Why pilot flew plane into side of mountain killing all 150 onboard as documentary airs

A new documentary has aired 10 years after a pilot deliberately crashed Germanwings Flight 4U 9525 into a mountain, killing all 150 on board.

The budget airline, once owned by Lufthansa, had one of its planes fly out from Barcelona, Spain to Düsseldorf, Germany on 24 March, 2015.

Despite taking off as normal, about 30 minutes into the flight, the plane started to rapidly descend and everyone on board was killed 10 minutes later.

There were no survivors as the plane crashed into the French Alps near Prads-Haute-Bléone.

There were no survivors onboard Germanwings Flight 4U 9525, which crashed into the French Alps near Prads-Haute-Bléone (PATRIK STOLLARZ/AFP via Getty Images)

How did the Germanwings crash happen?

Co-pilot Andreas Lubitz was alone in the cockpit due to Captain Patrick Sondenheimer briefly exiting to use the toilet. When the plane started to descend, he locked the cockpit door from the inside and manually set the plane to go into the mountains.

Crew members tried to break down the door when they realised what was happening, but sadly they were unable to get through.

This triggered a change in aviation safety rules, in which Lufthansa now requires at least two crew members to be in the cockpit of the plane at all times.

A year later, the BEA released a report on the incident and it was determined that Lubitz, 27, brought down the Airbus deliberately.

The BBC reports that Lubitz - who was from Montabaur, Germany - graduated from high school in 2007 and became a Lufthansa trainee the following year.

Germanwings: What Happened on Flight 9525? is the new documentary which takes a look at the tragic incident 10 years on (Francis Pellier MI DICOM/Ministere de l'Interieur/Getty Images)

Andreas Lubitz's struggles with mental health

Lufthansa CEO Carsten Spohr previously revealed that Lubitz took a break lasting several months prior to the crash. But BEA's final report found that the interruption was caused by depression, as he received treatment for a year and a half.

In that period, he considered suicide but was declared healthy, according to the outlet.

2013 was the year he joined Germanwings, first working as a flight attendant before becoming a co-pilot.

Prior to the crash, Lubitz flew for 630 hours and was '100 percent fit to fly without any restrictions or condition', Spohr claimed.

However, the crash report found: "On the day of the accident, the pilot was still suffering from a psychiatric disorder, which was possibly a psychotic depressive episode and was taking psychotropic medication. This made him unfit to fly."

Co-pilot Andreas Lubitz brought the plane down 'deliberately' (Getty Images)

Lubitz's chilling warning before the devastating crash

A former girlfriend of Lubitz told a German newspaper what he once said to her: "One day I will do something that will change the whole system, and then all will know my name and remember it."

"I never knew what he meant, but now it makes sense," she said.

10 years on from the incident, Sky Documentaries have released a new film titled Germanwings: What Happened on Flight 9525?, which includes interviews from 'experts and journalists'.

It is available to watch now on Sky Go.