Were the pilots to blame?

When the preliminary report was published after the crash, Ethiopian transport minister Dagmawit Moges appeared keen to exonerate Capt Yared Getachew and First Officer Ahmednur Mohammed Omar.

Yared was a relatively seasoned pilot, with more than 8,000 flying hours on his record, including some 1400 on 737s. His first officer, however, had just 361 hours, including 207 on Boeing 737s.

The Ethiopian government had taken charge of the investigation, which was helped by the relatively quick recovery and analysis of the aircraft’s flight recorders. It concluded that the crew had done nothing wrong.

“The crew performed all the procedures - repeatedly - provided by the manufacturer, but was not able to control the aircraft,” said Dagmawit .

Captain Yared Getachew Captain Yared Getachew

However, this conclusion has been challenged by people who believe the pilots committed a series of errors.

The trigger for the accident - and the root cause of another crash involving a near-identical 737 Max off Indonesia last year - is thought to have been the failure of a system known as Manoeuvring Characteristics Augmentation System (MCAS).

What now seems apparent with the benefit of hindsight is that MCAS had design flaws. Boeing itself has indicated as much.

MCAS is a piece of flight control software designed to make the new plane easier and more familiar to fly for pilots who were already used to the previous generation of 737 – reducing the need for potentially costly extra training.

It was meant to curb a tendency for the nose of the aircraft to rise too much, when it was already pitched up at a steep angle. It used the stabilisers - the horizontal wings on the tail of the aircraft, which are normally used to keep the aircraft balanced - to produce a nose-down movement.

However, the system relied on data from a single sensor on the outside of the aircraft to determine the angle at which the aircraft was flying. The failure of that one sensor could lead to the system deploying at the wrong time - forcing the aircraft into a descent.

It appears to have done just that on both the Boeings that crashed, forcing the nose of the aircraft down when the pilots were attempting to gain height.

The system was also very powerful, able to produce significant stabiliser movements, and capable of overriding the crews’ own inputs as it activated again and again.

After the second crash, authorities around the world grounded the 737 Max.

Boeing is working on modifications to the software, and the design will not fly again until regulators are satisfied it is completely safe. But if the aircraft control systems themselves were faulty, why have attempts been made - by Boeing and some US politicians - to cast a share of the blame onto the crew of the stricken aircraft?

Among the fiercest critics of the crew’s actions in the US has been Congressman Sam Graves, himself a qualified pilot and Ranking Member of the Committee on Transportation and Infrastructure in the House of Representatives. The Missouri Republican thinks that the crew of ET302 could have saved their plane.

Congressman Sam Graves Congressman Sam Graves

He has been a vocal presence at hearings in Washington involving both the Federal Aviation Administration (FAA) and the National Transportation Safety Board as part of the inquiries and investigations surrounding the 737 Max.

In May, Graves insisted that “facts in the preliminary report reveal pilot error as a factor”. He went on to claim that “pilots trained in the US would have successfully been able to control this situation”.

That is a controversial view, but one based on the way in which the pilots reacted to the situation confronting them. Nor is he alone in his opinion which is widely reflected in statements made by others online and in the media.

Before the Lion Air crash off Indonesia - in which 189 people were killed - the very existence of MCAS was unknown to airlines buying the 737 Max, or to their flight crews. The acronym did not appear in the flight manual.

After the accident, however, Boeing published a bulletin in which it described the effects of an MCAS malfunction, and instructed pilots to follow a particular “non-normal checklist” designed to help them cope with uncontrolled stabiliser movements.

This checklist - which is meant to be memorised by flight crew - instructed them to flip switches on the centre console, to turn off the stabiliser electronics, then balance the aircraft using manual trim wheels beside the pilots’ knees.

The Ethiopian crew tried to follow this procedure. They turned off the electronics and attempted to “trim” the aircraft - to bring it back into level, balanced flight - using the hand controls. But the preliminary report suggests that they were physically unable to do so.