers began to lose consciousness. The captain and co-pilot were the first to succumb to hypoxia. The aircraft then crashed into a mountainside, and there were no survivors.
