ANA to keep 787s grounded through May Japan's All Nippon Airways says all of its Boeing 787 Dreamliners will remain grounded until at least the end of May because of the plane's battery safety problems. ANA has more of the aircraft than any other carrier - 17 of the 50 that are operational. The financial cost of the grounding for Boeing and the airlines that own the jets has been estimated at 150 euros million a month. In January ANA lost around 11.3 million euros in revenue. No Dreamliners have flown for nearly five weeks and Boeing has not yet determined what caused the batteries to overheat on two planes last month. Despite that the planemaker has given US aviation regulators its plan to fix the problem. The proposal to the FAA includes measures to address a range of possible causes of short-circuits in the batteries, sources told Reuters. The proposed fix includes adding ceramic insulation between the cells of the battery to help keep cells cool and prevent a "thermal runaway" in which one cell overheats and triggers overheating in adjacent cells. It also includes building a stronger, larger stainless steel box with a venting tube to contain a fire and expel fumes outside the aircraft should a battery catch fire again, the sources said. In addition, the plan proposed wiring changes, self-torquing screws that will not come loose and battery alterations to prevent moisture and vibration problems, one of the sources said. But there was also a plan to use a different battery type or some other longer-term fix, the sources said. European rival Airbus had already signalled its plan to switch back to more traditional nickel cadmium batteries for its A350 airliner, but the 787 is far more dependent on electrical power, which would complicate any effort to change to a different type of battery. More about: Air transport, Boeing, Japan