WASHINGTON/SEATTLE (Reuters) - The U.S. aviation regulator will significantly change its oversight approach to air safety by July following two fatal Boeing Co MAX 737 passenger plane crashes, according to written congressional testimony seen by Reuters.

An aerial photo shows Boeing 737 MAX airplanes parked on the tarmac at the Boeing Factory in Renton, Washington, U.S. March 21, 2019. REUTERS/Lindsey Wasson

At a U.S. Senate Commerce subcommittee hearing on Wednesday, the Federal Aviation Administration (FAA) acting head Dan Elwell will say the agency’s oversight approach must “evolve” after the deadly crashes, according to the testimony.

While specific details on oversight changes were not clear, lawmakers are expected to question Elwell on how the regulator intends to change the process by which a manufacturer such as Boeing can to a large extent certify their own planes and flight software systems.

Anti-stall software on the Boeing 737 MAX plane is among the leading areas of focus for investigations into the two crashes. Investigators have pointed to “clear similarities” between the crashes, putting pressure on Boeing and U.S. regulators to come up with an adequate fix.

The aviation industry has been thrown into flux by a Lion Air crash in Indonesia last October that killed 189 people and an Ethiopian Airlines disaster on March 10 that killed 157, both involving Boeing’s 737 MAX single-aisle plane.

A spokesman for Ethiopia’s transport ministry, which is leading an investigation in Addis Ababa, said the preliminary crash report would very likely be released this week.

Boeing’s fastest-selling 737 MAX jet, with orders worth more than $500 billion at list prices, has been grounded globally by the FAA, although airlines are still allowed to fly them without passengers to move planes to other airports.

One such passenger-less Southwest Airlines Co Boeing 737 MAX 8 aircraft landed safely on Tuesday after declaring an emergency over an engine-related problem leaving Orlando, Florida, the FAA said. The issue was not related to a computer system on the 737 MAX, Southwest said.

Elwell’s testimony discloses that Boeing first submitted a proposed upgrade to its anti-stall software - the Maneuvering Characteristics Augmentation System, known as MCAS - to the FAA for certification on Jan. 21.

Boeing confirmed that in a statement on Tuesday, saying verification and certification flights took place on Feb. 7 and March 12, which is within the typical testing time period. The documentation to show FAA compliance was expected at the end of the week, Boeing said.

PILOTS WELCOME CHANGE

The Allied Pilots Association, which represents American Airlines pilots, said it welcomed steps toward increased safety.

“We’re also very interested in any changes to the FAA’s structure and certification process that would ensure that never again would a critical system like MCAS not be transparent to pilots,” the association said in a statement, highlighting some pilots’ concerns that they had been unaware of the existence of MCAS on MAX aircraft they were flying.

Chief Executive Dennis Muilenburg visited Boeing’s Seattle-area facilities on Tuesday and attended at least one all-hands meeting with engineers, two people familiar with the matter said.

Boeing is this week briefing airlines on software and training updates for the MAX, with more than 200 airline pilots, technical experts and regulators from around the world due to come to the Renton, Washington, facility where the 737 is assembled.

As well as FAA approval, any MAX software fixes will need a green light from governments around the world, a process that could take months.

Boeing’s software fix for the 737 MAX will prevent repeated operation of the anti-stall system and deactivate it altogether if two sensors disagree widely, two people familiar with pilot briefings told Reuters on Monday.

Upgrading an individual 737 MAX with Boeing’s new software only takes about an hour per plane, though the overall process could stretch on far longer as it is rolled out across the global fleet because of stringent testing and documentation requirements by engineers and regulators, according to a senior FAA official with knowledge of the process.

Boeing shares closed down slightly on Tuesday. They have lost about 12 percent and $29 billion in market value since the crash in Ethiopia.

Representative Peter DeFazio, who chairs the U.S. House of Representatives Transportation and Infrastructure Committee, sent a letter on Tuesday to the FAA, asking it to hold an independent, third-party review of Boeing’s proposed changes to the 737 MAX before it is returned to service.

The FAA’s review “must be thorough, deliberative and cannot be rushed,” he and Representative Rick Larsen wrote. The third party should include experts “to objectively advise” on the certification of “new and novel technology.”