The co-pilot of the Germanwings plane that crashed in the French Alps in March appears to have practised a rapid descent on a previous flight, a report by French investigators says, reports BBC News.
The report said Andreas Lubitz repeatedly set the same plane for an unauthorised descent earlier that day.
Lubitz is suspected of deliberately crashing the Airbus 320, killing all 150 people on board.
He had locked the flight captain out of the cockpit.
Lubitz appears to have practised programming a rapid descent on the outbound leg of the flight - from Duesseldorf to Barcelona on 24 March - the preliminary report by accident investigation agency BEA said.
It added that on several occasions - again with the captain out of the cockpit - the altitude dial was set to 100ft (30m), the lowest possible reading, despite instructions by air traffic control in Bordeaux to set it to 35,000ft and then 21,000ft.
It was also reset on one occasion to 49,000ft, the maximum altitude.
The changes apparently happened over a five-minute period at about 07:30 starting 30 seconds after the captain left the cockpit.
"I can't speculate on what was happening inside his head - all I can say is that he changed this button to the minimum setting of 100ft and he did it several times," BEA director Remi Jouty told Reuters news agency.
This conjures an image of the aircraft zig-zagging up and down while Lubitz pushed on a joystick, says BBC transport correspondent Richard Westcott.
But quickly turning a dial would not have led to dramatic changes on board, our correspondent adds, and the aircraft would have just kept descending as per instructions from air traffic control.
It was on the return leg on the same day - from Barcelona to Duesseldorf - that the fatal crash occurred.
Voice and data recorder findings suggest Lubitz locked the pilot out of the cockpit on the doomed flight before programming the plane to descend.
The co-pilot is known to have suffered depression in the past. Last month German prosecutors revealed that Lubitz had researched suicide methods and the security of cockpit doors.
The BEA's preliminary report also discloses more detail of what happened on board in the minutes before the crash.
The flight data recorder appears to show Lubitz increasing the aircraft's speed from 273 knots (505km/h, 314mph) to 345 knots on its descent.
On 14 occasions, air traffic control and French air defence tried to contact the plane.