Helios Airways Flight 522 was a scheduled passenger flight from Larnaca, Cyprus, to Prague, Czech Republic, with a stopover in Athens, Greece. Shortly after takeoff on 14 August 2005, Nicosia air traffic control (ATC) lost contact with the pilots operating the flight, named Olympia; it eventually crashed near Grammatiko, Greece, killing all 121 passengers and crew on board. It is the deadliest aviation accident in Greek history.

An investigation into the accident by Greece's Air Accident Investigation and Aviation Safety Board (AAIASB) concluded that the crew had failed to notice that the cabin pressurization system was set to "manual" during takeoff checks. A ground engineer had set it to "manual" to conduct testing, but had forgotten to restore it to "auto" afterward, and this configuration was subsequently missed by the crew during their pre-flight checks. This caused the plane to gradually depressurize as it climbed and resulted in nearly everyone on board suffering from generalized hypoxia, thus resulting in a "ghost flight". The negligent nature of the accident led to lawsuits being filed against Helios Airways and Boeing, with the former also being shut down by the Government of Cyprus the following year.