JAL resumed 787 service but more problem was found [Update 3]

[Update: Added AP report to the end of the post]
[Update 2: Nikkei confirms the repair was done by Boeing workers]
[Update 3: Added JAL explanation quote and plane registration info]

Japan Airlines (JAL) has resumed 787 service on June 1 2013 after four and a half months of grounding. The first 787-8 (788) flight was JL035 from Tokyo Haneda (HND) to Singapore (SIN). It was operated with JA825J, which happens to be the first 788 delivered to JAL. It departed at 01:02 from gate 113 and took off from runway 34L 01:23 and landed at SIN 5 minutes ahead of schedule. Japanese aviation site Aviation Wire has an article on JAL and ANA's first scheduled 788 service with some pictures. Above is JA828J departing from HND on June 1, I assume that's flight JL021 to Beijing (PEK).

JAL resumed 787 service with JL035 from Haneda to Singapore and it landed ahead of schedule according to status posted on Japan Airlines website.

However, the service resumption did not go as smooth as one has hoped. JAL has found pressure difference inside and outside the battery compartment in one of the 787s and had to take that out of service for further investigation.

JL021 was delayed due to battery problem found on the 787. JAL swapped the plane with a 767 according to its website.

According to Japanese news site jiji.com (via Yahoo!), the JAL discovered this battery problem during pre-flight check for JL021 from HND to PEK. JAL immediately took the plane our of service for investigation. Later, JAL found a tape used during the battery repair is still attached to the container and believe that's what caused the problem in the first place. JL021's departure was delayed for 1 hour 5 minutes as a result and JAL has changed the equipment back to 767.

Below is the explanation posted on JAJL 787 Modification and Flight Status page (Japanese, English):

JAL21 from Tokyo (Haneda) to Beijing departing at 09:10 has been operated by Boeing 767 instead of Boeing 787 due to aircraft inspection (see below). Return flight JAL22 from Beijing to Tokyo (Haneda) departing at 16:40 has also been operated by Boeing 767.
For other flights, no departure delay or flight cancel caused by airplane trouble and no battery problems were reported.

- During turn-round inspection between previous arrival and next departure, it was found that the indicator which showed the difference in pressure between inside and outside of the APU (Auxiliary Power Unit) battery enclosure box has activated, therefore the inspection was conducted.

As the result of the inspection, no battery failure was observed, but it was found that the two small holes (*) which are bored on the battery enclosure box were both obscured. The relevancy between holes obscured by tape and the indicator activation is under investigation.

This situation does not affect the battery system function or the gas venting function.

(*) The holes are adjusting the air flow so that the gas generated in battery failure can be vented effectively.

jiji.com reported that the plane that was taken out of service flew a roundtrip flight between Tokyo and Singapore on June 1 and no problems were found. Looking at the schedule, looks like it is the plane that operated JL719 from Tokyo Narita (NRT) to SIN and JL036 from SIN to HND the day before. [Update: Japanese site Aviation Wire confirms the plane in question was used to operate JL719 the day before and arrived HND at 05:40 without incident. The plane registration was JA827J. A total of 132 passengers were affected (load factor of ~70.0%). According to JAL website (Japanese, English), the modifications were performed on May 6 at Tokyo Narita (NRT) and confirmation flight was completed on May 11.]

While it's not clear who forgot to remove the tape, I believe the batteries were repaired by Boeing workers [Update: Nikkei confirms it's Boeing workers. See note below]. If that's the case, it is not the first time problems were found due to poor workmanship. 2 weeks ago, one of ANA's 787 electric distribution panel overheated and blackened during a test flight. Later, ANA found out that a nut wasn't properly tightened by Boeing workers. Luckily it did not cause any accident during the test flight.

JAL has finished inspection on the plane in question and it will return to service on June 3 to operate JL035, the red-eye service from HND to SIN. 

However this brought into question of the quality of the repair done to rush the 787 back into service. Hopefully this is an isolated incident otherwise it will be harder to convince people it is safe to fly on the 787 again.

[Update: According to AP (via usnews.com), there was a censor problem. It did not mention any tape being left behind. It's possible that jiji.com has a more updated story.]

[Update 2: According to Japanese news site Nikkei, JAL confirmed it was Boeing workers who left the tape behind. The tape was used during the repair to cover holes on the container so that they can adjust the airflow. JAL said no problems were found in the battery itself and it did not affect flight safety.]