WASHINGTON, U.S. — According to a Federal Aviation Administration (FAA) letter received on Monday, Boeing does not expect to obtain certification for the 737 MAX 10 before the summer of 2023, which raises more questions about the company's delivery schedule.

December is the deadline for Boeing deadline to secure regulatory approval for the largest (MAX 10) and the smallest (MAX 7) variants of its 737 MAX family jets. But the U.S. regulators and lawmakers insist that Boeing must meet the new modern cockpit-alerting requirements for the certification. If the manufacturer cannot get an extension from Congress, it may delay the aircraft's entry into service.

"With regard to the 737-10, Boeing's current project plan timeline has the 737-10 receiving an amended type certificate no sooner than summer 2023," two sources quoted acting FAA Administrator Billy Nolen as saying in a letter to Senator Roger Wicker, the top Republican on the Senate Commerce Committee.

Wicker suggested that Boeing be given time until September 2024 to obtain clearance for the two new types last week. He intends to include the proposal in a yearly military bill. But it's unclear whether Congress would be open to approving the plan.

The new cockpit alerting requirements are part of certification reform legislation that was passed in 2020 after two 737 MAX crashes killed 346 people and led to a 20-month grounding for the best-selling plane.

The additional cockpit alerting criteria were included in certification reform legislation that was passed in 2020 as a result of two 737 MAX crashes that killed 346 people and caused the best-selling aircraft to be grounded for 20 months.

Boeing has recently booked major MAX 10 orders from Delta Air Lines Canada's WestJet Group and other carriers.