The co-pilot who crashed his Germanwings plane into the Alps on March 24, killing all 150 on board, may have 'practised' the descent hours earlier on a previous flight, while the captain was out of the cockpit.

On Wednesday the French aviation safety authority BEA (Bureau d'Enquêtes et d'Analyses) published a preliminary report on its investigation of the crash of the A320 aircraft.

All 144 passengers and six crew members died in the crash on March 24.

The investigators found that co-pilot Andreas Lubitz rehearsed the descent on a flight from Dusseldorf to Barcelona on the same plane, earlier the same day, by repeatedly re-programming the autopilot to descend to an altitude of just 100 feet above ground.

He put the autopilot altitude back to its proper setting just seconds before the captain re-entered the cockpit.

The investigation revealed that, on the previous flight:

• about 7.20am there was the sound of the cockpit door opening and closing, corresponding to the captain leaving the cockpit while the plane was at cruise speed at 37,000 feet.
• 30 seconds later the flight was transferred to the Bordeaux control centre and the co-pilot was instructed to descend to 35,000 feet.
• Seconds later, the co-pilot put the plane into a descent.
• Over the next couple of minutes, the co-pilot changed the 'selected altitude' for the plane several times, and for around a minute and a half the selected altitude was just 100 feet.
• At 7.24am there was a buzzer sound as the captain requested to re-enter the cockpit. Some 14 seconds later there was the sound of the cockpit door being unlocked and the captain returned to the cockpit.

The 30-page report did not make any other dramatic new findings – it concluded that when the plane was cruising en route from Barcelona to Dusseldorf, co-pilot Andreas Lubitz was alone in the cockpit.

"He then intentionally modified the autopilot instructions to order the aeroplane to descend until it collided with the terrain," the report said. "He did not open the cockpit door during the descent, despite requests for access made via the keypad, the cabin interphone and knocks on the door."

On the fatal flight, the co-pilot again waited until the captain was out of the cockpit.

He then reset the autopilot's "selected altitude" to 100 feet, just as he had on the previous flight.

He ignored requests from the captain to re-enter the cockpit, and later knocking, then "violent blows" on the cockpit door.

Late in the descent, just a minute before the crash the co-pilot made slight adjustments to the flight path on his control stick, the investigators found.

The investigation found that 27-year old Lubitz, whom it did not name, had suspended his pilot's training for nine months in 2008/9 for medical reasons.

During his training his professional level was judged "above standard" by instructors and examiners.

In April 2009 his medical certificate was not revalidated by his employer, Lufthansa, due to "depression and the taking of medication to treat it". In July that year his request for a renewal of his medical certificate was refused.

However he got a new medical certificate in late July 2009, under the condition he undergo regular medical examinations.

Investigators found no significant mechanical problems with the plane, and no weather problems. They concluded that all of the plane's occupants died immediately in the crash.

Australia, Israel and Japan have appointed experts to follow BEA's safety investigation, since some of the victims came from these countries.

Investigators said they would continue to study the "systemic failings that may have led to this accident or similar events".

They would look at the "current balance between medical confidentiality and flight safety", aiming to explain how pilots can be in a cockpit with the intention of crashing a plane, despite the existence of regulations on mandatory medical criteria for flight crews.

They will also look at recruitment policies and training.

And they will investigate what they called "compromises" between cockpit security and flight safety, with a focus on cockpit door locking systems and access.