Carsten Spohr, chief executive of the German carrier, said he was "stunned" by findings that 28-year-old Andreas Lubitz appeared to have deliberately slammed the Airbus A320 into the French Alps.
All 150 people on board were killed in Tuesday's crash.
He told a news conference shortly after French officials gave an account of the flight's last minutes based on initial information gleaned from cockpit recordings that there was "no indication what might have led" to the young co-pilot's actions.
Cockpit employees are selected "very, very carefully" with much attention paid to their "psychological suitability", the Lufthansa chief, himself a former pilot, added.
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Lubitz began his training in 2008 and six years ago had taken a break during the programme before resuming again. He had passed his medical, flying and other tests and checks.
A Lufthansa spokeswoman said he had 630 hours of flight experience.
"He was 100 per cent airworthy, without reservation," Spohr said.
He added that no security "system in the world" could protect an airline from such a disaster.