WASHINGTON (AP) -- Federal regulators are telling airlines they can fly Boeing's 787 Dreamliners again as soon as they replace its problematic lithium ion batteries with a revamped battery system.

A Federal Aviation Administration safety order posted online Thursday applies to all U.S. airlines, but only one airline - United - currently has 787s in their fleet. They have six. The FAA estimated the repair costs for those planes at $2.8 million.

The planes have been grounded since mid-January, following a battery fire on a 787 parked at Boston's Logan International Airport, and a smoking battery that led to an emergency landing by another 787 in Japan.

There are 50 of the planes in service worldwide, but Boeing has purchase orders 840 more planes. Newly delivered will come with the revamped system.