WASHINGTON (Reuters) - The U.S. Federal Aviation Administration hopes civil aviation authorities around the world will decide at about the same time to allow the Boeing 737 MAX to resume flying, the agency told Congress on Tuesday in a letter seen by Reuters.

FILE PHOTO: Grounded Boeing 737 MAX aircraft are seen parked in an aerial photo at Boeing Field in Seattle, Washington, U.S. July 1, 2019. REUTERS/Lindsey Wasson

The FAA and other regulators grounded the plane in March after two fatal crashes in five months killed 346 people.

Acting FAA Administrator Dan Elwell said in letters to Senators Susan Collins and Jack Reed that the agency “hopes to achieve near simultaneous approval from the major civil aviation authorities around the world” but added that every regulator will make its own determination.

“We are working with our colleagues from the European Union, Canada and Brazil to address their concerns,” he wrote.

Collins will chair a Senate Appropriations subcommittee hearing Wednesday that will feature four senior FAA officials, including Ali Bahrami, who oversees aviation safety.

Boeing Co Chief Executive Dennis Muilenburg told analysts last week he was confident the MAX would be back in service as early as October after a certification flight in “the September time frame.”

Ryan Air Chief Executive Michael O’Leary said Monday that he had been told that flight would be delayed until October. Boeing on Tuesday reiterated Muilenburg’s recent comments.

Elwell said in his letter the FAA “will lift the 737 MAX grounding order only when it is safe to do so.”

The MAX’s return has been delayed as Chicago-based Boeing works to win approval for reprogrammed stall-prevention software and related training materials.

In late June, the FAA said it had identified a new risk as an agency pilot was running a flight simulator test seeking to intentionally activate the so-called MCAS stall-prevention system.

Boeing has said it is working on a fix to address the problem.

The FAA’s Technical Advisory Board, made up of experts not involved in the original 737 MAX certification, is also reviewing the MCAS software update and training requirements.

The European Aviation Safety Agency sent FAA and Boeing a list of concerns it wanted addressed before the MAX re-enters service, people familiar with the matter said.

Since the crashes, federal prosecutors, the Transportation Department’s inspector general, Congress and several blue-ribbon panels have been investigating how the FAA certifies new aircraft and its longstanding practice of delegating certification tasks to airplane manufacturers.

Elwell noted in his letter that on March 5 he created a new Aviation Safety Organization office on delegating authority. That office is in the process of selecting staff and is developing procedures “to conduct this important mission. No substantive changes to the existing (delegation) program have been made as a result of standing up this office.”