A preliminary report into the Ethiopian Airlines Boeing 737 Max 8 accident of 10 March has resolved some of the mystery behind flight ET302’s loss, but also raised new questions for me as FlightGlobal's test pilot – and who operates the type for a major carrier.
Firstly, the 737 Max family’s Maneuvering Characteristics Augmentation System (MCAS) is not a “stall-prevention” or “safety” feature. It is present to augment handling in certain parts of the flight envelope. Boeing certificated the re-engined twinjet with the feature, which I understand was not needed for compliance, but to make the re-engined model fly more like its earlier 737NG.
After reviewing the Ethiopian report, several issues arise.
The left stick-shaker activated after take-off. It seems the crew realised the warning was false, but the noise and column vibration no doubt created a distraction.
The master caution “anti-ice” and overhead panel “L alpha vane” was called out twice. In retrospect, this provides an indication of the faulty angle-of-attack vane – but at the time it would have been a further distraction.
The report states: “At 05:43:20, approximately 5s after the last manual electric trim input, an AND [aircraft nose-down] automatic trim command occurred and the stabiliser moved in the AND direction from 2.3 to 1.0 unit in approximately 5s.”
This was the stabiliser movement that placed the 737-8 into its fatal dive, but why was it commanded? MCAS could not have trimmed the stabiliser if the trim cut-out switches were still in cut-out. Had they been repositioned to “normal” by a crew member, allowing MCAS to re-engage? If so, the action was not announced.
Notably, the 737’s manual trim system is purely mechanical, and manual rotation of the stabiliser trim wheel at high speed would be very difficult with the elevator loaded up. It might even be necessary to relax the back pressure on the yoke – or even push it forward – in order to rotate the trim wheel.
The report also states: “The left overspeed warning activated and was active intermittently until the end of the recording.”
Proper response to an overspeed would be to retard the thrust levers and adjust pitch attitude. The crew was trying to raise the nose, yet thrust remained at 94% N1. Retarding the thrust levers and extending the speed brakes would have slowed the aircraft, and may have allowed the descent to be arrested by the elevator alone.