Boeing has uncovered a new problem with the 737 Max aircraft’s flight control computers, though the company does not anticipate the issue will upend its expectation to receive Max certification in the middle of 2020.

“During flight testing of the 737 Max’s updated software, an indicator light associated with the stabiliser trim system illuminated in the flight deck,” Boeing says on 6 February. “We determined that the illumination of this light was caused by differences in input data between the flight control computers.”

More specifically, the indicator light relates to a June 2019 software update that causes the Max’s two flight control computers to monitor each other – a feature Boeing calls “cross-compare redundancy”.

Max grounded in Washington

Source: Gary He/EPA-EFE/Shutterstock

Boeing has stored an estimated 400-plus 737 Max, including these aircraft parked at Boeing Field in Seattle, since the grounding took effect in March. It finally halted production in January.

The company provides few other details about that software update. But, in December 2019, a member of a Max review panel described the new monitoring system to lawmakers.

The monitors “cross-check pitch trim commands against each other” and cause the aircraft’s automatic trim system to be disabled upon detecting differences, said Matt Kiefer, a member of the FAA-convened Technical Advisory Board (TAB).

Boeing is now working on a software update that will ensure the indicator lights illuminates only “as intended”, it says. “Our current schedule assessment indicates that this change will not impact the present mid-2020 estimate for return to service.”

Boeing notified the FAA of the latest issue during the week of 20 January.

The FAA declines to say how the new software problem might affect certification. The agency has no “set timeframe” for certification, and approval will come only after the FAA’s safety experts “are fully satisfied that all safety-related issues are addressed to the FAA’s satisfaction”, the agency says.

In addition to reviewing the Max’s flight control systems – which contributed to two deadly crashes – regulators are looking closely at wire bundles on the Max due to concern about potential short circuits, according to reports.

Boeing calls speculation about the potential impact of the wiring concerns “premature”.

“We continue to perform the appropriate analysis including lab testing, fleet data assessment and third-party reviews”, the company says. “We are giving the technical team the time and resources they need to complete this analysis.”

“Our highest priority is ensuring the 737 Max meets all safety and regulatory requirements before it returns to service,” Boeing adds.