(Reuters) — All Nippon Airways, the biggest customer for Boeing Co.'s grounded 787 Dreamliner, will put its pilots through training to resume flights in June, sources told Reuters, after Boeing completed more than half of its tests to get its new battery system certified.

The Japanese carrier, known as ANA, is also likely to use the Dreamliner initially for cargo flights once the new battery system is installed, to reassure the public about safety before restarting passenger flights, one of the sources said.

Regulators grounded all 50 Dreamliners in use by airlines worldwide in mid-January after lithium-ion batteries overheated on two separate aircraft, on a Japan Airlines jet parked at Boston's Logan airport and on an ANA flight in Japan. ANA operates 17 of the carbon-composite jets and has cancelled more than 3,600 flights through the end of May.

Anticipating regulatory clearance, ANA will put its roughly 200 Dreamliner pilots through flight resumption simulator training so that they will be ready to fly the jets again in June, the sources with knowledge of ANA's operations said.

Since the Dreamliner was grounded, the pilots have been undergoing simulator training every month, but their next training will be specifically for flight resumption, the sources said. The training will start around mid-April, one of the sources said.

"The company is making as many assumptions as it can and is preparing based on them. In order to resume flights from June, it needs all 200 of the pilots ready to be flying by then," a source said.

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The sources declined to be identified because they were not authorized to speak publicly about the matter.

Without having found what caused the battery incidents in January, Boeing last month unveiled a new battery system and predicted the 787 could be back in the air within weeks, which drew skepticism from some experts and regulators.

ANA said last week that it was including Dreamliner jets in its June flight schedules.

"It's not that we have decided to resume flights, but rather that we have not decided on cancelling flights," spokesman Ryosei Nomura told Reuters.

He added that he had not heard anything about the flight resumption simulator training.

Boeing is conducting ground and flight tests to check the new lithium-ion battery system that it plans to install in the Dreamliner jets. The results will be submitted to the U.S. Federal Aviation Administration, which will decide whether to certify the fix.

"More than half of the testing is complete with the remaining ground and flight tests set to occur within the next several days," Marc Birtel, a spokesman for Boeing, told Reuters by email.

Boeing is planning to conduct one more test flight, and the data collected from the flight will be submitted to the FAA. Once the FAA certifies the fix, Boeing will have its engineers install the new battery system in the grounded jets.

"Our baseline plan is to deliver the new battery systems in roughly the same order as initial deliveries," Mr. Birtel said.

ANA, as the launch customer, will be the first to have its jets fixed.

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It is still unclear how long the FAA will take to approve Boeing's battery fix. After the FAA's certification, Japan's Civil Aviation Bureau is likely to certify the fix around the same time.

The U.S. National Transportation Safety Board, which is investigating the Dreamliner's battery trouble, is conducting a two-day forum on April 11-12 to examine the design and performance of lithium-ion batteries in transportation, as well as a separate hearing on April 23-24 on the 787 battery.

The NTSB is likely to make non-binding recommendations to the FAA at the end of an investigation. The two agencies work closely together.

A few dozen Boeing engineers are already in Japan so that they can start work on the battery fix as soon as approval is received, the sources said. ANA estimates it may take a month to install the new battery system for its 787 fleet.

Once the new systems are installed, ANA is likely to bring the Dreamliner back into the air first by flying a domestic cargo route between Tokyo and Naha, in the southern islands of Okinawa, one of the sources said.

"By making a track record, the company wants to provide a sense of security to passengers. What it is concerned about is whether passengers will fly the Dreamliner like they did before," the source said.

ANA spokesman Mr. Nomura said that initially flying the Dreamliner to carry cargo was among the carrier's options.

Before the grounding, ANA used the Dreamliner about twice a week to carry cargo between Tokyo and Naha.

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"We will probably conduct test flights before carrying passengers onboard. Some, though not all, of the pilots will have to fly in order to keep their pilot's qualifications," Mr. Nomura said. He declined to comment on details of the possible test flights.

Other airlines have so far kept the Dreamliner out of their flight schedules. United Continental Holdings has removed its six Dreamliner aircraft from its flying plans through June 5. Poland's LOT, which has received two of the jets, said it does not plan to fly its Dreamliners until late October.

The president of Japan Airlines, which owns seven Dreamliner jets, said last month that he was not thinking of exactly when flights would resume.