"As part of our ongoing efforts to determine the root cause of recent Boeing 787 lithium-ion battery incidents, the FAA will permit Boeing to conduct test flights of 787 aircraft to gather additional data," Transportation Secretary Ray LaHood and Federal Aviation Administration Administrator Michael Huerta said in a joint statement.
"The traveling public's safety is our highest priority. These test flights will be an important part of our efforts to ensure the safety of passengers and return these aircraft to service. ... The primary purpose of the test flights will be to collect data about the battery and electrical system performance while the aircraft is airborne," the statement said.
The flights will be conducted without passengers in defined airspace over unpopulated areas.
National Transportation Safety Board Chairwoman Deborah Hersman earlier said the FAA needs to rethink how it certifies batteries such as those used in the grounded 787 Dreamliner.
"The assumptions used to certify the batteries must be reconsidered," Hersman said.
Two fires aboard the new Boeing jets triggered the grounding of the 50 jets in commercial use after less than 100,000 hours of commercial flights, The New York Times reported Thursday.
The time element is significant, as Boeing had said in its certification application that the lithium ion batteries used in the jet -- and the source of the recent fires -- could be the source of a fire incident fewer than once in every 10 million hours of use.
Hersman said the investigation of the batteries "has demonstrated that a short-circuit in a single [battery] cell can propagate to adjacent cells and result in a fire." What is unknown, however, is what caused the short circuit in the single cell in the first place.
She said the short circuit resulted in a "thermal runaway," with the temperature running up to 500 degrees, then spreading through the rest of the battery.
"We have not yet identified what the cause of the short-circuit is. We are looking at the design of the battery, at the manufacturing, and we are also looking at the cell charging. There are a lot of things we are still looking at," Hersman said.
The FAA, meanwhile, might have taken a more cautious approach concerning the lithium ion batteries, since the agency has experience dealing with overheating issues in those batteries when they are used in cellphones and other devices.
In the case of the Dreamliner batteries, the agency relied on Boeing's tests, the Times said.
2014: The Year in Fashion [PHOTOS]
GM recalls 221,000 Cadillacs and Impalas