Boeing redesigned the wiring on the batteries of some early-production 787s, although the airframer says that there is no link between this and the battery failures that led to the global grounding of the aircraft in January.
The redesign was revealed by the Japan Transport Safety Board in an update to on the emergency landing of an All Nippon Airways 787 at Takamatsu airport, the event that triggered the grounding.
The JTSB says that the wing and tail navigation lights on the 787 involved in the incident remained on, even though the battery was switched off.
These lights only went off when the auxiliary power unit battery was disconnected, which led to the JTSB's finding that this battery was linked to the main battery bus.
"The design of this wiring has been changed after the 787 went into service," says the JTSB, adding that the wiring on this particular aircraft had not been changed.
"The main battery voltage data might be affected by the voltage of APU battery through the wiring. We, however, consider that the wiring had no relation to the occurrence of the incident, and that it did not compromise the safety of flight."
Boeing has reiterated the JTSB's finding that there was no relationship between this issue and the battery failure on the 787. It adds that no other cases of wiring issues were found during the investigation. A spokesman adds that only some early aircraft had this issue.
"Boeing designed a solution and asked airlines to incorporate it at the next convenient maintenance period, as this is not considered a potential safety concern," he says.
"If these lights stayed on for an extended period of time, they could drain the battery in excess of set limits, which would result in a maintenance message and would disable battery charging. The battery would have to be replaced."