As Japanese airlines begin installing the battery fix developed by Boeing for its grounded 787s, the carriers are turning their attention to flight tests and the eventual resumption of scheduled service with the aircraft.

Both All Nippon Airways and Japan Airlines confirm that the battery modifications on their 787s started today. The two rivals account for the majority of the 787s delivered so far, and they have been hardest hit by the aircraft’s grounding due to problems with its lithium-ion batteries.

An ANA spokesman tells Aviation Week that five support teams from Boeing are each working on one of the carrier’s aircraft, and the modification will take about a week per plane. All 17 of ANA’s 787s are scheduled to be finished by the end of May. The spokesman stresses that ANA still needs a revised Technical Circular Directive to be issued by the Japan Civil Aviation Bureau before it can begin test flights of its 787s.

Boeing has already conducted flight testing for the battery fix, and on April 19 the FAA granted approval for the modification design. A formal directive allowing 787s to return to flight with the modification is expected this week. JCAB will likely follow with its own directive.

Some news reports, quoting unnamed sources, stated that ANA plans to conduct 100-200 test flights on the 787s within Japan. However, the ANA spokesman says that “nothing is decided yet and we will continue to communicate closely with the Japanese agency and Boeing [as to] how many flights we fly before the 787 returns to service.”

ANA had already announced in February that it would extend the cancelation of 787 flights to May 31, to provide some certainty to its scheduling. The spokesman says this plan remains in effect, which signals that the 787s will return to scheduled service at the beginning of June.

This would allow 787s to resume flying on routes where they have been temporarily replaced with other aircraft. Frequencies have had to be reduced, and some routes cut altogether to accommodate the groundings.

Some of the international routes that have been suspended due to the groundings may take longer to be reintroduced, says the ANA spokesman. “It is not decided yet, but it seems we need more time to resume services to both Seattle and San Jose,” he says.

When ANA first announced that the 787 cancelations would extend to May 31, the carrier told Aviation Week that if the planes were cleared before then it may consider using some for special “relief flights,” to boost the domestic schedule where necessary. The ANA spokesman confirms there is still a possibility that such flights may occur during May.

Japan Airlines was operating seven 787s on international routes before the grounding. It has arranged for alternate aircraft on these routes through the end of May, and has also had to cut some frequencies. The carrier says it will make a decision about when its 787s will return to service after JCAB has issued its clearance.

The carrier has also yet to decide when test flights will begin, a spokesman says. JAL had to postpone the start of a new route to Helsinki due to the 787 problems. There is no word on a revised launch date.