WASHINGTON • Boeing has acknow-ledged that a cockpit alert notifying pilots of a sensor malfunction linked to two fatal accidents - in Indonesia and Ethiopia - was not working as represented on every 737 Max plane.
The plane manufacturer said it did not purposefully deactivate a warning meant to show if two angle-of-attack (AOA) vanes provided conflicting data to flight computers. But the alert functioned only if operators purchased a separate optional indicator, it said in a statement on Monday.
"The disagree alert was intended to be a standard, standalone feature on Max airplanes," Boeing said. "However, the disagree alert was not operable on all airplanes because the feature was not activated as intended."
The disclosure adds a new mystery about the design of Boeing's best-selling jet, which has been grounded since an Ethiopian Airlines 737 Max 8 crashed in March.
Boeing is working to convince regulators that the Max will be safe once an update is installed to the so-called manoeuvring characteristics augmentation system (MCAS) software, which played a role in both accidents after being activated by erroneous angle-of-attack data from a single sensor.
The AOA vanes measure the tilt of an aircraft against onrushing wind.
Southwest Airlines, the largest Max operator, said it first learnt from Boeing about the problem with the disagree alert after a Lion Air 737 Max crashed off the coast of Indonesia last October.
Southwest had thought the warning sign was standard on every Max, as it had been on the previous 737 NG models.
As part of a software update, Boeing will activate the alert on the Max rolling out of its factory, and retrofit planes in the field for no additional charge, Boeing chief executive officer Dennis Muilenburg said on Monday.
"The AOA disagree signal in the cockpit is not something that drives pilot actions," he said, referring to the equipment by its acronym. All information needed to safely operate the plane is provided in the flight deck and flight-deck display, he added.
In both 737 Max crashes, the jets misfired and repeatedly pressed the noses of the planes down until the flight crews lost control. All told, the accidents killed 346 people.
WASHINGTON POST, BLOOMBERG
