WASHINGTON (Reuters) - Federal Aviation Administration acting chief Dan Elwell told lawmakers on Wednesday he expects Boeing Co to submit a software fix for the grounded 737 MAX involved in two fatal crashes for approval soon, and said he was concerned by the planemaker’s lengthy delay in disclosing a software anomaly.

At a congressional hearing, the chairman of the U.S. House Transportation and Infrastructure Committee told the FAA it must “get it right” in deciding when to allow the Boeing 737 MAX to fly again.

“The world is watching and the FAA and Boeing must get it right,” Democratic Representative Peter DeFazio said, adding the incidents have raised concerns about how the FAA certifies aircraft.

The Boeing 737 MAX plane was grounded worldwide in mid-March after two crashes in October and March killed 346 people.

Elwell said the agency expects to get the software upgrade and training update from Boeing in the “next week or so.” He said the FAA will only allow the plane to resume flights when it is “absolutely safe to do so ... It’s important we get this right,” Elwell said.

Elwell said Boeing should not have waited 13 months to tell the FAA that it inadvertently made an alarm alerting pilots to a mismatch of flight data optional on the 737 MAX, instead of standard as on earlier 737s.

Elwell said he was “concerned” by the delay. “We’re going to look into that,” Elwell said. “Thirteen months is too long.”

The FAA is planning a May 23 meeting in Fort Worth, Texas, with air regulators from around the world to update them on the reviews. U.S. airlines have canceled flights as a result of the 737 MAX grounding into August.

Elwell said he hopes the international aviation community will work together. “My hope is that they have the confidence in our work and our analysis to make their ungrounding decisions if that’s where the discussion is as close to our decision as possible,” he said.

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Democratic Representative Rick Larsen, who chairs the aviation subcommittee that held Wednesday’s hearing, said the FAA “has a credibility problem. The FAA needs to fix its credibility problem.”

Boeing has said its software upgrade and associated pilot training will add layers of protection to prevent erroneous data from triggering the system called MCAS.

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The system activated in the Ethiopian Airlines crash in March and also during a separate Lion Air crash in Indonesia in October. Elwell said Boeing should have included more details on MCAS in its own manuals on the Boeing 737 MAX.

Committee leaders said they still expect to call Boeing to testify at a future hearing but expressed some frustration they have not yet received any documents from Boeing as they probe what went wrong with the 737 MAX.

The U.S. planemaker has been trying for weeks to dispel suggestions it made airlines pay for safety features after it emerged that an alert designed to show discrepancies in Angle of Attack readings from two sensors was optional on the 737 MAX.

Erroneous data from a sensor responsible for measuring the angle at which the wing slices through the air - known as the Angle of Attack - is suspected of triggering a flawed piece of software that pushed the plane downward in two recent crashes.

Boeing said last week it only discovered once deliveries of the 737 MAX had begun in 2017 that the so-called AOA Disagree alert was optional instead of standard as it had intended, but added that was not critical safety data.

Boeing said a Safety Review Board, convened after a fatal Lion Air crash in Indonesia last October, corroborated its prior conclusion that the alert was not necessary for the safe operation of commercial aircraft and could safely be tackled in a future system update.

Federal prosecutors, the Transportation Department’s inspector general and lawmakers are investigating the FAA’s certification of the 737 MAX 8 aircraft.