Ethiopian Airlines tragedy: the fatal flaws that killed 157

The pilots followed the right procedure to overcome the anti-stall system, but were unable to save the aircraft

The captain and first officer of Ethiopian Airlines flight ET302 did everything they could to save the aircraft, but were unable to prevent the crash in which 157 people died.

The official report into the crash of the Boeing 737 Max reveals a struggle between the two men and the technology installed on one of the world’s most modern aircraft.

The Aircraft Accident Investigation Bureau of Ethiopia (AIB) says that one minute after take-off from Addis Ababa, “the airspeed and altitude values from the left air data system began deviating from the corresponding right side values”.

Before the 737 had been aloft for two minutes, a faulty sensor triggered a false warning of a potential stall, beginning a fatal chain of events.

For the remainder of the flight, the pilots were battling against the Maneuvering Characteristics Augmentation System (MCAS) – software installed as a safety measure to prevent a high-speed stall.

MCAS automatically pushes the nose of the aircraft down when a sensor detects a potentially dangerous “angle of attack” – the angle between the wing and the airflow.

But if the sensor is faulty, MCAS can force the nose down and place the aircraft in danger.

When a Lion Air Boeing 737 Max was lost shortly after take off from Jakarta in Indonesia, MCAS was implicated in a crash that cost 189 lives. The interim report suggested that the pilots may have failed to respond appropriately. Boeing issued updated instructions to Boeing 737 Max pilots to spell out the procedure for disengaging the anti-stall system and controlling the trim of the aircraft manually.

But the AIB report into the Ethiopian Airlines tragedy indicates that the captain and first officer carried out the stipulated procedure to correct an incorrect MCAS – but were still unable to save the aircraft.

The anti-stall system forced the nose down four times. As they talked to air-traffic controllers, the crew correctly put the “stab trim cutout switch” to the cutout position. But the prescribed manual override of the tail stabiliser failed to work because the aircraft was travelling at very high speed.

Unable to correct the trim manually, the pilots switched the power to the stabiliser once again, to try to remedy the problem using electrical switches. But once again MCAS forced the nose down.

Six minutes after take-off, the Boeing 737 Max flew at around 575mph at an angle of 40 degrees into the ground.

“This accident was not survivable,” the investigators conclude.

In line with international convention, the accident report does not seek to assign blame. The International Civil Aviation Organization says: “The sole objective of the investigation of an accident or incident shall be the prevention of accidents and incidents; it is not the purpose of this activity to apportion blame or liability.”

But the report recommends that Boeing reviews “the aircraft flight control system and that “aviation authorities shall verify that the review of the aircraft flight control system related to flight controllability has been adequately addressed by the manufacturer before the release of the aircraft to operations”.

In response to the report, the president and chief executive of Boeing Commercial Airplanes, Kevin McAllister, said: “We will carefully review the AIB’s preliminary report, and will take any and all additional steps necessary to enhance the safety of our aircraft.”