Boeing spent much of the first half of last year battling problems with its 787 Dreamliner airplane — battery problems led to a number of fires, which led the plane to be grounded entirely in the US and Japan for months while the company worked on a fix. Unfortunately for Boeing, the 787's battery is back in the headlines today. Maintenance workers at Japan's Narita airport noticed smoke and an unidentified liquid coming from a 787's main battery two hours before the plane was scheduled to take off. According to Reuters, alarms in the cockpit confirmed there was an issue with the battery's power pack and charger.

Boeing is claiming that what these maintenance workers saw is all part of last year's battery's redesign. On its official Twitter account, Boeing wrote that the "787 issue on ground at Narita appears to involve venting of single battery cell during maintenance." A follow-up tweet claims that "improvements to 787 battery system appear to have worked as designed." What isn't clear yet is what exactly went wrong to cause the smoking battery in the first place — the airline is currently investigating the incident, and Boeing says that it is working with Japan Airlines to return the plane to service.

Update: The National Transportation Safety Board tweeted this afternoon that it's willing to help with the ongoing investigations in Narita: