US air safety regulators have notified Boeing that the aviation giant's newest widebody plane will require more than two years of additional testing and analysis before it can be certified.
The Federal Aviation Administration formally set the timetable for the 777X, saying the jet is "not yet ready" to advance to the next stage of evaluation, according to a letter to the company reviewed Monday by AFP.
Based on inputs from Boeing on the 777X and the test flights conducted so far, certification is "realistically going to be mid to late 2023," said Ian Won, an acting manager in the FAA's aircraft certification service, in the May 13 letter.
Boeing has pushed back the timeframe for the new aircraft several times and previously discussed a "late 2023" schedule for certification. The Seattle Times first reported on the FAA letter which it described as "sternly worded."
The exchange comes as aerospace giant and the FAA face lingering scrutiny on Capitol Hill after the lengthy grounding of the 737 MAX following two fatal crashes.
In the letter, Won pointed to a December 8 "uncommanded pitch" event in which the 777X jet performed unexpectedly and said it requires a "root cause" investigation by Boeing.
"We are expecting Boeing to complete comprehensive validation, verification reviews and document the process improvements and lessons learned," he said, calling for a "robust process" to ensure "this is not a systemic issue."
Won also cited a number of other outstanding questions on the new plane.
The company said it was working with the FAA.
"Boeing remains fully focused on safety as our highest priority throughout 777X development" a company spokeswoman said.
"As we subject the airplane to a comprehensive test program to demonstrate its safety and reliability, we are working through a rigorous development process to ensure we meet all applicable requirements."
Shares of Boeing fell 3.4 percent to $239.33 in midday trading Monday. The decline came amid a pullback in travel stocks due to worries over the Delta variant of the coronavirus.