Boeing did not tell its largest 737MAX customer Southwest Airlines, that a standard safety feature designed to warn pilots about malfunctioning sensors had been deactivated.

The safety feature is an alert that lights up in the cockpit if a plane’s angle-of-attack sensors transmit faulty data about the pitch of the plane’s nose. This feature is known as an angle-of-attack disagree light and was included in previous versions of the 737.

Southwest did not know about the change until after the fatal crash of a Lion Air flight in Indonesia.

Southwest said Boeing told the airline that the disagree lights were inoperable only after the Lion Air crash. The airline subsequently took action to turn the alerts on.

Southwest Airline’s statement:

Upon delivery (prior to the Lion Air event), the AOA Disagree lights were depicted to us by Boeing as operable on all MAX aircraft, regardless of the selection of optional AOA Indicators on the Primary Flight Display (PFD). The manual documentation presented by Boeing at Southwest’s MAX entry into service indicated the AOA Disagree Light functioned on the aircraft, similar to the Lights on our NG series. After the Lion Air event, Boeing notified us that the AOA Disagree Lights were inoperable without the optional AOA Indicators on the MAX aircraft. At that time, Southwest installed the AOA Indicators on the PFD, resulting in the activation of the AOA Disagree lights – both items now serve as an additional crosscheck on all MAX aircraft.

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