What it does is relatively simple. It moves the horizontal stabilisers - the tail wings of the aircraft, which are normally used to help the aircraft maintain level flight - to bring the nose downward.

This has two effects: it increases pressure on the pilot’s control column, making the handling feel more familiar to experienced 737 pilots; and it reduces the risk of the nose rising too far and triggering a stall - although Boeing insists it is not an anti-stall system, and is simply there “to enhance the pitch stability of the airplane - so that it feels and flies like other 737s”.

What now seems apparent with the benefit of hindsight is that MCAS had design flaws.

First, it relied on data from a single angle of attack sensor, even though the aircraft had two of them. This type of sensor, in the nose of an aircraft, measures the angle at which it is encountering the airstream.

But relying on only one of them meant that if it failed, the system could deploy at the wrong time, and push the nose of the aircraft down when it was supposed to be climbing, for example.

Second, although the pilot could use a thumb control to correct the pitch of the aircraft, MCAS would deploy repeatedly in cycles, forcing the nose down again and again.

MCAS worked in the background, and it wasn’t until after the Lion Air crash that Boeing explained what it was and how it could be deactivated. It was not specifically mentioned in the flight manual - which is meant to give pilots the information they need to fly the aircraft safely.

Recent reports in the US media suggest that flight crews themselves were deeply unhappy they had not been kept informed. An audio recording obtained by CBS News purportedly contains an exchange between members of American Airlines’ pilots union and a company official, which took place a month after the first crash. In it, one pilot angrily informs the official that “we flat out deserve to know what is on our airplanes”.

Boeing emphasises that there are set procedures for pilots to follow, which are meant to help them deal with uncontrolled stabiliser movements, whatever their cause. These were contained in the manual, and should have been memorised by the pilots as well. It says the manual did tell crews to expect automatic stabiliser movements as the plane approached stalling speed, the “same type of aircraft behaviour” caused by MCAS.

It has also pointed out that the day before the Lion Air crash, the same aircraft experienced similar problems, but was able to continue safely to its destination.

On that flight, within seconds of take-off, the airspeed and altitude indicators gave sharply different readings. Then the nose of the aircraft began moving down of its own accord.

But this time, the pilots worked out what to do about it. They cut off power to the electronics operating the stabilisers and began controlling them manually.

They may have been lucky, though. According to media reports, they were told what to do by a third pilot who happened to be in the cockpit, having hitched a ride aboard the plane.

Since these incidents, serious questions have been raised about the design of the MCAS software and whether the 737 Max was allowed into service despite a potential catastrophic flaw.

So what were the circumstances in which it was certified as safe to fly in the first place - and why was it not grounded when that flaw first came to light?

Many analysts are now pointing the finger at the close, even symbiotic, relationship between the company and the regulator responsible for aviation safety in the US, the Federal Aviation Administration (FAA).