Boeing doesn't expect federal regulators to approve its changes to the grounded 737 Max until this summer, according to two people familiar with the matter.

That timetable would be five or six months longer than Boeing predicted late last year, and is the latest of several delays in the plane's approval process.

It would also create more headaches for airlines by pushing the Max's return further into the peak summer travel season. Even after federal officials approve the plane for flight, airlines will need more time to train pilots on changes that Boeing is making after two crashes killed 346 people.

The latest timetable is based on work remaining to be done before the Federal Aviation Administration will allow the Max back in the sky, according to the two people familiar with the matter, who spoke Tuesday on condition of anonymity to discuss timing that Boeing has not publicly announced.