The NTSB said it will investigate a December flight in which a United Airlines plane descended to within less than 800 feet off the ocean surface after taking off from Hawaii. The NTSB said it expects to issue a preliminary report in two to three weeks. The agency said it was asking United questions about the incident before deciding whether to launch a formal investigation. The Boeing 777 dropped more than 14 hundred feet before regaining altitude and completing the December 18th flight from Maui to San Francisco.