Incident: A warning written, then never sent

Module 9 · Incident file · About 35 minutes

What happened

In September 2020 the majority staff of the US House Committee on Transportation and Infrastructure published its final report on the Boeing 737 MAX (House Committee on Transportation and Infrastructure, "Final Committee Report: The Design, Development & Certification of the Boeing 737 MAX", September 2020). Two MAX aircraft crashed: Lion Air flight 610 on 29 October 2018, killing all 189 people on board, and Ethiopian Airlines flight 302 on 10 March 2019. The report counts 346 deaths. Most of it is about the design and certification of MCAS, the flight control software. This file follows one thread from its chapter 6, on a cockpit warning, because it is a failure of communication rather than of engineering.

The AOA Disagree alert tells pilots when the aircraft's two angle-of-attack sensors disagree "by more than 10 degrees for more than 10 continuous seconds". It was certified as a standard feature. In software, it had been tied to an optional display that fewer than 20 percent of buyers chose, so it did not work on "more than 80 percent" of the fleet. Boeing found this in August 2017.

The report states that Boeing "prepared a Fleet Team Digest to inform its customers", but "never sent it". A Boeing manual for Lion Air dated 16 August 2018 still described a working alert. By November 2018 about 250 MAX aircraft had been delivered, roughly 200 of them without a working alert. Boeing told the FAA after the Lion Air crash. Boeing and the FAA both maintain the alert was not safety-critical; the report calls a working alert one of "three missed opportunities" to find the faulty sensor before flight 610 left. At a hearing in October 2019 Boeing's then chief executive said: "We made a mistake."