Boeing needs to pay more heed to how pilots react to emergencies in its safety assessment of the 737 Max plane, US transport chiefs have said.
The 737 Max has been grounded since March following two fatal crashes.
The US Federal Aviation Administration (FAA) must decide if the plane is safe.
But according to the US National Transportation Safety Board (NTSB), the crews in the fatal crashes “did not react in the ways Boeing and the FAA assumed they would”.
The 737 Max plane has not flown commercially since an Ethiopian Airlines aircraft crashed shortly after take-off on 10 March, killing 157.
It followed a Lion Air crash on 28 October last year which killed 189.
In both incidents, investigators have focused on the role played by a software