Boeing officials revealed they are unsure if software changes requested by government regulators for the grounded 737 MAX fleet will further delay the return of the planes, Donald Wood of Travel Pulse reported.

According to the Seattle Times, Boeing released a statement saying it was in the final stages of the necessary software upgrades to its 737 MAX aircraft, but the Federal Aviation Administration has not completed its audit of the second flight-control computer used on each plane.

One of the issues raised by FAA regulators is that Boeing presented the MCAS documentation in a similar format as it had been in the past, but officials wanted it in a different form. The plane manufacturer is currently making the necessary changes.

The issues center around the MCAS flight-control system on the MAX fleet, which has been blamed for the two crashes that left 346 people dead. Despite the requested changes, Boeing is still looking for FAA approval before the end of the year.

Boeing may already be reworking its software for FAA regulators, but the European Union Aviation Safety Agency said Monday it believes the grounded MAX fleet will return to service during the first quarter of 2020.

Air Canada officials also believe the return of the MAX could stretch well into 2020, following approval by Transport Canada.

According to the Seattle Times, Boeing expects the FAA to clear the 737 MAX as safe to fly again in late December, but the FAA likely won’t finalize a new MAX pilot training regimen until January.

That means Boeing may be able to resume deliveries of the jet to airlines before year end, but that the airplanes won’t be cleared to carry passengers until as much as a month later, the report said.

The airlines need at least a further month to get the planes out of mothballs and their pilots trained. This explains why both American and Southwest on Friday took the MAX out of their flight schedules until early March.

Boeing also laid out the specific milestones that must be completed ahead of the MAX’s return to service, and said the first was completed last week. That clear view of the limited steps remaining boosted Boeing stock to close at US$366.96, up US$16.41 or nearly 4.7%.

Once the US airlines have full clearance to fly the MAX, they will need 30 to 40 days to get the planes ready and to put thousands of pilots through the training. They will then introduce the MAX only slowly into their schedules, the report said.

While it’s possible that the MAX’s return to service could slip further, omitting the MAX from flight schedules until March will provide certainty for passengers booking now for the early part of the busy spring break period.