Guenter Lubitz said he believed his son was innocent.
Families of victims of the Germanwings airline crash have criticised the father of the co-pilot for claiming on the second anniversary of the crash that his son was not responsible.
German prosecutors concluded in January that Andreas Lubitz, 27, was suicidal and flew the plane into the ground, killing all 150 people on board.
But Guenter Lubitz said on Friday his son was not depressed at the time.
Family members criticised the timing of his announcement.
His news conference fell on the same day that relatives are commemorating two years since the death of their loved ones.
Mr Lubitz’s statement is the first public statement by a member of Lubitz’s close family since the 2015 disaster.
He said: “Up to now, everyone has believed the theory of a co-pilot who was depressed for a long time, who deliberately crashed his plane into a mountain in a planned act. We are convinced this is false.”
Mr Lubitz, 63, said he has been working alongside a journalist, Tim van Beveren, who he called “an internationally recognised aerospace expert”.
The two men plan to introduce a theory that a carbon monoxide leak in the cabin disabled Lubitz while the captain was locked out of the cockpit.
The cockpit voice recorder aboard the aircraft captured the sound of the captain attempting to break through the door with an axe as the aircraft plummeted to earth.
The crash killed all 150 people on board.
Elmar Giemulla, a lawyer for several of the victims’ families, said Mr Lubitz’s actions were “irresponsible”.
“I imagine that Mr Lubitz wants to promote a theory that would absolve his son of any responsibility,” he told Germany’s Rheinische Post newspaper.
Mr Lubitz told Die Zeit magazine: “Our son was a very responsible person. He had no reason to plan and carry out a suicide, and certainly not to take another 149 innocent people with him.”
About 500 people are expected to travel to the French Alpine town of Digne-les-Bains for a church service to commemorate the victims.