The U.S. National Transportation Safety Board (NTSB) is investigating why an Asiana Airlines Boeing 777-200ER arriving from South Korea undershot the runway at San Francisco International airport on July 6.

The aircraft, carrying 291 passengers and 16 crew, was operating flight 214 from Seoul to San Francisco and landed short of the threshold of runway 28L at 11:36 a.m. local time. The 777 was badly damaged in the initial crash landing and was subsequently gutted by fire that engulfed the fuselage. Passengers evacuated using principally the left doors L1 and L2, from which slides were deployed, as well as aft left doors. Doors were also opened on the right side of the fuselage though it is not yet known if the fire on the right side of the aircraft prevented any of them being used for evacuation. Of the 307 people onboard, 181 were taken to local hospitals, 49 of them with serious injuries. Yet, despite the severity of the impact and the ensuing blaze, there were only two confirmed fatalities and one person still unaccounted for seven hours after the crash.

The aircraft hit the low seawall which separates the airport from the waters of San Francisco Bay. Images of the debris field indicate the 777 made an initial impact to the right of the centerline, losing its tail section and parts of the landing gear before sliding down the runway and slewing off into the grass to the south of the normal touchdown area. Eyewitnesses report the 777 struck the wall ahead of the displaced threshold area in a nose high attitude, causing the entire empennage to detach aft of the pressure bulkhead. The vertical and horizontal tail were scattered in the displaced threshold area, just ahead of major sections of the landing gear.

Images of the wreck show the left Pratt & Whitney PW4090 engine had been dislodged from its mounting and most of the trailing edge and the outboard section of the leading edges on the left wing was missing. There was no immediate indication of where the remains of the left engine had come to rest. The right engine was visible lying alongside the fuselage forward of the wing root, while the right wing tip section also showed severe damage. The fuselage was buckled forward of the wing-body join as well as aft, close to the exposed rear pressure bulkhead which was also ruptured by the impact.

Weather at the time of the accident was good with light winds of 6-7 kt. from the southwest and visibility of 10 nm. or more. Investigators will focus on several areas including the performance of the aircraft’s equipment, engines, systems and flight crew, as well as other factors concerning the dynamics of the approach. One item of particular focus for investigators is expected to be the status of runway approach guidance facilities. An FAA Notam (notice to airmen) for San Francisco indicates that, at the time of the accident, the instrument landing system glideslope for runway 28L was declared out of service from June 1 to Aug. 22.

The Asiana accident represents only the third hull loss for the 777 since the aircraft entered service in 1995. The first of these was the January 2008 short landing accident of a British Airways 777-200ER at London Heathrow which was traced to fuel starvation caused by ice build-up in the engine’s fuel-oil heat exchangers. The second was a flight deck fire which occurred on an Egyptair 777-200ER whilst on the ground in July 2011. The Asiana accident is also the first involving a 777 which has resulted in a passenger fatality.

The Asiana aircraft written off in the accident, HL7742, was one of 12 777-200ERs operated by the Korean carrier. The aircraft first flew in February 2006 and was registered to the airline the following month.