Nearly twenty years on from the deadliest aviation accident in Greek history, the series of fatal mistakes which led to a crash that killed everyone on board are still no easier to come to terms with.

The 115 passengers and six crew members onboard Helios Airways Flight 522 were tragically killed on 14 August, 2005, after the plane crashed into a hillside near the village of Grammatiko before bursting into a fireball.

Only one person remained completely conscious throughout the ordeal, while holidaymakers and airline staff were knocked out as the ‘ghost plane’ was stuck in a loop at 35,000ft in the air.

The seemingly routine one-hour-and-45-minute flight on the Boeing 737-300, nicknamed ‘Olympia’, was flying from from Larnaca in Cyprus, to Prague, when it turned into one of the most bizarre aviation disasters the world has seen.

After air traffic control were unable to get in touch with Flight 522 for more than an hour after take off, the Greek military decided to send two F16 fighter jets up to determine what was going on.

Chillingly, they noticed the first officer slumped in his seat at the controls, while the captain’s seat was empty – and inside the cabin, passengers were unconscious, despite oxygen masks dangling down.

Those onboard Helios Airways Flight 522 met a grisly end (Milos Bicanski/Getty Images)

The fighter jet pilots helplessly watched on as the Helios Airways jet flew aimlessly around in the sky, well aware of the inevitable disaster which awaited when it finally came to a stop one way or another.

And it was all down to a single switch which set off a series of catastrophic events, an investigation by Greece’s Air Accident Investigation and Aviation Safety Board (AAIASB) found.

Captain Hans-Jürgen Merten, 58, was sat in the cockpit with first officer Pampos Charalambous, 51, and the duo boasted thousands of hours of flying as well as decades of experience between them.

But for some reason, the accomplished pair decided to ignore the cabin altitude warning horn which sounded as they reached an altitude of 12,040ft, which should have encouraged them to stop their ascent.

It is thought they may have believed it was instead the take-off configuration warning, which indicates that the aircraft is not ready for take-off, as both sounds are identical.

Several warning lights then lit up in the cockpit over the next few minutes – but again, the pilots did not click on.

The plane continued its climb, while the crew were completely unaware that the aircraft was experiencing a gradual loss of cabin pressure, which caused oxygen levels onboard to plummet – triggering the masks in the passenger cabin to deploy.

The Boeing 737-300 burst into flames after smashing into a Greek hillside (LOUISA GOULIAMAKI/AFP via Getty Images)

Captain Merten contacted the Helios operations centre to report that ‘the take-off configuration warning was on’, as well as that the ‘cooling equipment was normal and alternate off line’ and the ‘cooling ventilation fan lights were off’.

He was asked to confirm that the cabin pressurisation system was set to auto rather than manual, but he disregarded the question and continued to discuss the issues – as he was already beginning to experience hypoxia.

This term refers to people experiencing low levels of oxygen in their body tissues which causes confusion, restlessness, difficulty breathing, rapid heart rate bluish skin, and loss of consciousness.

Soon, the pilots and everyone on board were completely conked out, and Flight 522 remained soaring in its holding pattern in autopilot for a number of hours.

The fighter jets were sent up, and their only hope of avoiding an even larger tragedy was flight attendant Andreas Prodromou, who had managed to stay conscious thanks to a portable oxygen supply.

He entered the cockpit and plonked himself in the captain’s seat in a desperate attempt to regain control of the plane.

Prodromou held a UK Commercial Pilot Licence, but was not qualified to fly the Boeing 737, and despite his best efforts, his fate was sealed – he waved at the fighter jets briefly, before the left engine faltered and the plane began to drop.

121 people died in the tragedy (Milos Bicanski/Getty Images)

Even though he would have been aware he wouldn’t make it out alive, the 25-year-old made the selfless decision to try and divert the plane away from Athens to avoid any more casualties and instead directed it towards a rural area.

Just 10 minutes after the left engine failed, the right engine also flamed out, and the Helios Airways aircraft plummeted into a hillside situated about 25 miles from Athens.

It burst into flames upon impact, meaning the large majority of bodies were burned beyond recognition, with autopsies later revealing that those onboard were likely still alive at the time of the crash.

An investigation by Greek authorities and Helios Airways ensued, which found that a switch was responsible for the chaos which erupted in the skies, shocking Europe.

When the plane had arrived at Larnaca from London earlier that day, the original flight crew had complained about a frozen door and hearing strange noises emerging from the service door.

They requested a full inspection of it, seeing a ground engineer conduct a pressurisation leak check, and to do this without the engines being on, he had to set the pressurisation system to ‘manual’.

But he never switched it back to ‘auto’ after carrying out the test – so as the plane climbed up through the clouds, it depressurised without any crew members realising what was going on.

As well as this, several incidents of issues with cabin pressure had been recorded onboard a number of prior flights.

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

Featured Image Credit: Milos Bicanski/Getty Images/LOUISA GOULIAMAKI/AFP via Getty Images