Europe’s aviation safety agency, which is conducting its own independent review of Boeing’s grounded 737 MAX, is not satisfied with a key detail of Boeing’s fix to the jet. It wants Boeing to do more to improve the integrity of the sensors that failed on the two fatal crashes in Indonesia and Ethiopia, killing 346 people.

And it’s demanding that Boeing demonstrate in flight tests the stability of the MAX during extreme maneuvers, not only with Boeing’s newly updated flight-control system but also with that system switched off.

These were among the disclosures in a presentation Tuesday to the European Parliament by Patrick Ky, executive director of the European Union Aviation Safety Agency (EASA). Ky listed what appear to be more stringent EASA requirements than those of its U.S. counterpart, the Federal Aviation Administration (FAA).

Boeing has publicly said it hopes for FAA clearance for the MAX in October so that it can return to passenger service in the U.S. this year.

Typically, overseas regulators follow the FAA’s lead. But after the MAX crashes revealed shortcomings in the FAA’s certification process, that’s no longer certain.

One of Ky’s slides cited a letter EASA sent to the FAA on April 1, less than three weeks after the MAX was grounded, that laid out four conditions for it to allow the MAX to return to service.

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The first condition stipulated is, “Design changes proposed by Boeing are EASA approved (no delegation to FAA).”

The second is that EASA complete an “additional and broader independent review” of the aircraft, beyond the specific design changes to the flight-control system that went haywire on the crash flights.

If the FAA moves ahead and clears the MAX to fly while EASA holds off until later, it would create an unprecedented divergence in worldwide regulation that would gravely complicate the schedules of many airlines flying internationally.

FAA approval would apply only to U.S. airlines flying domestically. European airlines flying the MAX, such as Norwegian Air, require clearance from EASA.

And it will put Boeing in a very awkward position if the FAA says the MAX is safe to fly while others hold back approval.

Both MAX crashes were initiated by faulty sensors that measure the plane’s Angle of Attack, the angle between the oncoming air flow and the wing. That fault then activated a new flight-control system — a piece of software known as the Maneuvering Characteristics Augmentation System (MCAS) — that on each of the crash flights repeatedly pushed the nose of the jet down.

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Although Boeing has updated MCAS so that it now takes input from both Angle of Attack sensors on the MAX instead of only one, and won’t operate if they disagree, Ky indicated EASA finds this insufficient.

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One of his slides states that while Boeing’s proposal has improved the Angle of Attack system, there is “still no appropriate response to Angle of Attack integrity issues.”

On Thursday, EASA elaborated a little via email:

“We can confirm that EASA is not yet satisfied with the proposed solution by Boeing on the improved architecture and logics for the AOA system,” the agency wrote. “We are following a methodical approach to assess the overall safety of the flight control and associated functions of the aircraft, as well as the pilot interaction with the systems, to take account of the human factors involved.”

And EASA wants stringent flight tests that prove the MAX’s safety with or without MCAS.

Boeing engineers designed the original MCAS to smooth out the feel of the yoke in the pilot’s hands during certain extreme high-speed turn and stall maneuvers.

Before the MAX is cleared to fly passengers again, both EASA and the FAA will require flight tests of the new updated software. In addition, Ky said, EASA will require Boeing to demonstrate the stability of the jet in flight tests that include high-speed turn and stall maneuvers with MCAS switched off.

The latter requirement should go some way to satisfying one gnawing public concern about the MAX. On the Internet, many Boeing critics have expressed concern that the jet is “inherently unstable” with engines that are too big, and that a software “band-aid” isn’t good enough to fix that. The EASA requirement to fly safely without MCAS should demonstrate otherwise.

On Wednesday, the FAA declined to clarify if the EASA requirements are stricter or in line with its own.

“We aren’t going to comment on specific details about ongoing discussions,” the FAA said in a statement. “The FAA has a transparent and collaborative relationship with other civil aviation authorities as we continue our review of changes to software on the Boeing 737 MAX … Each government will make its own decision to return the aircraft to service based on a thorough safety assessment.”

A safety official within the FAA, who asked for anonymity because he spoke without agency approval, said that the U.S. regulator has worked through the MAX approval process, looking for system flaws “with a fine-tooth comb, like they never have before.”

“People know it’s perhaps something they should have caught the first time around,” he said. “They want to make sure it doesn’t happen again.”

Still, the official wasn’t aware of any lingering concern at the FAA with the Angle of Attack sensor system. He said that the software and system changes Boeing has proposed have been all but agreed upon within the FAA and that only the level of pilot training that will be required remains undecided.

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While U.S. pilots have said they are satisfied that some computer-based training is sufficient, overseas regulators may require full flight-simulator training. The FAA official said that both EASA and India’s aviation regulator, the Directorate General of Civil Aviation, have so far balked at agreeing to computer-based training alone.

Ky’s presentation confirms that, for EASA, the amount of pilot training required before the MAX flies passengers is still “a work in progress.”

Ky said that EASA communicated to Boeing and the FAA in July a list of significant technical issues, which included system failures insufficiently monitored; forces needed to move the manual trim wheel too high; and a risk of crew confusion in some failure cases, especially an Angle of Attack single failure at takeoff.

A slide presenting the “latest status” of the process indicates that the pilot training and Angle of Attack system remain in play.

In a statement Wednesday, Boeing declined to comment on discussions with regulators. “We continue to work with the FAA and global regulators on addressing their concerns in order to safely return the MAX to service,” the company said in a statement.

On Tuesday, Alexandre de Juniac — head of the International Air Transport Association, the global trade group representing the world’s airlines — told Reuters in Chicago that “with the 737 MAX we are a bit worried … because we don’t see the normal unanimity among international regulators that should be the case.”

“We see a discrepancy that’s detrimental to the industry,” de Juniac added, urging regulators to make any changes to the single certification process “collectively,” according to Reuters.

Ky’s parliamentary presentation the same day, also briefly cited by Reuters, made that discrepancy plain.