Germanwings Crash Pilot May Have Attempted a Trial Run During His Previous Flight

A preliminary report released by the French prosecutors examining the Germanwings crash has revealed that co-pilot Andreas Lubitz apparently practised a rapid descent on his previous flight.

The report was released on Wednesday by the French Civil Aviation Safety Investigation Authority (BEA) and details the technical information discovered through an analysis of the downed plane's black boxes.

It shows that during the flight immediately before the one that crashed — on a route between Düsseldorf and Barcelona — the pilot left the cockpit for 4 minutes and 30 seconds towards the end of the flight, after the plane had entered its final descent, travelling at 37,000ft. While he was gone Lubitz altered the selected altitude to 100ft several times, each time quickly adjusting it back to the correct altitude before the plane's course was altered. The correct altitude setting was restored before the captain re-entered the cockpit.

Related: Germanwings Co-Pilot Appears to Have Deliberately Crashed Plane, According to French Prosecutor.

His actions could indicate the acting out of a chilling practice run for his next flight, which he crashed into a mountainside, killing everyone on board. They could also suggest he had planned to bring down the Düsseldorf-Barcelona flight, but could not go through with it at the crucial moment.

On the following flight, the selected altitude was changed from 38,000ft to 100ft while the co-pilot was alone in the cockpit. The aeroplane then began a continuous and controlled descent.

Germanwings flight 4U9525 crashed into the French Alps on March 24, with 144 passengers and six staff on board. It later emerged that the plane was deliberately crashed by co-pilot Lubitz, a 27-year-old German citizen.

Speaking at a press conference in March, French prosecutor Brice Robin talked through the events that led to the tragedy. After taking off from Barcelona, the two pilots operating the Airbus A320 had a "cheerful and polite" conversation, according to Robin. Some 20 minutes after take-off the captain gave instructions to Lubitz about landing in Düsseldorf, to which the copilots' answers were "laconic."

Robin then explained that the captain had to leave the cockpit to go to the bathroom. During that time, Lubitz initiated the plane's descent by "pushing the flight monitoring system's buttons." When the captain returned he was locked out of the cockpit.

"The most plausible interpretation is that the co-pilot, through a deliberate omission, refused to open the door of the cockpit to the captain and activated the loss of altitude button for a reason that we are totally unaware of, but that can be interpreted as an intention to destroy this aircraft," Robin said.

Related: Pilot Accused of Deliberately Crashing Germanwings Plane "Hid an Illness from his Employers."

The BEA preliminary report comes as part of an ongoing safety investigation. It began the same day as the disaster, when a team of seven BEA investigators traveled to the crash site on March 24.

According to the preliminary report, this investigation will focus on two key aspects. Firstly, the "current balance between medical confidentiality and flight safety." The second focus will be cockpit security, including a focus on "cockpit door locking systems and cockpit access and exit procedures."

Other restated findings in the latest report include that Lubitz had suffered an episode of depression, which delayed the renewal of his copilot's class 1 medical certificate between April and July 2009.

The report also said the Marseille control center had called the flight crew on 11 occasions and on three different frequencies during the descent period, without receiving an answer. Another three contact attempts came from the French military defense system, which again received no reply.

The captain sounded the cockpit's buzzer in an attempt to regain access 4 minutes and 7 seconds after he had left. He then attempted to use the interphone to plead to be allowed in, though no answer was elicited.

Again, the preliminary report stated that the sound of Lubitz's breathing could be heard on one of the retrieved black boxes, and that it only disappeared a few seconds before the flight hit the Alps.

Related: German Plane Crashes in French Alps with 150 People on Board.

Follow Sally Hayden on Twitter: @sallyhayd