It has been called "the most scrutinised transport aircraft in history", but some critics believe Boeing's 737 Max is still not safe.
It was cleared to fly passengers again by US regulators last year, having been grounded following two catastrophic accidents.
Since then, however, a number of potentially serious problems have been reported during 737 Max flights.
Boeing insists the aircraft is both safe and reliable.
On 14 October, a 737 Max took off from Boeing Field airport in Seattle, bound for Brussels. It was a delivery flight, taking the brand-new plane to start work for its owners, the travel group Tui.
But minutes into the 5,000-mile journey, the pilots reported an urgent "flight control problem" and had to turn back. The aircraft landed safely shortly afterwards.
The issue, linked to the autopilot, was rectified relatively quickly. The plane set off for Brussels again the following day, and has been flying regularly since then.
However, this was not an isolated incident.
Emergencies
Whenever a US carrier or repair station discovers any serious failure, malfunction or defect aboard an aircraft, it has to inform the US regulator, the Federal Aviation Administration (FAA), via a so-called "service difficulty report".
There have been more than 180 such reports since the 737 Max returned to service.
Most faults were found on planes that were on the ground. But on 22 occasions, they occurred in-flight, and on four of those the pilots declared emergencies.
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US pilots also declared emergencies on two further occasions due to engine failures. These events do not currently appear in the FAA's database, but have been reported on the Aviation Herald website, which lists accidents and incidents in commercial aviation.
The problems occurred in a US fleet that is still relatively small, with fewer than 160 aircraft delivered as of mid-October - some of which were grounded for several weeks early in the year after the discovery of electrical issues.
Service difficulty reports are also made public by the Canadian regulator, Transport Canada. Its database shows that the Canadian fleet of 56 737 Max aircraft has generated 19 reports, five of them relating to in-flight incidents.
The 737 Max remains under intense scrutiny. The plane was grounded for 20 months from March 2019, after being involved in two major accidents, in which 346 people died.