2005-08-14
Helios Airways
Flight ZU522
Departure
Larnaca Airport, Larnaca, Cyprus
Destination
Athens International Airport, Athens, Greece
Causation Breakdown
Recurrence Likelihood Today
Low
Post-maintenance pressurization checklists have been strengthened. Cabin altitude warning systems have been reviewed for distinctiveness. Improved pre-flight checks for pressurization mode position are now standard.
Summary
Helios Airways 522 climbed to cruising altitude with the pressurization in ground mode, silently filling the cabin with hypoxia. The crew was incapacitated within minutes of passing 18,000 feet. The aircraft flew on autopilot for two hours over Greece as military jets escorted it. A flight attendant, surviving on emergency oxygen, entered the cockpit but the aircraft crashed near Athens when it ran out of fuel. All 121 aboard perished.
Cause & Investigation
Determined cause
Crew incapacitation from hypoxia — pressurization in manual mode
Following a maintenance check, the pressurization system was left in manual/ground mode. As the aircraft climbed, the cabin altitude rose without automated warning. Crew and passengers gradually lost consciousness from hypoxia. The aircraft flew on autopilot for nearly two hours while Greek military jets intercepted and observed the unresponsive crew. A flight attendant with emergency oxygen briefly entered the cockpit but could not prevent the crash when the aircraft ran out of fuel.
Investigation body
Hellenic Air Accident Investigation and Aviation Safety Board (AAIASB), Greece.
Timeline & Safety Improvements
Flight ZU522 disaster
Boeing 737-31S operating for Helios Airways. 121 fatalities, 0 survivors.
Formal investigation opened
Conducted by: Hellenic Air Accident Investigation and Aviation Safety Board (AAIASB), Greece.
Cause determined: Crew incapacitation from hypoxia — pressurization in manual mode
Following a maintenance check, the pressurization system was left in manual/ground mode. As the aircraft climbed, the cabin altitude rose without automated warning. Crew and passengers gradually lost consciousness from hypoxia. The aircraft flew on autopilot for nearly two hours while Greek military jets intercepted and observed the unresponsive crew. A flight attendant with emergency oxygen briefly entered the cockpit but could not prevent the crash when the aircraft ran out of fuel.
Improvement implemented
Mandatory checklists for pressurization mode verification after maintenance were strengthened globally.
Improvement implemented
Cabin altitude warning systems were reviewed and their audibility standards raised.
Improvement implemented
Post-maintenance test flight procedures were overhauled to include pressurization mode verification.
Improvement implemented
Helios Airways was shut down by Cypriot authorities following the investigation.