Crash of Germanwings flight 4U 9525: Timeline of Terror

It started off like any other flight - with a routine morning ascent over a calm Mediterranean Sea that gave no hint of the terror soon to engulf Germanwings flight 4U 9525.

Typical turned to tragedy when the Airbus A320 smashed into the French Alps. Days later, sorrow turned into anger and shock when French prosecutors said the cockpit voice recorder revealed that the co-pilot may have deliberately crashed the plane.

Here's a timeline of the final moments of the flight:

10.01 am (local time)

Flight 4U 9525, a 24-year-old aircraft, took off from Barcelona bound for Dusseldorf. First Officer Andreas Lubitz and his captain, who has not been named, were "friendly" and "spoke naturally", said French prosecutor Brice Robin, citing evidence from the plane's cockpit voice recorder.

"For the first 20 minutes, they spoke in a normal fashion, courteous, like normal pilots. There was nothing abnormal."

Then the captain discussed the landing with Lubitz, whose replies became short. "There is no real exchange," said Mr Robin.

10.27am:

The aircraft gently levelled off at its cruising altitude of 11,558m, giving the captain an opportunity to leave the cockpit, presumably to use the toilet. He was heard handing command of the aircraft to Lubitz and then the sounds of a seat backing up and the cockpit door opening and closing were heard.

10.29am:

As the plane reached the French coast, passing over the port town of Toulon, Lubitz initiated an unapproved, straight-line descent that was initially gentle and gradual, but turned rapid in the final moments. He appeared to have left the autopilot engaged, turning a small dial on the centre of the instrument panel to select a lower altitude while leaving the direction unchanged.

"This button is a button that turns many times to descend," Mr Robin said. "This action can only be deliberate...It would be impossible to turn the button by mistake. If you passed out and leaned over on it, it would only go a quarter-way and do nothing."

The captain then tried to get back into the cockpit. "We hear several calls from the pilot to access the cockpit," the prosecutor said.

But Lubitz never responded, even when the captain was heard knocking on the door.

If a pilot becomes incapacitated in the cockpit, a code entered on a keypad outside the cockpit will release the lock after a pre-programmed delay - but not if the double lock has been activated from within the cockpit. The double lock cannot be overridden from the passenger cabin.

"We are speaking of a deliberate action to refuse to open the door," Mr Robin said.

10.32 am:

During the final eight minutes before the flight crashed into the French Alps, Lubitz

was apparently calm and silent, breathing normally and showing no sign of panic.

"He did not say a single word. Total silence," Mr Robin said.

Air traffic controllers in Marseille noticed the plane's rapid descent, which was well below its planned cruising height, and tried to contact the plane. But there was no response.

The controllers then sent an electronic "squawk" - code 6700 - to the plane, which gives pilots who have lost radio contact the chance to make an emergency landing without coming too close to other planes. Again, there was no response.

As the plane continued to descend, an aural altitude warning sounded. That would have alerted the rest of the crew that they were now dangerously close to the Alps.

The next sound on the cockpit voice recorder was of someone trying once more to forcefully open the flight deck door but failing to succeed.

10.40am:

In the final moments before impact, the cries of passengers could be heard on the flight recorder. Then came the impact. "I think the victims were only aware at the very last moment. The screams are heard only in the last instant before the impact," said Mr Robin.

SOURCE: NBC NEWS, AGENCE FRANCE-PRESSE