Both the European Union Aviation Safety Agency (EASA) and the UK's Civil Aviation Authority (CAA) have lifted a 22-month ban on flights of the Boeing 737 MAX after a design and pilot training overhaul in the wake of crashes that killed 346 people.
EASA confirmed a provisional approval given in November, but dropped calls for an extra flight-angle sensor to back up a system implicated in crashes.
"Let me be quite clear that this journey does not end here," EASA executive director Patrick Ky said in a statement. "We have every confidence that the aircraft is safe, which is the precondition for giving our approval. But we will continue to monitor 737 MAX operations closely as the aircraft resumes service."
Regulators worldwide grounded the MAX in March of 2019 after crashes in Indonesia and Ethiopia.
The UK, which is no longer in EASA after leaving the EU, followed the agency's lead on Wednesday January 27.
The UK's CAA's lifting of the ban on the MAX is its first significant aviation safety decision since Britain formally left the European Union and the EASA on December 31, 2020.
"This is not a decision we have taken lightly and we would not have allowed a return to service for UK operators, or lifted the ban on the aircraft operating in UK airspace, unless we were satisfied that the aircraft type is airworthy and can be operated safely," CAA chief executive Richard Moriarty said.
Most of the work done on the MAX was completed last year while Britain was still part of EASA, and the CAA said that it worked alongside EASA on the process.
UK airlines will be able to operate passenger flights with the MAX subject to close oversight, including pilot training, the CAA said, adding that it is in close contact with travel group TUI, which is the only UK operator of the aircraft.
Criticisms By Relatives Of Crash Victims
Relatives of some victims of the 737 MAX crashes have criticised the move to clear the aircraft, which is the latest version of the world's most-flown jet.
Crash investigations show bad data from a single faulty sensor triggered a barely documented software system that ordered repeated dives and overwhelmed both accident crews.
Boeing has said that data from both "Angle of Attack" sensors on the MAX will be tracked in the modified aircraft, instead of just one as in the past. But EASA has suggested a third sensor system to act as a jury in case one of the main sensors fails.
The proposal, which has been opposed by the US Federal Aviation Administration, triggered a regulatory tussle over whether existing modifications will allow pilots to cope with any sensor outage, or whether a further safety net was needed.
Ky said in September that Boeing had agreed to install the digital equivalent of a third sensor on the next version, the 737 MAX 10, followed by retrofits on other models.
However, in a document alongside the ungrounding order, EASA dropped the proposal for a third "synthetic" sensor on the grounds that Boeing had promised other ways of securing data.
It said that Boeing had agreed to develop further changes "within two years" to improve fault-monitoring and allow pilots easily to select the right data.
An EASA spokesperson said that the solution now being considered by Boeing is different from a third sensor but "broadly aligned". She declined further comment on proprietary details.
A Boeing spokesperson said, "We will address all regulatory requirements, technical needs and testing requirements."
In comments to EASA released on Wednesday January 27, Virginie Fricaudet, who lost her sister on Ethiopian flight 302 and who heads a France-based relatives association, said that the MAX is "aerodynamically unstable" and should have a third sensor.
Naoise Ryan, who lost her husband, the global deputy chief engineer of the UN World Food Programme, in the same crash, called the MAX a 'bastard-type' aircraft with modern modifications bolted onto a 1960s aircraft design.
EASA acknowledged that the aircraft's technical roots would hinder the addition of complex new systems.
"Due to the legacy...architecture of the Boeing 737, the installation of an additional AOA sensor would require a significant engineering effort," it said, adding that Boeing has nonetheless demonstrated that its approach was viable and safe.