The flight crews of two crashed Boeing 737 Max aircraft did not react to the emergency in the ways the manufacturer assumed they would, according to transport safety bosses.

A Lion Air crash in October 2018 and an Ethiopian Airlines accident in March this year, both of which involved a Boeing 737 Max aircraft, killed a total of 346 passengers and crew and caused global authorities to order the grounding of all of the aircraft type. The planes have still not returned to the skies.

Investigations into both crashes found the 737 Max’s Manoeuvring Characteristics Augmentation System (MCAS) received conflicting data from sensors, which erroneously triggered the system to force the noses of the planes down on take-off to avoid stalling.

Ethiopian Airlines insists that the investigation into the March accident shows its pilots followed the recommended procedure to try to regain control of the aircraft, but their efforts failed.

The US’s National Transportation Safety Board (NTSB) said its own investigation of the accidents suggested the pilots may have been overwhelmed by a number of alerts and warnings that were not included in simulator tests, during which Boeing only assessed responses to a generic MCAS failure and not problems that could lead to such a failure, such as the sensor issue.

Chairman Robert Sumwalt said: “We saw in these two accidents that the crews did not react in the ways Boeing and the FAA assumed they would. Those assumptions were used in the design of the airplane and we have found a gap between the assumptions used to certify the Max and the real-world experiences of those crews, where pilots faced with multiple alarms and alerts at the same time.”

The NTSB’s report notes that the organisation’s examination did not analyse the actions of the pilots involved in either accident but instead focused on the process by which the 737 Max was certified to fly.

It has recommended that future system safety assessments for the 737 Max consider the effect of alerts and indications on pilot response and address any gaps in design, procedures and training. It also called on manufacturers to “more clearly present failure indications to pilots”.

There is still no estimate on when the FAA will recertify the 737 Max to return to service, but the European Aviation Safety Agency (EASA) has already said it will not accept the US’s decision and will instead run its own tests on the aircraft before it will approve it to operate commercial flights again.

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