Under fire from lawmakers on Capitol Hill over the two deadly Boeing crashes, the head of the Federal Aviation Administration on Wednesday defended the FAA's practice of relying on aircraft makers to help certify their own planes for flight.

Acting FAA Administrator Daniel Elwell said the strategy has "consistently produced safe aircraft designs for decades." And he said the agency would need 10,000 more employees and an additional $1.8 billion a year to do all the work now done by designated employees of the companies it regulates.

Under the self-certifying program, these employees perform tests and inspections needed to win safety approvals, with the FAA overseeing their work. The approach is credited with holding down government costs and speeding the rollout of new models.

But in the wake of the air disasters involving Boeing's new 737 Max jetliner in Ethiopia and Indonesia, that practice has been seized on as evidence of an overly cozy relationship between the FAA and the industry.

Sen. Richard Blumenthal, D-Conn., said at a Senate subcommittee hearing that delegating safety work to the companies puts "the fox in charge of the henhouse."

"The fact is that the FAA decided to do safety on the cheap, which is neither safe nor cheap," Blumenthal said. He vowed to introduce legislation to change the system.

At the same hearing, the Transportation Department's inspector general, Calvin Scovel III, said the FAA plans to significantly revamp its oversight of aircraft development by July. But the department gave no indication it intends to abandon the collaborative approach.

Scovel said the changes would include new ways to evaluate training and self-audits by aerospace companies.

In Seattle, Boeing said the process by which it designs, develops and tests planes has led to safer and safer air travel, and it sees no need for an overhaul.

The FAA and the industry say that deputizing private employees to do safety-related tasks is vindicated by the nation's safety record — one passenger accident death in the U.S. in the last 10 years and millions of flights. Elwell said the approach is "part of the fabric of what we have used to become as safe as we are today."

The Max, featuring bigger, more efficient engines, went into service in 2017. The software in a new automated system that can push the plane's nose down to prevent an aerodynamic stall is suspected of playing a role in the Oct. 29 crash of a Lion Air jet in Indonesia and has also come scrutiny in the March 10 plunge of an Ethiopian Airlines Max. In all, 346 people died.

Boeing is working on new software to be installed in the Max, which has been grounded worldwide in the meantime.

Sen. Ted Cruz, R-Texas, who chaired the hearing, said the "close relationship between industry and regulators" threatens to erode the confidence of the flying public.

Transportation Secretary Elaine Chao, who oversees the FAA, has asked Scovel to look into the way the agency certified the Max as airworthy. Also, the Justice Department is investigating possible criminal violations involving the airliner's certification.

In addition, the Transportation Department watchdog plans to examine the widespread use of automated systems to fly airplanes and whether pilots are being adequately trained for when such features fail.

Meanwhile, in Seattle, Boeing's vice president of airplane development, Mike Sinnett, outlined the changes he company is making to the flight-control system and pilot training, and he repeated Boeing's confidence in the safety of the Max.

Boeing also invited about 200 pilots from several airlines to its Seattle-area facility to explain to them the changes in the Max.

"We are working with customers and regulators around the world to restore faith in our industry and also to reaffirm our commitment to safety and to earning the trust of the flying public," Sinnett said.

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Koenig reported from Dallas, Krisher from Detroit. Associated Press writer Gene Johnson in Seattle also contributed to this story.