Boeing wrongly assumed pilots would quickly trim out MCAS

Boeing wrongly assumed pilots would quickly trim out MCAS - Flight Global

"Boeing incorrectly predicted the manner in which 737 Max pilots would respond to the activation of the Manoeuvring Characteristics Augmentation System, by assuming they would initially pull back on the control column and then trim out the force to maintain level flight....

"Failure to re-trim the aircraft during a series of repeated MCAS activations would result in the stabiliser gradually shifting to its maximum deflection, with the crew attempting to keep the nose up with increasing force on the control column.

"When the 737 Max was being developed, simulator testing during functional hazard assessment 'never considered' the scenario of repetitive MCAS activation incrementally driving the stabiliser to its maximum limit.

"Boeing had believed repetitive MCAS activations to be 'no worse' than a single activation, because of its assumption that the pilots would trim out the forces each time, says the inquiry. It had also assumed that the crew would respond correctly, and within 3s."

Boeing's MCAS test did not simulate other cockpit effects - Flight Global

"Indonesian investigation authority KNTK says Boeing's preliminary hazard assessment of MCAS, carried out on a full-flight simulator in 2012, examined crew responses to uncommanded MCAS activation 'regardless of underlying cause'.

"This focus on the pilots' response to MCAS - rather than the reason MCAS might be triggered - meant that specific failure modes 'were not simulated', says the inquiry, and therefore neither were the cockpit effects of those failure modes.

"KNKT says a failure such as erroneous angle-of-attack sensor data, leading to unreliable airspeed alerts, stick-shaker activation, and other alarms in the cockpit were not part of the simulation....

"[A post-crash simulator] exercise [recreating cockpit conditions] found that crews could not maintain altitude with control column force alone if short activation of electric trim resulted in an accumulating mis-trim from the MCAS nose-down commands.

"'Repeated MCAS activations increased the flight crew workload and required more attention to counter it,' says the inquiry. Communicating with air traffic control was 'distracting', it adds, and crews found the non-normal checklist 'hard to get through'."