WASHINGTON, March 18 (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.