Boeing said in a statement that the fire was traced to a battery unit that helps power electrical systems when the engines are idle -- typically while a plane is being serviced or cleaned.

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Boeing said the fire was traced to a battery unit that helps to power electrical systems when the engines are idle -- typically while a plane is being serviced or cleaned.

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Each unit can also serve as a source of electrical power for the military, or emergency response, something always in short supply in forward operating positions.

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