In the last five years, statistics for fatal accidents to commercial passenger jets were so low they looked set to prove that a permanent zero fatal accident target was achievable.

Technology is accepted to be the main contributor to these remarkable safety performance improvements. The superb engineering and smart systems in the latest jets made them as different from their predecessors as today’s generation of automobiles is from cars of the 1970s.

But, on 29 October 2018, Lion Air flight JT610 crashed only about 12min after take-off from Jakarta, Indonesia. The aircraft was a Boeing 737 Max 8 that was delivered by the manufacturer to the airline less than three months before, one of 11 of this new marque in its fleet.

That was a shock, but when on 10 March this year another almost new 737 Max 8 also crashed within a few minutes of take-off from Addis Ababa, Ethiopia under circumstances that appear similar, a chill went through the entire aviation community.

Ethiopian Airlines has grounded its 737 Max fleet, Singapore has banned Max operations in its airspace, and the Chinese aviation authority CAAC has grounded all Maxes registered there – almost sixty of them. And on 12 March Australia, Ireland, France, Germany and the UK added themselves to the rapidly growing list of those who had banned operation of the type. Late on 12 March the biggest blow fell: European Union body the European Aviation Safety Agency has banned all 737 Max 8s and 9s from its skies except to fly, empty, to maintenance bases. The agency argued that it cannot be ruled out that the Ethiopian accident was caused by the same failure as that which appears to have caused the Lion Air crash. And, shortly before midnight, India had joined the doubters.

Now Latin America has begun a wave of groundings and, as a result, by the end of the Western European day on 12 March more than a third of all Maxes in service around the world had been affected by effective groundings. There has never been an event like this, where the original certificating authority has declared an aircraft airworthy but much of the rest of the world has decided it is not so confident.

Back to the accident issues. The two take-off airports couldn’t have been more different, one at sea level, the other at an elevation of more than 7,000ft, but in both cases it was daylight and the weather conditions were benign.

Both aircraft were seen to dive to impact.

The Indonesian investigator (NTSC) issued a preliminary factual report that doesn’t pretend to provide a verdict on the cause of the Lion Air crash, but suggests that a factor in the sequence of events leading to it was a faulty angle of attack (AoA) sensor. This device, says the report, sent false signals to a new stall protection system unique to the Max series of 737s, known as the manoeuvring control augmentation system (MCAS). According to the report, these signals wrongly indicated a very high AoA, and the MCAS triggered the horizontal stabiliser to trim the aircraft nose-down.  The crew seems not to have known how to counteract this nose-down control demand.

The NTSC did, however, provide fine detail about malfunctions on same airframe on the previous day (28 October), when almost exactly the same sequence of events occurred, including the signal from the faulty AoA sensor to the MCAS. But on that occasion the captain stopped the nose-down stabiliser trim rotation by selecting the STAB TRIM switches to CUT OUT, and then proceeded safely to the scheduled destination.

Some pilot associations in the USA whose members operate the Max have professed publicly that there was a widespread ignorance among Max-qualified pilots of the very existence of the MCAS, and also among them was an assumption that a runaway trim could be dealt with in exactly the same way as it was for all the earlier 737 marques. Actually the drill is different for the Max, as Boeing and the US Federal Aviation Administration (FAA) have pointed out.

The MCAS was developed for the Max because its more powerful engines are heavier and fitted further forward than those on earlier marques, affecting the aircraft’s centre of gravity and thus its behaviour at low speeds approaching the stall, so the manufacturer wanted to boost stall protection. It looks as if Boeing had either not foreseen the potential effect of a false high AoA indicator input to the MCAS, or it had failed to warn pilots clearly what that effect could be and how to react. The FAA also, it appears, had not anticipated this.

After the Lion Air crash the FAA put out an emergency airworthiness directive requiring operators of the Max to make clear to pilots the procedures for dealing with a runaway stabiliser trim. Boeing maintained that information was already available.

Pilots converting from earlier 737 marques to the Max are not required to undergo a new full type rating course or simulator sessions, because all 737s are deemed to have sufficient commonality to operate under the same type rating. Thus 737-rated pilots being prepared for the Max are required only to undergo a brief academic “differences course”. For example Southwest Airlines pilots had done their differences course entirely online, and American Airlines the same.

On 11 March, a day after the Ethiopian crash, the FAA revealed it has required Boeing to solve the software problem – and if applicable the hardware – that at present means that a false AoA input can trigger the MCAS stall protection when it is not needed, effectively causing a stabiliser pitch trim runaway. Meanwhile it has declared that the 737 Max series is airworthy.

But if it were to be found that there is a common cause of these two Max crashes – whatever that cause is determined to be – the implications for the manufacturer and the airlines are significant, given the massive size of the order book for 737 Max series aircraft.