Major rule was introduced after pilot deliberately crashed plane with 150 onboard and it’s still used today

It's been 10 years since all 150 people on board Germanwings flight 9525 were killed after the co-pilot crashed the plane into the French Alps, leading to new safety rules being introduced.

What ought to have been a standard flight from Barcelona to Düsseldorf on 24 March, 2015, ended in tragedy when co-pilot Andreas Lubitz waited until the plane's captain stepped out of the cockpit, before locking everyone else out and directing the plane towards a mountain.

Flight 9525 - the subject of Sky documentary Germanwings: What Happened on Flight 9525? which aired on Wednesday (7 May) - hurtled into a mountain at 430mph, with investigators ruling that it was a deliberate crash caused by the co-pilot.

Around half an hour into the flight Captain Patrick Sondheimer left the cockpit to use the toilet, and it was at this point Lubitz locked him and everyone else out.

The plane's black box recorded Lubitz telling the captain he could go for a break 'any time' and saying 'you can go now'.

The co-pilot deliberately crashing the plane after making sure he was alone in the cockpit resulted in some changes (ANNE-CHRISTINE POUJOULAT/AFP via Getty Images)

As the captain left the cockpit he told the co-pilot he was in charge of radio communications and the door could be heard closing.

Lubitz then changed the plane's altitude and ignored communication attempts, with the recording showing that the co-pilot remained silent for the remainder of the time while Sondheimer could be heard banging on the door as he tried to get back in and stop the plane from crashing.

The captain could be heard shouting 'open the damn door', while in the final moments before Lubitz crashed the plane the passengers could be heard screaming.

Lubitz had been diagnosed with severe depression by a doctor and ruled unfit to fly, but he'd kept it hidden so he could still be in the cockpit.

How did Flight 9525 affect aviation safety?

The disaster led to a new rule being introduced by Lufthansa, the parent company of Germanwings, which required at least two crew members to be in the cockpit of the plane at all times.

A memorial erected at the site of the crash which contains 149 elements. One for each person killed in the crash, apart from Lubitz (Denis Thaust/SOPA Images/LightRocket via Getty Images)

A number of other airlines decided to follow suit and adopt this rule to ensure that no plane could have someone in the cockpit by themselves.

Under the guidelines, a member of cabin crew has to be in the cockpit with the pilot if the second pilot needs to leave for a bathroom break.

Earlier this year, a challenge to the two pilot rule met some resistance, as Politico reported that new technologies could see a return to one pilot being allowed to be in the cockpit by themselves.

The EU Aviation Safety Agency (EASA) cooled its approval to single pilot operations, which doesn't mean the plane flying with only one pilot but instead means situations where there is only one person in the cockpit would be permitted again.

Single pilot operations are also referred to as extended minimum crew operations (eMCOs), which might be tempting to airlines for budgetary reasons.

Under current EASA rules, flights that last longer than a pilot's shift need to have four pilots on board.

A study into 'new advanced flight deck technologies' and 'smart cockpits' is being set up to look into the matter, but pilots themselves are concerned about the safety risks of reducing the amount of them on a flight.