United Airlines Flight 1

United Airlines flight 1 encountered severe convective turbulence during cruise over the Republic of the Philippines, while enroute from San Francisco International Airport (KSFO), San Francisco, California, to Singapore Changi Airport (WSSS), Republic of Singapore. As a result of the turbulence, two flight attendants (FAs) sustained serious injuries, three FAs and one passenger sustained minor injuries. The airplane was not damaged, and the flight continued to its destination without further incident. The Civil Aviation Authority of the Philippines - Accident Investigation and Inquiry Board, delegated the investigation to the NTSB as the State of Operator and State of Registry.

A review of weather records revealed areas of scattered convection were present to the northeast of the accident location. The flight crew reported that prior to the accident the flight was smooth, and they had been deviating around small, scattered returns noted on the cockpit weather radar as they flew southwest toward the intended destination.

Figure 1. The most intense scattered convective activity noted north/northeast of accident site (data fusion satellite imagery provided by United Airlines).

The weather provider noted that there were no significant meteorological information (SIGMETs) or flight plan guidance advisories for turbulence or convective activity for the area, due to the scattered nature of the convection. Fusion satellite and lightning data showed isolated convective cells near the accident location and time, with lightning development and likely updrafts.

The flight crew reported that prior to the accident, the airplane was at flight level 400 in a thin cirrus cloud layer, there was no moonlight and the flight was smooth. The seatbelt sign was off. Immediately before the turbulence encounter, cockpit weather radar displayed no returns ahead of the airplane at any range. There were no messages or alerts about weather from dispatch, nor any indication of weather activity from their 3rd party data sources (Skypath and WSI). The flight crew described the turbulence as a sudden and severe jolt that lasted 3-5 seconds, during which it was difficult to read the instrument panel. Immediately after the turbulence subsided, the flight was smooth.

The FAs working in the aft galley (where the serious injuries occurred) described the turbulence as sudden and brief, the airplane dropped beneath them (or they were lifted off their feet) and they fell to the floor with severe force.

The other cabin crew solicited assistance from passengers with medical experience and rendered aid to those injured. They enlisted a non-revenue FA to replace one of the injured crew, to cover one of the doors during landing.

The captain coordinated with the cabin crew, dispatch and the remote medical advisory service, and the first officer. After considering a diversion to Manila, they chose to continue to WSSS, which was about 80 minutes away. The flight crew declared an emergency, landed uneventfully, and medical personnel met the airplane at the gate. One FA sustained a broken ankle requiring surgery and another was hospitalized for longer than 48 hours with a head injury.

Verbatim NTSB analysis and probable cause from the NTSB dataset