'Freakiest flight in history' and final words of attendant who tried to fly plane

The history of aviation has seen many strange and shocking plane crashes but there are few that are as mysterious as the fate of Helios Airways Flight 522.

It became known as the "Greek Ghost Flight" when it crashed into hilly terrain near Grammatiko, Greece - after passengers and pilots fell unconscious.

Later in the investigation, experts discovered one simple mistake resulted in the crash. It led to a manslaughter verdict and a 10-year prison sentence for Helios engineer Alan Irvin. British born Irvin was later exonerated and his name cleared.

Air Aviation expert David Lermount investigated and reported on air crashes for over four decades. He told The Mirror: "It was an appalling decision, because in the end, It's one of the checks the pilots should have done."

David a consulting editor on Flightglobal magazine added: "The whole aviation system, is based on the idea that one single slip should not cause a disaster, because there are a whole load of people who check these things and if they don't, the whole system fails. I think it was sheer carelessness from the pilots. And it led to the most freakiest air accidents I can ever remember."

Twenty years ago on Thursday August 14 2005 at 9.07am, Flight 522 took off from Larnaca Airport for Athens.

The flight time was one hour and 45 minutes, but more than two hours since takeoff and the plane was still in the air, with 121 people on board.

It eventually crashed into a rural hillside after the plane ran out of fuel - killing everyone on board on impact.

So what happened?

Helios was a charter airline with low cost fares to Greece. It was the height of summer and the plane is filled with families.

Flight 522 was piloted by Hans-Jürgen Merten a German contract pilot hired by Helios for the busy holiday season. His copilot, Pampos Charalambous from Cyprus, worked exclusively for Helios for the last five years.

The take off was normal but minutes into the flight, while the plane was still climbing to its cruising altitude, an alarm blared in the cockpit. The takeoff configuration alarm normally only sounds on the ground.

The captain radioed the Helios Operations Centre at Larnaca airport in Cyprus about the takeoff configuration alarm and then another alarm the Master Caution alarm - which means the system is over heating sounded while he was speaking to the engineer Alan Irwin.

Meanwhile, oxygen masks dropped down for surprised passengers - unbeknown to the pilots who were trying to work out what was wrong with ground control.

Since the crew didn't think they were having cabin pressure problems, they focused on the plane's cooling systems.

Irwin, then 44, was recorded as asking the captain to confirm that the pressurisation panel was selected to auto. The captain did not reply.

It is believed he may have been feeling the effects of hypoxia but if he had answered Irvin’s question, the mistake would have been identified and disaster avoided.

Instead, the captain got up from his seat to look for the plane’s cooling circuit breakers - to fix the issue of overheating - which he wrongly believed was the problem.

Within minutes, radio contact with the pilots was lost. The worry became the plane had been hijacked.

The Greek Air Force deployed fighter jets to investigate the Helios plane.

They were horrified to see no sign of the captain and the co pilot slumped in his seat. There was no movement from the passengers but then one of the jet pilots spotted a figure walking into the cockpit and sitting in the captains’s seat .

Suddenly, the plane made a sharp left and began dropping altitude fast. Just after 12, almost three hours after it took off from the island Helios Flight 522 slammed into the ground.

The chief coroner visited the crash site where pieces of the plane were spread amongst the charred remains of victims and later described as "the worst experience of his life".

Most of the bodies were burned beyond recognition, but autopsies showed that they were all alive but .unconscious at the time of impact. This included the captain who investigators believe collapsed due to lack of oxygen -after leaving his seat.

Investigators discovered other tissue samples found in the remains of the cockpit which belonged to Helios flight attendant Andreas Prodromou. Andreas had his commercial pilot's license and was hoping to becoming a captain for Helios.

He had been the figure the jet pilot had seen sitting down in the captain's seat.

But all the training in the world wouldn't have helped him land the plane safely. The plane veered to the left because shortly after they saw Andreas enter the cockpit, the left engine had run out of fuel.

The cockpit voice recording was recovered and Andreas was heard breathlessly repeating Mayday - but this was never heard by anyone on the ground - probably because the radio was still tuned to Larnaca, the airport on Cyprus where the flight had taken off.

Each passenger had enough oxygen for 12 minutes at high altitudes. In this time pilots are able to get the plane down to a low altitude. But Helios 522 had no pilots at the helm.

Oxygen masks do not automatically release into the cockpit. So the pilots passed out of hypoxia before the passengers. The autopilot would have put the jet in a holding pattern - with noone flying the plane.

There were four portable oxygen bottles on aboard the 737 which could each last an hour. All four bottles were found at the crash site and three of them appeared to have been used.

This is how investigators believe Andreas survived the hypoxia long enough to attempt to fly the plane.

As autopsies showed, everyone on board apart from Andreas had passed out. Although they had a heartbeat at time of impact, they were mostly likely in an irreversible coma due to the lack of oxygen to the brain.

The reason was sadly due to one flick of a switch.

The crash investigators discovered that the pressurisation selector switch had been left in “manual” rather than “auto” after a pre-flight safety check.

This was attributed to human error and British born Alan Irwin who was the engineer who had done the check was held largely responsible. The final investigation findings said he did not return the switch to its correct position after his check. The final report also blamed the pilots who had failed to spot the error.

Boeing, which made the plane, released a statement during the crash inquiry: “Helios’s ground engineers did not follow Boeing’s correct procedure. At least 16 separate mistakes were made by the ground staff, the flight deck crew and the passenger cabin crew. If any one of these mistakes had not been made, the accident would not have happened.”

The accident investigation report, published in 2006, concluded that the crash had three direct causes: the switch being in manual; the failure to identify the cabin warning; and the crew being incapacitated by hypoxia.

Air safety expert David Learmount added: "Many things played a part in the crash but ultimately by doing their checks properly, the pilots could have and should have prevented this accident."

In 2007, the families of eight Cypriots who died in the crash filed for €76m (£69m) in compensation from Boeing, for having “the same alarm in place for two different types of dysfunction”, according to their lawyer, Constantinos Drougas. The case was settled out of court and the families were paid an undisclosed sum.

Irwin, along with other Helios workers, was charged with manslaughter, found guilty, and sentenced to 10 years' imprisonment and remained free on bail pending an appeal.

By 2013, Irwin succeeded in his appeal. The other defendants lost their bids for exoneration.