Helios Airways Flight 522 leaves Larnaca, Cyprus on August 14, 2005, bound for Athens with 121 passengers and crew aboard, and stops responding to air traffic control somewhere over Greek airspace. This episode of the Air Crash Investigations series reconstructs how a cabin pressurization switch left in the wrong position led to hypoxia that incapacitated both pilots while the plane flew on, effectively unmanned, on autopilot. Greek F-16 fighter jets scramble to intercept the airliner and their pilots report seeing the co-pilot slumped at the controls and a flight attendant, Andreas Prodromou, attempting to reach the cockpit and fly the plane himself. Investigators work through black box data, wreckage analysis, and simulations to explain why a routine pre-flight check failed to catch the error and why the warning horn was mistaken for an unrelated alarm. The episode lays out the maintenance and training lapses that let a single overlooked switch setting turn into a fatal chain of events, ending with the plane's crash near Grammatiko.