BOEING Co yesterday began installing reinforced lithium ion batteries on five grounded 787 jets owned by launch customer All Nippon Airways, starting a process that should make the first commercial Dreamliners ready to fly again in about a week.

Teams of Boeing engineers are working on the ANA jets at four airports in Japan, including Tokyo's Haneda and Narita hubs, Ryosei Nomura, a spokesman for the carrier said.

The Dreamliners have been parked since regulators in the United States and elsewhere ordered all 50 planes out of the skies in mid-January after batteries on two of them overheated.

ANA is the world's biggest operator of the carbon-composite aircraft with 17 of the planes. After ANA, the biggest 787 operator is local rival Japan Airlines Co with seven jets, followed by United Continental Holdings Inc's United Airlines and Air India with six each.

ANA plans about 100 to 200 round trip test flights in May of its repaired aircraft before carrying passengers again from June, sources familiar with ANA's operations said last week. The flights will check the safety of the aircraft, and allow ANA's 180 Dreamliner pilots to get accustomed to flying it again and renew their licenses after a break of over three months.

ANA has not said how much the 787's grounding has cost it to date, though it has said it lost about US$900,000 in revenue per plane in the last two weeks of January. The grounding has cost Boeing an estimated US$600 million.

The Federal Aviation Administration in the US on Friday approved a Boeing plan to encase the 787's lithium ion batteries in steel box, install new battery charges, and add a duct to vent gases directly outside the aircraft that could cause overheating.

Investigators in the US and Japan have yet to unravel what caused a 787 battery onboard an ANA jet in Japan and one on another JAL Dreamliner parked at Boston's Logan Airport to overheat.