Hersman has shown that same steely persistence over the last week as the public face of the investigation into the Asiana Flight 214 crash in San Francisco.

Since becoming the National Transportation Safety Board's youngest chairman at 39 in 2009, the daughter of a former air force test pilot has been at ease in front of television cameras while working behind the scenes to get her agency access to crash sites and witnesses.

The NTSB chairman is bound to rub some people the wrong way. Without the power to regulate, the chairman's authority is limited to investigating, recommending and, at times, scolding.

That can include pointing out the failings of federal agencies, local governments and major industries including airlines, carmakers and shipping operators.

Hersman was tough enough to make her points, while empathetic enough to maintain contact with families of crash victims long after investigations had been completed, said Mark Rosenker, who held the NTSB post before Hersman took over. "You're not going to make everyone happy," Rosenker said. "You're going to make some manufacturers and some operators uncomfortable. You do it in the belief you're going to make transportation safer."

Hersman is licensed to drive motorcycles and school buses, according to her official biography. While no Jedi, she knows something about warriors in fast aircraft. Her father is retired US Air Force Brigadier General Walt Hersman, who flew fighter jets in Vietnam and was an experimental test pilot.

Befitting her toughness and willingness to be unpopular if need be, she served as president of her homeowners' association for five years and has competed in triathlons.

She is married to Niel Plummer, a software engineer, and is raising three school-age boys in suburban Virginia.

Hersman first worked as a 19-year-old intern in Wise's congressional office while studying political science at Virginia Tech.

She became his senior legislative aide and later advanced to the staff of the Senate Committee on Commerce, Science and Transportation.

Along the way, she earned a master's degree in conflict resolution from George Mason University in Fairfax, Virginia.

As the senior Democrat staff member on the Surface Transportation subcommittee, she delved into railroad issues, trucking, buses and the maritime industries, said David Strickland, head of the National Highway Traffic Safety Administration, who worked on the committee at the time.

Both were hired by former South Carolina Senator Fritz Hollings, a Democrat known for being an "energetic, excitable, high-volume guy," Strickland said. "At his most energised moments, Debbie was always cool and calm and collected, and the senator loved her because she was so poised."

Hersman was named an NTSB board member by President George W. Bush in 2004 and is in her second two-year term as chairman, after being elevated to that role by President Barack Obama in 2009.

Her terms on the board and as chairman expire this year.

Hersman isn't averse to standing up to a powerful company if she perceives its actions as violating protocol or getting in the way of her agency's work.

The agency rebuked Boeing in March for holding a press conference in Tokyo about plans to get its grounded 787 Dreamliner flying again.

The company spoke without telling safety-board officials and provided its own conclusions about the causes of battery fires while the investigation was ongoing, it said.

In the Asiana investigation, Hersman has tangled with the Air Line Pilots Association, the world's largest union for pilots with more than 50,000 members at 33 airlines in the US and Canada.

The group accused Hersman of releasing information too early in the probe and feeding speculation about crew errors. Hersman didn't back down when asked about the union's criticism.

The agency was providing facts, while cautioning that no conclusions had been reached, she said. "We are the advocate for the travelling public," Hersman said. "We think it's important to show our work."

When Hersman suggested in December 2011 that distracted driving had become endemic in cars, buses, trucks, boats and aircraft - based on several crash investigations across all modes - she recommended a ban on handheld portable electronic devices, including mobile phones.

This year, Hersman's call to lower the standard for what qualifies as drunken driving from 0.08 per cent blood-alcohol content to 0.05 per cent drew the anger of the restaurant industry.

The industry argued Hersman was wrongly targeting average diners when the vast majority of fatalities are caused by habitual drinkers, said Sarah Longwell, managing director of the American Beverage Institute in Washington.

The agency cast its lot with "anti-alcohol zealots" instead of focusing on practical solutions, she said.

Senator Jay Rockefeller, a West Virginia Democrat and the current Commerce Committee chairman, pushed Obama to name Hersman US transportation secretary after Ray LaHood announced he would step down this year. Obama nominated Charlotte, North Carolina, Mayor Anthony Foxx.

Rockefeller advocated for Hersman "because she was so good," he said. "She's thoroughly energetic, thoroughly detailed and always accurate."

Even as a committee aide, Hersman showed the qualities needed to be a good NTSB chairman, Rockefeller said.

"You have to be methodical," he said. "You have to be detailed. You can't make mistakes. You have got to be fair, and she is. But she's also very, very tough."

Wise, who later became West Virginia's governor, has watched his former staffer this week and sees "vintage Debbie".

He said: "I'm watching today what I watched 20 years ago - a quiet but determined approach; wanting to know all the facts; listening to people, but also letting folks know the status quo isn't sufficient.''