Boeing expects its 737 Max to begin flying again in January, a month later than it had originally hoped.
But the planemaker said that it was possible that Max deliveries to airlines could resume in December.
The group is still hoping for certification from the Federal Aviation Administration (FAA) within this quarter, which would allow it to resume deliveries by the end of the year.
In a statement Boeing said: “In parallel, we are working towards final validation of the updated training requirements, which must occur before the Max returns to commercial service, and which we now expect to begin in January.”
Boeing’s 737 Max has been grounded since March after two separate crashes months apart killed 346 people.
The group said it is working with the FAA and other regulatory authorities.
It said: “While the FAA and other regulatory authorities will determine the timing of certification and return to commercial service, Boeing continues to target FAA certification of the MAX flight control software updates during this quarter.”
Boeing needs to complete a number of certifications before the Max is deemed safe to fly. The company received certifications for three of the five last week and is now working towards the FAA line pilots evaluation and the FAA certification flight test.