No timetable has been set for when the Boeing 737 Max will be allowed to return to service.
US Federal Aviation Authority acting director general Dan Elwell said if it took a year for the grounding order to be lifted “so be it”.
International aviation regulators from 33 countries met on Thursday in Texas to discuss the 737 Max’s return to service.
The new generation aircraft was grounded globally in March after two crashes in five months in which 346 people died in crashes in Indonesia and Ethiopia.
Ryanair chief executive Michael O’Leary said earlier this week that he expected the 737 Max to receive approval by late June or early July.
The FAA is waiting for Boeing to submit its final software fix to prevent future air disasters involving the aircraft’s manoeuvring characteristics augmentation system (MCAS).
Elwell was asked by reporters whether it was realistic that the 737 Max could be flying again by the summer.
“If you said October I wouldn’t even say that, only because we haven’t finished determining exactly what the training requirements will be,” he said.
“If it takes a year to find everything we need to give us the confidence to lift the [grounding] order so be it.”
He said discussions with Boeing over approving the safety update were “a constant give and take until it is exactly right. It’s taking as long as it takes to be right. I’m not tied to a timetable”.
“I’m not going down the timetable road,” he said.
But Elwell told CNBC television that airlines that have taken the grounded aircraft out of their schedules until August do not need to extend those flight cancellations, a signal that the FAA may approve the 737 Max to fly again as early as late June.
“It definitely could be a month, two months,” he told CNBC, adding “it’s all determined by what we find in our analysis of [Boeing’s] application, and we’re pretty confident that the application is in good shape”.
Once the order is lifted, it will take between 100-150 hours of preparation before the 737 Max will be ready for flying, according to officials for American Airlines, United Airlines and Southwest Airlines.
Boeing said: “We appreciate the FAA’s leadership in taking this important step in bringing global regulators together to share information and discuss the safe return to service of the 737 Max.
“Our team, our airline customers, and regulators place the highest priority on the safety of the flying public.
“Once we have addressed the information requests from the FAA, we will be ready to schedule a certification test flight and submit final certification documentation.”