Boeing has admitted it knew about a problem with a system on its 737 Max aircraft a year before two fatal crashes but did not take action.

The aircraft manufacturer said yesterday that it had inadvertently made an alarm feature optional rather than standard, but insisted that this did not affect the 737 Max’s safety.

Known as the Angle of Attack (AOA) Disagree alert, the feature is designed to let pilots know if two sensors are reporting conflicting data. Boeing claimed it had intended to make this a standard system on the aircraft but only realised after deliveries had started that the feature was only included if airlines paid extra for it.

The firm said it intended to fix the problem in a later software update, which it began working on after a Lion Air crash involving a 737 Max in October last year, in which 189 people were killed.

Five months later, an Ethiopian Airlines flight operated by the same model of aircraft crashed, killing 157 people.

Investigations into both accidents found the pilots may have struggled to regain control of the aircraft after the Manoeuvring Characteristics Augmentation System (MCAS) forced the nose of the plane down several times owing to erroneous AOA data.

Boeing maintains that the absence of the AOA Disagree alert “did not adversely impact airplane safety or operation”. At an earlier press conference, CEO Dennis Muilenburg said: “As in most accidents, there are a chain of events that occur. It is not correct to attribute that to any single item.”

The US Federal Aviation Administration (FAA) told Reuters that Boeing had not made it aware of the software issue until November 2018 – one month after the Lion Air crash. Although it said the problem was “low risk”, it added that Boeing could have helped to “eliminate possible confusion” by letting it know earlier.

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