WASHINGTON (Reuters) - A United Airlines Boeing 787 jetliner on a January 24 flight bound for Washington from Lagos suffered a failure of two key parts for navigation before a sudden unexpected aircraft movement led to passenger injuries, investigators said.

The National Transportation Safety Board said the left and then right inertial reference units failed, which was followed by the autopilot disconnecting automatically and sudden altitude changes. There were 16 injuries, including one serious injury and the flight safely returned to Nigeria.