The US aviation regulator has issued a list of changes still required to allow the Boeing 737 Max to return to service.
The Federal Aviation Administration document details actions it wants to be made before the new generation aircraft can fly again commercially.
The 737 Max has been grounded since March 2019 following two fatal crashes in Indonesia and Ethiopia which killed 346 people.
Boeing hopes to restart 737 Max services early next year after the changes are made.
Proposed changes include updating flight control software, revising crew procedures and rerouting internal wiring.
In a related report also published on Monday, the FAA said that Boeing’s own recommendations had sufficiently addressed the problems that had contributed to the two fatal crashes.
Once the proposals become official, Boeing can then make the changes and ready the aircraft for flight.
The design updates will need to be made to all 737 Max aircraft delivered to airlines along with those not yet ordered or built.
The FAA will not speculate when the work will be completed.
“The agency continues to follow a deliberate process and will take the time it needs to thoroughly review Boeing’s work,” it said.
“We will lift the grounding order only after FAA safety experts are satisfied that the aircraft meets certification standards.”
A Boeing spokesman told the BBC: “We’re continuing to make steady progress towards the safe return to service, working closely with the FAA and other global regulators.
“While we still have a lot of work in front of us, this is an important milestone in the certification process.”
Boeing is expected to carry out 737 Max simulator pilot training at Gatwick.
Airlines affected by the grounding of the 737 Max include Tui and Norwegian Air while Ryanair has faced delays in delivery of the aircraft.
British Airways parent company IAG signed a letter of intent to buy 200 737 Max aircraft last year.