US regulators said Wednesday Boeing must address a new "potential risk" in the Boeing 737 MAX, further clouding the timeframe for resuming service on the planes after two deadly crashes.
The issue, which surfaced during FAA simulator testing, concerns the ability of pilots to quickly reassert control of the plane if an automated flight handling system pushes the plane downward, said a person familiar with the matter.
The Federal Aviation Administration "will lift the aircraft's prohibition order when we deem it is safe to do so," the agency said in an email.
"The FAA recently found a potential risk that Boeing must mitigate."
Boeing said the software fix for the 737 MAX that it has been developing for the last eight months does not currently address the matter.
"The Boeing Company agrees with the FAA's decision and request and is working on the required software to address the FAA's request," Boeing said in a securities filing.
"Boeing will not offer the 737 MAX for certification by the FAA until we have satisfied all requirements for certification of the MAX and its safe return to service."
In a media statement on Wednesday, the company also said addressing the "condition" would cut pilots' workloads "by accounting for a potential source of uncommanded stabiliser motion."
Shares of Boeing fell 0.7 percent to $372.50 in after-hours trading.
Boeing's global fleet of 737 MAX planes has been grounded since mid-March following two crashes which claimed 346 lives.
A key step in the certification is an FAA test flight, which has still not been scheduled until at least the week of July 8, said a person familiar with the matter.
Even before this latest issue surfaced, the outlook for getting the planes back in the air was uncertain, in part because the FAA would like other regulators to approve the plane's re-entry soon after the US agency.
Some regulators have expressed support for requiring simulator training for pilots on the 737 MAX, an idea that was also endorsed last week by retired pilot Chesley "Sully" Sullenberger at a congressional hearing.
A requirement to have pilot simulator training would add cost and time to a resumption, in part because there are only four 737 MAX simulators out on the market now. Still, some panellists at the hearing noted simulators on earlier Boeing 737 models could potentially be used.
US carriers such as American Airlines and Southwest Airlines recently pushed back their timeframe for flying the planes again until the end of summer.
Much of the attention since the Lion Air and Ethiopian Airlines crashes has focused on the on the Manoeuvring Characteristics Augmentation System (MCAS).
In both of the MAX crashes, the MCAS pointed the plane sharply downward based on a faulty sensor reading, hindering the pilots' effort to control the aircraft after takeoff, according to preliminary findings from crash investigations.