US Federal Aviation Administration flight tests of the Boeing 737 Max have uncovered a data processing issue that affected government pilots' ability to perform the procedure for counteracting so-called "runaway stabiliser".
The runaway stabiliser procedure is the method by which Boeing has said pilots should respond to erroneous activation of the 737 Max's manoeuvring characteristics augmentation systems (MCAS) – the software that activated prior to two 737 Max crashes.
"During simulator testing last week at Boeing, FAA test pilots discovered an issue that affected their ability to quickly and easily follow the required recovery procedures for runaway stabiliser trim," a source familiar with the FAA's 737 Max evaluation tells FlightGlobal. "The issue was traced to how data is being processed by the flight computer."
Asked to comment, Boeing says, "The safety of our airplanes is Boeing’s highest priority. We are working closely with the FAA to safely return the MAX to service”.
The FAA confirms it has identified "a potential risk that Boeing must mitigate".
"The Federal Aviation Administration is following a thorough process, not a prescribed timeline, for returning the Boeing 737 Max to passenger service. The FAA will lift the aircraft prohibition order when we deem it is safe to do so," says the agency.
"We continue to evaluate Boeing's software modification to the MCAS and we are still developing necessary training requirements," the FAA adds.
Additionally, the FAA is addressing recommendations it received from a "technical advisory board" convened to review its 737 Max work, says the agency.
Regulators worldwide grounded the 737 Max in March after the crash of an Ethiopian Airlines 737 Max 8. That crash followed the October 2018 crash of a Lion Air 737 Max 8.
The accidents killed a combined 346 people.
Boeing adds that the FAA "identified an additional requirement that it has asked the company to address through the software changes that the company has been developing for the past eight months".
"Boeing agrees with the FAA's decision and request, and is working on the required software," the company adds. "Addressing this condition will reduce pilot workload by accounting for a potential source of uncommanded stabiliser motion."
"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," says Boeing.
Story updated on 26 June to include comments from Boeing in the preceding three paragraphs.