The return of Boeing’s 737 Max has been delayed again. On Tuesday, the company said it did not expect regulators to approve the jet to fly again until the middle of the year. American Airlines, United and Southwest had already taken Max flights off their schedules until June, and if this new timeline holds, it would further push back when the plane will be available for commercial flights.

The announcement reflects Boeing’s new appreciation for the challenges facing the company in its effort to persuade regulators that the Max is ready to fly. Boeing and the Federal Aviation Administration have continued to find new flaws with the Max that go beyond an automated software system known as MCAS. The software contributed to two accidents, in late 2018 and early 2019, that killed 346 people and led to the worldwide grounding of the Max in March.

But the new estimate is conservative and intended in part to give Boeing some leeway with airline customers, who became frustrated over the past 10 months after the company repeatedly missed its own optimistic deadlines for the plane’s return to service, according to three people familiar with the matter. If regulators find no new problems with the plane, they could lift the grounding by the spring, the people said.

Once the plane is cleared to fly, it will take airlines weeks to reintegrate them into their commercial schedules. The three United States carriers that fly the Max — American, United and Southwest — had already pushed back Max flights until June. They did not immediately change those schedules in response to Tuesday’s announcement.