U.S. aviation regulators expect to receive Boeing Co.’s proposed software fix for the grounded 737 Max as soon as next week and will then begin a review that will include test flights and input from a technical advisory board.

“We will not allow the 737 Max to fly in the U.S. unless it is absolutely safe to do so,” Daniel Elwell, acting administrator of the Federal Aviation Administration, told lawmakers Wednesday without offering an estimate for how long the review would take.

A technical advisory board named by the agency will provide a “third set of eyes,” and its recommendations will directly affect the timing of the grounded Max’s return to service, Elwell said at a hearing in Washington.

Boeing is also working on a service bulletin describing the flight control system revisions, Elwell told the House aviation subcommittee at a hearing on the status of Boeing’s best-selling jet, which has been grounded since March after two crashes in a five-month span.

The FAA has come under fire for approving a feature known as the Maneuvering Characteristics Augmentation System, or MCAS, and for giving the planemaker too much authority to oversee itself. After a sensor on 737 Max jets in Indonesia and Ethiopia malfunctioned, MCAS continually pushed down the aircraft nose until pilots lost control. Boeing is redesigning the system to make it less prone to operate in error.

Sensor Failure

Lawmakers grilled Elwell during the hearing, quizzing him about the certification process that allowed the plane to fly. His counterpart at the National Transportation Safety Board also appeared at the hearing.

Transportation and Infrastructure Committee Chairman Peter DeFazio, an Oregon Democrat, complained that Boeing hasn’t turned over any records sought by House investigators.

DeFazio said he was disappointed that a single sensor’s failure had led to the crashes. “We shouldn’t have to be here today,” DeFazio said.

Asked for comment on DeFazio’s statement regarding records, Boeing spokesman Charles Bickers said, “Boeing continues to support the ongoing accident investigations and is committed to working closely with Members of Congress, their staff and relevant officials. Safety is our top priority when we design, build, deliver and maintain Boeing aircraft.”

Elwell said the agency was directly involved in approving the flight-control system, participating in a test flight of the system that drove down the nose in the two accidents.

The FAA’s acting chief criticized Boeing for not disclosing to the FAA or to airlines for more than a year that a 737 Max display supposed to show whether a sensor was malfunctioning wasn’t working.

“I think that’s an issue, sir,’’ Elwell said under questioning by DeFazio. “It shouldn’t take a year for us to find out.’’

While Elwood expressed frustration with Boeing’s tardy disclosure, he said the so-called angle-of-attack sensor disagree light was “advisory” on the 737 and useful for maintenance teams—but wouldn’t have made a difference in either crash. The alert lights up when twin vanes that measure a plane’s nose against the air stream provide divergent readings to flight control computers.

‘They Deserve Answers’

DeFazio said the committee is still in the early stages of its review of how the plane was certified by FAA. But the tragedies are shocking, including for families of victims, he said.

“They deserve answers and accountability, as does the general flying public,” DeFazio said.

The highest ranking Republican on the committee, Representative Sam Graves of Missouri, urged caution before blaming Boeing for the accidents. In his opening statement, he listed what he called multiple errors by pilots and airline maintenance workers in the accidents that he said should be considered along with Boeing’s design.

“To focus on one single factor misses the forest for the trees,’’ Graves said.

The FAA is hosting a meeting of foreign aviation regulators May 23 in the Dallas-Fort Worth area and Elwell said part of the purpose was to repair what he called a perception that there is a “crisis of confidence’’ in the agency’s leadership.

Regulator Meeting

Because so many nations grounded the 737 Max in March before FAA felt it had enough data to do so, Elwell said the aviation system wasn’t as collaborative as it had historically been. The FAA plans at the meeting to discuss what it knows about the efforts to fix the plane and the steps it plans to take before approving it for flight.

The NTSB is assisting Ethiopian and Indonesian authorities in their investigations of the two crashes, Chairman Robert Sumwalt testified.

The 737 Max, Boeing’s best-selling aircraft, was grounded on March 13 after it became clear that an Ethiopian Airlines crash three days earlier had similar underlying causes to a Lion Air crash on Oct. 29 near Jakarta.

MCAS was added to the 737 Max to make it less likely to enter an aerodynamic stall. It automatically commands a relatively modest dive if it senses a plane’s nose has gotten too high. In the accidents, it repeatedly pushed down the nose despite efforts by the pilots to counter it. The crashes killed 346 people.

Senate Hearing

At a separate hearing Wednesday, senators pressed the nominee to become FAA administrator, Stephen Dickson, on whether the agency needs to change procedures that led to certification of the 737 Max. Several lawmakers focused on the use of aircraft manufacturers’ employees to sign off on designs.

“Whatever corrective actions need to be taken or process changes need to be put in place, I can guarantee you that those will be accomplished,’’ Dickson said.

He also said it’s “very important not to jump to conclusions.”

“Working with the private sector with the proper controls and protocols is going to allow the regulator to be much more effective and add a lot more safety value that just throwing extra resources at it,” Dickson said.