The co-pilot on Germanwings Flight 4525 tried a controlled descent on another flight that morning, French air accident investigators have said.
The BEA investigation agency said in a report released today that co-pilot Andreas Lubitz repeatedly set the plane into a descent, then brought it back up again on a flight on the same A320 jet from Dusseldorf to Barcelona.
The report said that the pilot appeared to have left the cockpit during that flight as well.
Prosecutors say Lubitz intentionally crashed the plane on its return flight from Barcelona to Dusseldorf on March 24, killing all 150 on board.
Cockpit data shows Lubitz put the earlier flight into descent mode five times in a four-and-a-half minute period.
The report only outlines interim findings, and BEA said it is continuing to look at the "systemic failings that may have led to this accident or similar events".
The investigators said their main focus is on "the current balance between medical confidentiality and flight safety" and the "compromises" made on security after the September 11 attacks in the US, notably on cockpit door locking systems.
Lufthansa spokesman Helmut Tolksdorf said the airline has not yet had time to analyse the new details released today and plans no immediate comment. Lufthansa is the parent company of Germanwings.
The report showed the pilot left the cockpit on the Dusseldorf-Barcelona flight for about four-and-a-half minutes.
Shortly after the pilot left, the "selected altitude" of the flight changed repeatedly, including several times as low as 100ft.
A chart released by the BEA showed the plane did not descend sharply during this period, suggesting that passengers and crew might not have noticed any change.
It would be highly unusual for a pilot to repeatedly set a plane for such a low altitude for no apparent reason.
The BEA report did not analyse why Lubitz repeatedly tried to descend the plane.
French prosecutors are conducting a separate criminal investigation into the crash.