Boeing has admitted it had to correct flaws in its 737 MAX flight simulator software.
In a statement, it said: "Boeing has made corrections to the 737 MAX simulator software and has provided additional information to device operators to ensure that the simulator experience is representative across different flight conditions."
It, however, did not indicate when it first became aware of the problem, and whether it informed regulators.
"Boeing is working closely with the device manufacturers and regulators on these changes and improvements, and to ensure that customer training is not disrupted," it added.
The planes were grounded worldwide after two crashes, which killed 346 people, was blamed on a defected anti-stall system.

US House rejects war powers resolution, backs Trump on Iran war
GCC and EU ministers urge immediate halt to Iranian attacks
Trump wants say on Iran's next leader
British PM Starmer to send four Typhoon jets to Qatar
Bombing of Tehran intensifies as war enters day six
Azerbaijan vows to respond after four injured by Iranian drones
72 killed in Israeli attacks on Lebanon as it warns residents to leave south
Nepal goes to the polls; voters seek change after youth-led protests
