Helios Airways flight 522 crashed on August 14, 2005 after a lack of oxygen incapacitated the aircraft's crew members and led to the plane eventually running out of fuel.
The plane reportedly arrived in Cyprus with a possible pressure problem. Helios technicians did a pressure check but failed to reset the pressurization system from manual back to auto after the test was completed. When the plane was put back into service, flight crew managed to overlook the error during the pre-flight procedure, the after-start check, and the after take-off check.
The aircraft departed Larnaca International Airport at 9:07 a.m., heading to Prague with a stopover in Athens. As the aircraft continued to climb, the air pressure slowly dropped and cabin altitude warning horn sounded.
Due to hypoxia, the judgement of the plane's pilots was impaired and they misunderstood the true nature of the warning signal. Ground controllers lost contact with the plane at 9:20 a.m. At the same time, oxygen masks were deployed inside the cabin as air pressure reached dangerous levels."
Two Hellenic Air Force F-16 fighter jets were deployed to establish contact when the aircraft failed to answer calls from Athens ATC. The jets observed that the pilot was nowhere to be seen and the first officer was seen slumped forward in his seat, unconscious, while the oxygen masks were seemingly unused.
At 11:49 a.m., flight attendant Andreas Prodromou, who was able to remain conscious using a portable oxygen tank, tried to land the plane but was too late when the engines flamed out due to fuel exhaustion.
At 12:04 pm, the aircraft crash landed into hills near Grammatiko, a village in East Attica, Greece. There were no survivors. It was the deadliest aviation disaster in Greek history with 121 fatalities and is the fourth-deadliest involving a Boeing 737-300.