Boeing chairman, president and CEO Dennis Muilenburg has said a system introduced on its 737 Max 8 aircraft is not entirely at fault for two fatal crashes.

Muilenburg told investors and the media that the system was only one factor in a series of events that led to the Lion Air and Ethiopian Airlines accidents, which killed 346 people in total, though he admitted one common factor in both incidents was faulty data from a sensor triggering the aircraft’s Manoeuvring Characteristics Augmentation System (MCAS).

He said: “As in most accidents, there are a chain of events that occur. It is not correct to attribute that to any single item.”

However, evidence gathered in the investigations of both crashes have raised questions about the safety of the system. A report on the Lion Air accident found the nose of the aircraft was pushed down more than 20 times before crashing, while investigators claim the pilots of the Ethiopian Airlines aircraft followed procedures set out by Boeing to keep the aeroplane from nose-diving.

Investigators have said the pilots may have struggled with the MCAS on the 737 Max, which was designed to keep the aircraft from stalling if sensors detected the angle of attack on take-off is too high by pushing the nose of the plane down.

Boeing began working on a software update for the 737 Max following the Lion Air crash and said following the Ethiopian Airlines incident that it would no longer charge airlines extra to install an additional warning system for the MCAS that alerts pilots if the system’s sensors produce contradictory readings.

Southwest Airlines, one of the manufacturer’s biggest customers, reportedly told US media that it had not been made aware that this “AOA Disagree” alert – which came as standard on the previous 737 models – had become a paid-for option until after the Lion Air crash last October.

In a statement released following Muilenberg’s press conference, Boeing said: “The disagree alert was intended to be a standard, stand-alone feature on Max aeroplanes. However, the disagree alert was not operable on all aeroplanes because the feature was not activated as intended. The disagree alert was tied or linked into the angle of attack indicator, which is an optional feature on the Max. Unless an airline opted for the angle of attack indicator, the disagree alert was not operable.”

The head of the UK’s Flight Safety Committee has claimed that airlines and aircraft manufacturers are doing the “absolute minimum” pilot safety training to keep costs low.

Meanwhile, whistleblowers have contacted the US Federal Aviation Administration (FAA) to voice concerns about Boeing’s manufacturing processes, with one saying they saw damage to wiring used by the sensor implicated in both crashes. The FAA is investigating these claims.

Boeing has since reported a £773 million drop in revenues due to the 737 Max grounding, though the full scale of the impact is yet to be determined. The company has temporarily dialled back production of the Max to deal with a pause in deliveries while a software update is tested and approved by the FAA.

There is no estimate for how much longer the 737 Max will be grounded.

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