Investigation Summary
The accident was initiated by a maintenance error where the cabin pressure control system was left in the 'MANUAL' position following a leak test. During the climb, the flight crew failed to notice the incorrect switch position. As the cabin altitude rose, a warning horn sounded, which the crew misinterpreted as a takeoff configuration warning. Hypoxia quickly set in, rendering the pilots unconscious while the aircraft continued on autopilot until it ran out of fuel near Athens.
Final Conclusions
The primary cause was the failure of the flight crew to recognize that the cabin pressurization system was set to manual. Contributing factors included inadequate pre-flight checks, misidentification of the cabin altitude warning horn, and the failure of maintenance personnel to reset the system to 'AUTO' after testing.
Video Analysis
The Ghost Plane | Helios Airways Flight 522 — An investigation into how a simple switch and a series of missed warnings led to the tragic 'ghost flight' of Helios 522.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Cabin Pressure Control System (CPCS)
Regulates cabin altitude by controlling the outflow valve; it was set to MANUAL instead of AUTO.
Cabin Altitude Warning Horn
An aural alert that triggers when cabin altitude exceeds 10,000 feet.
Passenger Oxygen System
Chemical oxygen generators that provide approximately 12-15 minutes of air once masks drop.
Interesting Facts
- 01The aircraft was known as the 'Ghost Plane' because it flew for over two hours with an incapacitated crew.
- 02The cabin altitude warning horn and the takeoff configuration warning horn used the same sound on this 737 model.
- 03A flight attendant, Andreas Prodromou, remained conscious using portable oxygen and attempted to save the plane.
- 04Maintenance had performed a pressure leak test earlier that morning and failed to return the switch to AUTO.
Safety Actions & Advisories
Warning Horn Differentiation
Boeing issued a requirement to install distinct warning lights or sounds to differentiate between takeoff configuration and cabin altitude warnings.
Checklist Procedures
Enhanced emphasis on the 'Pressurization' item during pre-flight and after-takeoff checklists to ensure the system is in AUTO.