In the fatal crash, Andreas Lubitz, 27, locked his captain out of the cockpit and put the Airbus A320 into a continual descent.
The plane, which was en route to Dusseldorf from Barcelona, crashed into a mountain killing all 150 people on board.
In an interim report yesterday, the French air accident bureau, BEA, said Lubitz repeatedly set the Airbus into a descent then brought it back up again on an earlier flight on the same A320 jet from Dusseldorf to Barcelona on the morning of 24 March.
The report said the other pilot had appeared to have left the cockpit during that earlier flight as well.
Investigators said cockpit data showed Lubitz put the earlier flight into descent mode five times in a four-and-a-half-minute period.
The BEA said it is continuing to look at the “systemic failings that may have led to this accident or similar events”.
BEA director Remy Jouty said: “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.”
Cockpit door security was strengthened on passenger planes after the 9/11 attacks in the US.
Voice recorder findings suggest Lubitz locked the pilot out of the cockpit on the doomed flight.
Evidence has shown that the captain tried to break down the cockpit door.
There have been various reports about Lubitz’s mental state and his fitness to fly.
Yesterday, the BEA said its main focus was on “the current balance between medical confidentiality and flight safety” and the “compromises” made on security after 9/11, notably on cockpit-door locking systems.
Last month, German prosecutors revealed that Lubitz had researched suicide methods and the security of cockpit doors.
The BEA is expected to release its final report in a year.
Three Britons died in the disaster. One was Paul Bramley, 28, who was originally from Hull. He was studying hospitality and hotel management at Cesar Ritz College in Lucerne and was about to start an internship.
Another of the Britons who died was father-of-two Martyn Matthews, 50, a senior quality manager from Wolverhampton.
Also killed was seven-month-old Julian Pracz-Bandres, from Manchester, who died alongside his mother, Spanish-born Marina Bandres Lopez-Belio.