American aeroplane manufacturer Boeing has admitted that it knew about and failed to correct a problem with its 737 Max jets a year before the aircraft was involved in two fatal accidents.

The company said an important alarm feature that may have prevented both accidents was classified as being “optional” instead of “standard” and insisted that leaving the early warning system off most of the planes did not jeopardise flight safety.

A worldwide fleet of nearly 400 737 Max planes was grounded in March after an Ethiopian Airlines flight crashed and killed all 157 people onboard only five months after a Lion Air crash in Indonesia claimed the lives of all 189 passengers.

In both crashes, erroneous data was fed to the jet’s Manoeuvring Characteristics Augmentation System (MCAS), an anti-stall system designed to improve the handling of the aircraft. The MCAS system relied on data from a single sensor that may have inadvertently triggered the crash.

Boeing did not require that a new feature which alerts pilots when different sensors were reporting conflicting data to MCAS, known as the Angle of Attack (AOA) Disagree alert be included on the planes.  Boeing now argues they intended to deal with the problem in a later software update. It is unclear if the AOA feature would have prevented the two accidents. The fact that the information that the sensors gave different signals does not automatically allow the pilots to disable the MCAS.

During the second accident, the pilots did follow the procedure on how to deal with MCAS failure set by Boeing but were unable to maintain control of the aircraft, which has led Boeing to insist that the AOA alert was not necessary.

The US Federal Aviation Administration claims that Boeing had not informed the agency of the software issue until November 2018.

The fact that Boeing was aware of the underlying issue with the MCAS system and failed to inform any of the airlines who bought the plane has left the company open to being held legally responsible for the accidents.