MONTREAL (Reuters) - The U.S. Federal Aviation Administration invited continued feedback from global regulators at a Boeing 737 MAX briefing on Monday about the steps needed to return the grounded passenger jet to flight after two fatal crashes.

FILE PHOTO: An aerial photo shows Boeing 737 MAX aircraft at Boeing facilities at the Grant County International Airport in Moses Lake, Washington, September 16, 2019. REUTERS/Lindsey Wasson

The closed-door meeting, on the eve of a United Nations aviation assembly in Montreal, brought together representatives from more than 50 countries with airlines that fly that MAX and those that will have incoming flights of the aircraft.

Airlines have urged regulators to coordinate with one another in a bid to avoid damaging splits over safety as they evaluate software changes undertaken by Boeing Co to return the MAX to flight.

Some countries have already vowed to run their own independent validation studies before restoring flights.

FAA Administrator Steve Dickson told the regulators in Montreal “accidents in complex systems rarely are the result of a single cause; rather, they often happen due to a complex chain of events and interaction between man and machine.”

He said all countries must “foster improvements in standards and approaches for not just in how aircraft are designed and produced, but how they are maintained and operated.”

Boeing’s best-selling jet was grounded globally in March, days after the crash of an Ethiopian Airlines flight that followed a similar Lion Air disaster in Indonesia in October. The two crashes took 346 lives.

The U.S. manufacturer has spent months working to update critical flight control software at the center of both crashes, in hopes of winning FAA approval for the planes to fly again in the United States between October and December.

The European Union Aviation Safety Agency (EASA), which is examining the 737 MAX design, said recently there was still no appropriate response to issues with the integrity of the aircraft’s angle of attack system.

In both crashes, erroneous data triggered the activation of an automated system that repeatedly pushed down the plane’s nose.

Paul Njoroge, who lost his wife, three children and mother-in-law in the Ethiopian crash, and Chris Moore, who lost his daughter, held pictures of victims outside the meeting.

The FAA has said it wants input from EASA and international regulators from Canada and Brazil before it conducts a certification test flight, a key step before final approval.

At the meeting, the FAA’s top aviation safety official provided details on activities to certify the aircraft since the regulators met in Texas in May. A senior Boeing executive provided a technical briefing on company efforts to address safety regulators’ concerns, the FAA said.

Transport Canada said on Monday that human factors and pilot workload were among areas it was continuing to assess before allowing the return to service of the aircraft.

The 737 MAX grounding is not on the agenda of the assembly of the International Civil Aviation Organization, which sets global standards for 193 member countries and runs from Tuesday until Oct. 4.

However, the United States, Canada, Peru and Trinidad and Tobago jointly presented a paper urging the U.N. aviation arm to study how to improve minimum pilot training standards in what would be the first broad review of training requirements.

Separately, the U.S. Office of Special Counsel, an office that reviews whistleblower allegations, said the FAA appears to have been “misleading in their portrayal of FAA employee training and competency” in providing Congress information about some safety inspectors that were involved in assessing training requirements for the Boeing 737 MAX, according to a letter released Monday.

The letter was reported earlier by the Washington Post. The FAA said in a statement to Reuters it remains “confident in our representations to Congress and in the work of our aviation safety professionals.”