US lawmakers have requested additional information from Boeing, United Technologies and the Federal Aviation Administration about a defective 737 Max sensor alert that Boeing did not address with urgency until after the fatal October 2018 Lion Air crash.

The request seeks more information about the aircraft's angle-of-attack (AOA) disagree alert, which was defective on some 737 Max. Faulty AOA sensors caused flight control software to activate unexpectedly on two 737 Max, both of which crashed.

Boeing has previously said its engineers discovered the AOA alert issue in 2017, several months after 737 Max deliveries started. But the company determined at that time that the fault would not affect safety, deferring a fix until the next planned software update.

"The Committee obtained information suggesting that Boeing decided in November 2017 to defer a software update to correct the AOA disagree alert defect until 2020, three years after discovering the flaw, and only accelerated its timeline after the October 2018 Lion Air accident," says a media release from the House Committee on Transportation and Infrastructure.

Committee chair Peter DeFazio and aviation subcommittee head Rick Larsen have now sent new letters to Boeing, UTC and the FAA "requesting a timeline and supporting documents related to when they became aware that the angle of attack disagree alert on some Boeing 737 Max planes was defective, as well as when airlines were notified about the defect".

The committee is among several entities investigating the 737 Max's safety and certification. In early April it first requested information related to the 737 Max from Boeing and the FAA, and held a related hearing on 15 May. The committee confirms it plans to hold a second hearing on 19 June.

The AOA disagree alert warns pilots when the aircraft's AOA sensors have different readings. After Boeing began Max deliveries it realised the AOA disagree alert did not work on aircraft that lacked an optional AOA indicator – a separate cockpit reading.

Boeing did not notify the FAA about the inoperable alert until after the October 2018 Lion Air crash, more than a year after it learned of the problem, the House Committee says.

Boeing initially concluded the "absence of the AOA disagree alert did not adversely impact airplane safety or operation," it says. "Accordingly, the existing functionality was deemed acceptable until the alert and the indicator could be delinked in the next planned display system software update."

"Based on the safety review, the update was scheduled for the MAX 10 rollout in 2020," it says. Boeing acknowledges it "fell short in the implementation of the AOA disagree alert and are taking steps to address these issues so they do not occur again".

The FAA confirms it received word of the AOA alert problem in October 2018 and deemed the issue to be "low risk".

"However, Boeing’s timely or earlier communication with the operators would have helped to reduce or eliminate possible confusion,” the FAA says.

Source: Cirium Dashboard