At first, only China and a few smaller carriers grounded their Boeing 737 Max fleets in the response to the 10 March crash of Ethiopian Airlines flight ET302, in which 157 people were killed.
While Chinese carriers had a combined fleet of 97 aircraft, that could still, just, be ignored – put it down to politics, maybe – but then over the coming days, a steady drip became a trickle, which became a torrent, as regulators across the globe ordered the 737 Max grounded.
When the European Union Aviation Safety Agency (EASA) finally acted on 12 March – a number of its national constituents already having moved unilaterally to ground the Max – followed by its Canadian counterpart early the next day, that left just the US regulator as a hold-out.
In fact, the US Federal Aviation Administration had rejected action for three days, but eventually fell into line on 13 March, when President Donald Trump announced that the USA would order that both current in-service Max variants – the 737-8 and -9 – be removed from operations.
"Those planes are grounded effective immediately," Trump said. "The safety of the American people, of all people, is our paramount concern."
In the days preceding the presidential intervention, the FAA had repeatedly issued statements stressing that it lacked sufficient evidence to ground the Boeing narrowbody.
But at the forefront of everyone's minds were the possible links to the loss of a Lion Air-operated 737-8 on 29 October last year, flight JT610, which killed all 189 on board.
The occurrence of two fatal accidents involving newly delivered aircraft just five months apart is virtually unprecedented in the modern era.
In the case of the Lion Air crash, Indonesian investigators believe erroneous angle-of-attack data triggered the Max's Maneuvering Characteristics Augmentation System (MCAS) – a piece of software designed to bring the aircraft's nose down if it appears to be approaching a stall.
The Lion Air crew battled unsuccessfully to regain control of the Max as its horizontal stabilisers repeatedly trimmed the nose down.
MCAS was introduced on the Max to counter a nose-high tendency caused by tweaks to the configuration over previous-generation 737s that in certain conditions created a greater risk of stalling.
Boeing, even while recommending the "temporary suspension of operations" of the Max out of "an abundance of caution and in order to reassure the flying public of the aircraft's safety", maintains that the type is safe.
It had previously noted that the circumstances surrounding the Ethiopian crash remain uncertain, and that pilots, by following established cockpit procedures, can easily disable the MCAS software suspected of playing a role in the Lion Air crash.
But while stopping short of directly linking the two accidents, the FAA, when announcing that the 737-8 and -9 were being removed from service, pointed to new evidence informing its decision.
"The investigation of the ET302 crash developed new information from the wreckage concerning the aircraft's configuration just after take-off that, taken together with newly refined data from satellite-based tracking of the aircraft's flightpath, indicates some similarities between ET302 and JT610," the FAA's grounding order states.
The new details "warrant further investigation of the possibility of a shared caused for the two incidents that needs to be better understood and addressed," it adds.
Boeing recently disclosed it is working on an "enhancement" to the MCAS software, and the FAA says it intends to mandate such improvements in an airworthiness directive issued by April. Roll-out of the upgrades was reportedly delayed by the US government shutdown in late 2018 and early 2019.
The piecemeal approach and lack of consensus over the grounding has prompted a welter of opinions on the handling of the situation, some touching on long-standing concerns related to pilot training and advanced cockpit technology.
President Trump, of course, has a view, tweeting on 12 March that "airplanes are becoming far too complex to fly".
Depending on which side of the fence you sit, the FAA's days of inaction are either the agency kowtowing to Boeing's influence, or the right approach, while the rest of the world acted prematurely.
US pilots have deemed the 737 Max safe to fly, thanks to second-to-none pilot training and additional warning sensors.
The Allied Pilots Association, while faulting Boeing for not disclosing the existence of the system until after the Lion Air crash, has said its pilots are adequately prepared to deal with in-flight MCAS issues.
"We know we have the skill-set and knowledge to combat" MCAS problems, says Dennis Tajer, communications chair at the union, which represents pilots at American Airlines. "We are ready and we are prepared."
Despite the deluge of negative publicity, few details of the fatal Ethiopian crash have emerged.
Flight ET302 departed Addis Ababa at 08:38 local time, heading for Nairobi; it came down 6min later near Bishoftu, around 25 miles (40km) southeast of the capital.
According to the airline's chief executive Tewolde Gebremarium, the 737-8's captain had requested a return having experienced problems after take-off.
In addition, "maintenance checks did not reveal any problems", says Gebremarium. The jet (ET-AVJ) was delivered on 15 November 2018 and had since accumulated around 1,200h.
Ethiopia is controlling the investigation, and information has so far trickled out at a pace slower than typical, says aviation safety consultant and former National Transportation Safety Board member John Goglia.
Investigators retrieved the aircraft's flight-data and cockpit-voice recorders within one day of the crash, but analysis was not scheduled to begin until 15 March, by France's BEA accident investigation body.
"Why are we taking so long to read the recorders?" Goglia asks.
The worldwide grounding now heightens pressure on Boeing to convince regulators the aircraft is safe. It also raises real questions about Boeing's plan to boost 737 Max production.
At present, the manufacturer is continuing to build new 737 Max jets, although it has suspended deliveries.
But, perhaps to Boeing's gain, the FAA's order does allow the agency to grant "special flight permits" for purposes including moving aircraft for storage and performing flight-testing.
Regardless, Boeing's plan to boost 737 output this year to 57 from 52 aircraft monthly faces new challenges.
The 737 Max represents perhaps the largest chunk of Boeing's future, and the company, locked in a narrowbody race with Airbus, has in the last year worked feverishly to get more and more 737s out of the door.
A prolonged grounding could eventually cause a back-up of aircraft at Boeing's facilities, potentially leading the company to suspend 737 production, says Richard Aboulafia, an aerospace analyst with Teal Group.
But he thinks the airframer has the financial strength to weather the storm, and he doubts that the grounding will affect the 737 programme's commercial success, noting Boeing's thousands-strong order backlog and a seemingly unceasing demand for narrowbodies.
"This is why only very big companies build jetliners," he says.
For now, the impact on the world's airlines remains modest, due largely to the relatively small size of the 737 Max fleet.
The US Max operators – American Airlines, Southwest Airlines and United Airlines – have a combined 72 examples, about 3% of those carriers' total 2,400 aircraft, according to Cirium's Fleets Analyzer.
"Given the size of the American, United and Southwest fleets, we expect those companies to have little issues adjusting schedules and backfilling aircraft with 737-800s," says a 12 March analysis from investment bank Cowen.
Other carriers face greater levels of disruption: WestJet's 13 737 Max, for instance, represent about 10% of its fleet. Norwegian's inventory of 18 737-8s is 14% of its 128-strong narrowbody fleet, and the budget carrier was operating several transatlantic services with the type. It has indicated that it expects compensation from Boeing over the issue.
Another 54 airlines worldwide operate 737 Max, but most of those have fewer than a dozen of the type, Fleets Analyzer data shows.