Some regulators around the world plan to complete their own review of the Boeing 737 Max, threatening to set back the company's anticipated timeline for the aircraft's service return, Boeing chief executive Dennis Muilenburg said on 11 September.

Muilenburg insists Boeing continues "targeting" the aircraft to receive regulatory clearance to fly "early in the fourth quarter".

But he concedes apprehension among regulators worldwide, and notably in Europe, could throw off the fourth quarter target.

"The risk to that [timeline] continues to be regulatory alignment… around the world," Muilenburg tells attendees at a Morgan Stanley investor conference.

He notes the European Union Aviation Safety Agency (EASA) "has brought up some question… we are working our way through," including queries about the 737 Max's angle-of-attack system.

"It creates timeline uncertainty," Muilenburg adds. "A phased ungrounding of the airplane among regulators around the world is a possibility."

His comments come as EASA confirms it will not rubber stamp the Federal Aviation Administration's certification of the Max. Instead, EASA will scrutinise the aircraft itself and perform test flights to EASA standards with its own pilots.

"EASA intends to conduct its own test flights separate from, but in full coordination with, the FAA," that agency says. "The test flights are not scheduled yet. The date will depend on the development schedule of Boeing. We will send our own pilots."

The additional scrutiny marks a break from the historic precedent of regulators from one country delegating aircraft certification to the country in which an aircraft is manufactured.

The European regulator is also eying the 737 Max's angle-of-attack (AOA) indicator system, which uses only two AOA indicator vanes.

"EASA has concerns regarding the consequences of angle-of-attack sensor failures… and the ability of the flight crews to cope with the situation in critical phases of flight, such as take-off," EASA says.

European airworthiness regulations do not require three AOA vanes, but two are "the bare minimum requirement to meet the safety objectives", EASA says. "An architecture with three vanes can more-easily be found compliant with the regulation."

EASA says Boeing must prove its two-vane system meets safety standards. It could possibly do so

through changes to "flight crew procedures and training, or through design enhancements", the agency says.

Muilenburg stressed the same on 11 September.

He says EASA's AOA concern "doesn't necessarily mean" Boeing must change AOA hardware, which could be a lengthy effort. Rather, changes to processes, software or training may be sufficient, he adds.

"We have had good, productive discussions with EASA," Muilenburg says. "We have had EASA flying in our simulators."

The company is now testing a final version of a revised flight control software and is preparing to roll out a new pilot training package, according to Muilenburg.

"We are making solid progress on the software update to the airline," he says. "We are working through all the deliverables with the FAA."

In the months after the March crash of an Ethiopian Airlines 737 Max 8, Boeing redesigned the manoeuvring characteristics augmentation system (MCAS) to prevent the type of wayward activation that played a major role in two crashes. The other crash, in October 2018, involved a Lion Air 737 Max 8.

Boeing then began making broader tweaks to the aircraft's flight control software to address regulatory concern about a data processing bug, the FAA has said.

Meanwhile, a group of global regulators, working as part of an FAA Joint Authorities Technical Review panel, is preparing findings related to a review of the Max's certification.

The panel could release those findings any day.