The US aviation regulator has indicated that the Boeing 737 Max might return to service later than airlines had hoped.
US Federal Aviation Authority (FAA) acting director general Dan Elwell said if it took a year for the grounding order to be lifted “so be it”.
International aviation regulators are meeting on Thursday to discuss the 737 Max’s return to service.
The plane was grounded in March after two crashes in five months in which 346 people died.
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.
Mr 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.
“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”.
One decision the FAA has yet to make is whether or not to require pilots to undergo simulator training for the safety update.
Aviation regulators from 33 countries, including the UK, Europe and China, are meeting in Texas.
The meeting, led by the FAA, could set out a timetable for when the aircraft can return to service.
The regulator said it would provide its “safety analysis that will form the basis for our return to service decision process”.
The FAA also said it “will provide safety experts to answer any questions participants have related to their respective decisions to return the fleet to service”.
Boeing has developed a software update for the Manoeuvring Characteristics Augmentation System (Mcas) on the 737 Max – a new feature on the jet designed to improve the handling of the plane and to stop it pitching up at too high an angle.
Mcas has been linked to both the Ethiopian Airlines crash in March, which killed 157 passengers and crew, and the Lion Air disaster in Indonesia at the end of October, in which 189 people perished.
However, Boeing has not formally submitted the software fix to the FAA.
The FAA is expected to conduct a certification flight in the coming weeks.
The executive chairman of the International Pilot Training Association, Captain Tilmann Gabriel, told the BBC: “The FAA current acting director general has made it very clear that he is not committing to October, which was the real date [for the reintroduction], but there is so much to do.
“The credibility of the FAA and Boeing is at stake here. I’m convinced that there is a fix found, but this has to be now properly introduced.
It is also not clear when regulators outside the US in other countries will allow the plane back in the air.
China was the first country to ban the 737 Max from its skies following the Ethiopian Airlines crash. Other nations including the UK, Australia, New Zealand and the European Union soon followed suit.
The US was one of the last countries to ground the 737 Max in March following the Ethiopian Airlines crash. Southwest Airlines and American Airlines are the biggest operators of the 737 Max globally.
The FAA has said that the issue of whether to make pilot training on 737 Max simulators a requirement before the plane can return to service is “still under review”.