WASHINGTON - Failures to spot and anticipate safety flaws during certification of new aircraft have been linked to 70 percent of U.S. airline-crash deaths in the past 20 years, according to data compiled by Bloomberg.

Boeing's tests finding the 787 Dreamliner's lithium-ion batteries couldn't catch fire are renewing questions about whether complexity of new aircraft can outpace manufacturers' and regulators' ability to spot problems during design and certification.

"We don't know what we don't know," Bernard Loeb, who retired as head of the National Transportation Safety Board's aviation division in 2001, said in an interview. "We're still highly dependent on the knowledge and capability of the human being, and human beings are fallible."

Improved certification standards have been one reason there hasn't been a fatal U.S. crash involving a major airline since 2001, NTSB Chairman Debbie Hersman said in an interview.

"But there are occasions where those assumptions are incorrect or not conservative enough."

In the absence of regulations for planes and components using new technology, the Federal Aviation Administration creates rules known as "special conditions," as it did in certifying the Dreamliner's batteries in 2007.

That approval, which the NTSB will examine at a hearing next month, illustrates the need to modernize standards for approving new aircraft, Kevin Hiatt, president of the Alexandria, Va.-based nonprofit Flight Safety Foundation, said in an interview.

Boeing is confident in its 787 battery fix proposal, Chairman and Chief Executive Officer Jim McNerney said at a conference in Washington Thursday.

Boeing plans to conduct a flight test with the revamped battery within days, McNerney said.

The history of airline accidents since 1993 is dominated by cases in which manufacturers and aviation regulators didn't foresee how a plane might fail, according to NTSB accident findings and its 2006 report on the issue.

Five such crashes occurred in that period, according to NTSB findings, including the three most deadly of the era: USAir Flight 427 on Sept. 8, 1994, killing 132; Trans World Airlines Inc. Flight 800 on July 17, 1996, killing 230; and American Airlines Inc. Flight 587 on Nov. 12, 2001, killing 265 people.

Investigators in those cases discovered a hidden flaw in a hydraulic device that could send a plane plunging out of control, explosive fuel tanks exposed to sparking electrical equipment during routine operation, and vulnerability to icing in a plane approved to fly in weather conditions conducive to ice formation.