Boeing Co. doesn’t expect its 737 Max 10 model to be ready for certification until next summer at the soonest, US aviation regulators told a key lawmaker.

The planemaker’s latest plan was relayed in a letter Monday from Federal Aviation Administration acting Administrator Billy Nolen to Senator Roger Wicker, a Mississippi Republican. The comments provided the latest indication that the plane won’t meet a crucial deadline to finalize its approvals by the end of 2022.

Congress in 2020 had required that newly built airliners have more modern safety alerting systems in the wake of the twin fatal crashes on a similar plane, the Max 8, after assurances the last remaining 737 models would be cleared to fly by this year. But both the Max 7 and Max 10 models of the jet are in danger of missing the upcoming deadline, Nolen said. The jets face costly redesigns unless Congress acts to change the law.

“I support my team taking the time they need to fully understand the human factors assumptions” on the planes, Nolen said in the letter, which was reviewed by Bloomberg.

Boeing declined to comment on the letter, referring instead to a prior statement that it’s “focused on meeting all regulatory requirements to certify the 737-7 and 737-10, and safety remains the driving factor in this effort.”

The planemaker’s shares climbed 4% at 9:54 a.m. Tuesday in New York. The FAA letter was earlier reported by Reuters.