The US National Transportation Safety Board says is investigating a December flight in which a United Airlines plane nose-dived to within less than 250 metres of the ocean surface after taking off from Hawaii. The NTSB said it expects to issue a preliminary report in about two weeks. The agency said that it had been asking United questions about the incident before deciding whether to launch a formal investigation.

The Boeing 777 dropped more than 470 m. in 18 seconds before regaining altitude and completing the Dec. 18 flight from Kahului Airport on the island of Maui to San Francisco, according to data from tracking service Flightradar24. No injuries were reported.

The entire incident reportedly lasted 45 seconds and the passengers had to withstand a 2.7G force of a plane descending at a speed of 2,600 m. per minute.

Chicago-based United said it is cooperating with authorities and the pilots are currently receiving additional training.