Reason passengers on 'ghost plane' were stuck in loop at 35,000ft for hours before everyone was killed

A series of fatal mistakes led to Greece's most deadly aviation accident to date that still haunts people 20 years on.

On 14 August 2005 Helios Airways Flight 522 departed Larnaca in Cyprus for Prague on a seemingly routine flight that turned into an utterly devastating crash.

After the Boeing 737-300, nicknamed 'Olympia' departed, air traffic control were trying to get in touch with it for over an hour after take off.

After no response, the Greek military decided to send two F16 fighter jets up to figure out what was happening.

The details of what these jets saw is utterly chilling. They noticed that the first officer was slumped at the controls and the captain's seat was totally empty, while back in the cabin passengers sat unconscious with oxygen masks dangling down in front of them.

The flight 522 tragically burst into flames on a Greek hillside (LOUISA GOULIAMAKI/AFP via Getty Images)

The fighter jets were helpless as they watched Flight 522 fly around aimlessly in the sky awaiting the inevitable tragedy that would happen when the plane eventually came to a stop.

Perhaps the most bizarre aspect of this accident was the fact that only one person remained conscious during the flight as the plane became stuck in a loop at a whopping 35,000ft in the clouds.

What led to this point then?

Despite the thousands of hours of experience held by both the first officer and captain, they decided to ignore the cabin altitude warning horn which went off as the reached 12,040ft. This alarm should have stopped them from ascending further into the clouds.

Some have theorised that the pair mistook the warning for a take-off configuration warning instead, as both have the exact same sound, however several warning lights continued to light up the cockpit in the minutes following.

The airline is now discontinued (Milos Bicanski/Getty Images)

As the plane continued to climb, it experienced a gradual loss of cabin pressure, causing oxygen levels to plummet.

This triggered the masks in the cabin to deploy and likely caused the passengers to pass out quickly.

The only person to maintain consciousness was flight attendant Andreas Prodromou. Although not qualified to fly a Boeing 737, Prodromou did hold a UK Commercial Pilot Licence.

It's reported that Prodromou waved at fighter jets before the left engine faltered and the plane began to descend.

Although Prodromou was not able to change the fate of the plane, he made the decision to steer it away from the populated city of Athens and toward a rural hillside to avoid further casualties.

The flight crashed into a hillside near the village of Grammatiko and burst into a fireball, tragically killing the 115 passengers and six crew members on board.

121 people tragically lost their lives in the crash (Milos Bicanski/Getty Images)

Greek authorities and Helios Airways launched an investigation swiftly after the incident, which eventually found that a switch was responsible for the tragedy.

Earlier that day the plane had landed at Larnaca from London, where the flight crew had complained about a frozen door and hearing strange noises emerging from the service door.

Upon these reports and before the flight departed for Prague, the plane was fully inspected. This meant that the engineer had to switch the pressurisation system to 'manual'. But this switch was never returned to 'auto', meaning flight 522 continued climbing through the air depressurising without any crew knowing.

Families who lost loved ones on the doomed flight filed a slew of lawsuits against both Helios Airways and Boeing, and the airline was swiftly shut down by the Cyprus government in 2006.