Boeing has submitted updated software for the 737 MAX jet's anti-stall system to the Federal Aviation Administration for review and certification testing, but it's unclear when the troubled airliner will return to commercial service.

Federal regulators want additional information about how pilots respond to cockpit controls and displays in varying flight conditions using the new software before beginning certification test flights.

"It's impossible to estimate how long that might take," Robert Mann, president of R.W. Mann and Company, an aviation consulting firm, told China Daily. "This will be a fairly complex process and the software is just step one. It's impossible to know what other jurisdictions will do. Historically, it would be just a rubber stamp but that won't happen under current circumstances."

Some analysts expect MAX jets to return to service by August, but that assumes quick approval of the updated software, quick installation and expeditious safety checks to bring the grounded aircraft out of storage and ready them for flight.

The FAA did not respond to a request for comment.

Boeing MAX jets were grounded worldwide following crashes March 10 in Ethiopia and October 29, 2018 in Indonesia that killed a total of 346 passengers and crew. Preliminary investigations suggest the aircraft's Maneuvering Characteristics Augmentation System (MCAS), an automated anti-stall device, apparently forced the noses of the Lion Air and Ethiopian Airlines flights down and into a fatal plunge when it erroneously determined the planes were about to stall. To avoid a stall, MCAS points the nose of the plane down to gain air speed.

Critics have said the FAA relied too heavily on Boeing's engineers when certifying the aircraft. At a Congressional hearing last week in Washington, Daniel Elwell, acting administrator of the Federal Aviation Administration, said review of the plane's safety took five years to complete and while the regulatory agency relied on Boeing for technical details, the FAA made the final decision to certify the plane.

"In the US, the 737 MAX will return to service only when the FAA's analysis of the facts and technical data indicate that it's safe to do so," Elwell told members of the US House Aviation Subcommittee last week. He did not say when that might be.

There are about 400 MAX jets in service worldwide, including about 97 in China. Boeing has met with Southwest, American and United — the US airlines that fly the 737 MAX — and European carriers to discuss safety and software updates for MCAS. Wall Street analysts expect Boeing to spend about $1 billion to update and install new software worldwide for the anti-stall system.

Boeing said it has flown the updated MCAS software on 207 flights totaling about 360 hours to test its reliability.

"We're committed to providing the FAA and global regulators all the information they need, and to getting it right," Dennis Muilenburg, Boeing's chief executive officer, said in a statement. "We're making clear and steady progress and are confident that the 737 MAX with updated MCAS software will be one of the safest airplanes ever to fly."

Boeing said it also has developed improved training materials now being reviewed by the FAA and regulators around the world.

Boeing has said flight simulator training is not necessary for the 737 MAX and federal regulators do not require it. Nevertheless, some airlines purchased flight simulators, land-based versions of a cockpit that replicates flying complete with hydraulics to mimic the aircraft's pitch and roll under specific conditions.

The New York Times reported that the flight simulators failed to replicate the conditions created by the anti-stall system at high speed. Boeing said it made software improvements to better reproduce the experience of flight under severe conditions. Boeing does not manufacture flight simulators.

"It would be hard to replicate the feel of the downed planes because there are lots of factors involved — especially inputs by pilots," John Cochran, president of Eaglemark, an aviation consulting firm, told China Daily. "There's data from the flight recorders, but you'd have to match all parameters, including altitude, speed and air pressure. It's difficult to go from an aircraft to a digital simulator, but you can get close."

Cochran said he expects Boeing 737 MAX jets to return to service by late August, assuming there are no unforeseen problems in the new software or certification by regulators around the world.

Ryanair, a discount airline serving Europe, said grounding of the 737 MAX means it will carry fewer passengers overall in older, less fuel-efficient airplanes.

New York investment bank Jefferies expects Boeing to delay introduction of a new airplane until 2028 from the originally planned 2025.