Pilot's chilling final words before deliberately crashing packed airliner into cliff

The horrific tragedy might never have occurred had the aircraft's 34 year old Captain Patrick Sondenheimer not needed to use the lavatory during the journey. Following the 9/11 attacks in the United States, aviation security had been substantially enhanced, with one of the most significant changes being a dramatic strengthening of cockpit doors.

As Captain Sondenheimer prepared to exit the cabin, he instructed his co-pilot Lubitz to begin preparing for the descent briefing. Rather than a clear "yes," the younger man responded with three words that, in hindsight, appear deeply unsettling: "Hopefully, we'll see."

As he piloted the aircraft across the Alps alone, air traffic control directed Lubitz to flight level 350 or 35,000 ft. "He read this back, set flight level 350 on the autopilot's flight control unit, and then initiated the descent," Petter says.

Approximately 30 seconds later, air traffic control issued another instruction and Lubitz adjusted the autopilot settings once more, momentarily selecting the minimum 100ft before following the directive to proceed to 21,000 ft just moments before the captain entered the code to re-enter the cockpit.

Petter says: "Four and a half minutes had passed since he left. And during that time, an internal battle had likely been fought in the cockpit. A battle where sense had finally won, but not by a big margin. and a fight of which the captain, passengers, and cabin crew obviously had no idea."

Lubitz had been struggling with his own mental health difficulties for some considerable time, stemming from concerns that his eyesight was deteriorating. Anything short of perfect vision threatened his capability to fly commercial aircraft - which was not merely his livelihood, but his greatest passion.

A search of his flat following the catastrophe uncovered several shredded sick notes, including one for the day of the ill-fated flight.

Lubitz had previously been diagnosed with depression, but was subsequently given the all-clear to fly again. However, by the end of March 2015, his suicidal thoughts seemed to have taken a particularly dark turn.

Post-crash investigations into Lubitz's internet search history revealed that in the days leading up to the incident, he had researched the locking mechanism for the Airbus A320's bulletproof cockpit door.

The deeply disturbed co-pilot had evidently been plotting the disaster well in advance: "For this man to start planning a mass murder suicide shows that something darker was in play here," Petter says. "Maybe he was angry at his employer or the state for some reason. Or maybe he just wanted his name to be remembered. We will never know."

After acknowledging a message from air traffic control, Captain Sondenheimer once again exited the cockpit, setting Lubitz's grim plan into action.

Petter elaborates: "Only about 20 seconds after the captain had left, the selected altitude on the flight control unit changed again down to 100 ft. And immediately after that, the aircraft went from maintaining its cruise altitude to an autopilot mode known as open descent, in which the engines will reduce back to idle thrust and the autopilot will pitch down as much as needed in order to maintain the commanded speed.

He adds: "This meant that the aircraft immediately left flight level 380 and started descending towards its new commanded target.

"The captain would have definitely have noticed this from the toilet as the engine noise and angle of the aircraft would have shifted quite substantially. And my guess would have been that he likely thought that this was strange, but not outside of the norm since we sometimes do get asked to change our cruising altitudes in order to facilitate traffic flow."

However, when Captain Sondenheimer returned to the front of the aircraft and discovered the cockpit door was locked, his concerns will have escalated rapidly.

After hammering on the door with growing desperation, the captain could be heard on the cockpit voice recorder shouting: "For the love of God, open this door."

By this point, air traffic controllers on the ground had realised something was seriously wrong with Germanwings Flight 9525 and a French Mirage fighter jet was scrambled from the Orange-Caritat Air Base to intercept the airliner.

However, by the time the AdlA fighter was airborne, it was already too late. Despite Captain Sondenheimer apparently finding a heavy object to assist in his attempts to break down the door, his efforts proved futile.

"I cannot think of a worse situation to be in than this one," Petter says. "Both for the captain who was doing whatever he could to try and save this horrific situation and for all of the poor innocent passengers who by this time would have definitely realised that something was seriously wrong."

Despite urgent radio calls from ground controllers and nearby aircraft, Lubitz remained silent. Just after 9:40am, the Airbus's emergency warning system began to sound "terrain, terrain, pull up, pull up" but Lubitz disregarded it.

The cockpit voice recorder data, which has never been officially released, captured the screams of the doomed passengers as, just after 9:41, Germanwings Flight 9525 crashed into a cliff face at an altitude of around 5,000 feet.

Due to the extreme violence of the impact, French Minister of the Interior Bernard Cazeneuve stated there was "little hope" of finding any survivors. All 150 people on board, including Lubitz, were soon officially declared dead.