July 15, 2013
Credit: Justin Sullivan
While NTSB investigators look closely into the actions of the flight crew for potential causes behind the July 6 Asiana Airlines Boeing 777-200ER accident, safety experts working on the other side of the cockpit door are already learning valuable new design lessons for crash survivability.
Although all but two of the 307 passengers and crew survived the ordeal, more than 180 were injured, several of them critically. Therefore, while looking into why the crash occurred in the first place, the investigation is focusing just as closely on why these injuries and deaths were sustained, and it has already unearthed troubling concerns about the functionality of the aircraft's main exit escape slides.
Video and eyewitness accounts testify to the violence of the aircraft's brief passage along San Francisco International Airport's Runway 28L before coming to an abrupt halt to the south side of the strip, adjacent to the touchdown zone markers some 1,500 ft. from the threshold. Although the aircraft did not cartwheel in the same devastating way as in the 1989 DC-10 crash in Sioux City, Iowa, it was massively damaged by an initial impact with the seawall and the displaced runway threshold, during which parts of the main landing gear and the entire empennage were ripped away. Traveling at over 100 mph, the aircraft departed the runway at the touchdown markers where it became partially airborne again, pirouetting in a complete 360-deg. circle around its nose section.
During this high-speed ground loop, the aft end of the tailless fuselage momentarily pitched up at around 40 deg., causing passengers and crew in the back section of the cabin to fall vertically as much as 100 ft. when the aircraft came to a rest with its nose pointed back toward the runway. Despite this pummeling, the fuselage structure remained substantially intact with the extensively damaged wings appearing to have borne the brunt of the impact loads. Although the forward two-thirds of the fuselage was gutted by the post-crash fire, the overall structural integrity of the cabin section was not immediately compromised by the impact itself, with significant buckling only evident in two zones: forward of the wing root and aft by Section 47/48 where the empennage was broken off.
The NTSB says the fire was caused by oil leaking from a ruptured tank onto the damaged remains of the No.2 (starboard) Pratt & Whitney PW4090 engine, which was ripped from its wing mountings and lay beside the fuselage. The left engine was detached during the initial ground roll and came to rest on the north side of the runway, just under 2,000 ft. from the threshold. While the fire makes it more difficult for investigators to assess the post-crash condition of the forward and mid-cabin sections, the intact aft cabin is yielding information about the survivability design aspects of seats, interior paneling, overhead bin structures, seat tracks, cabin floors, exits and escape slides.
Images released by the NTSB of the aft cabin, close to the buckled section by Doors L4 and R4, show how the seating, cabin floor and ceiling in some areas, were significantly damaged and dislodged. Already weakened by the initial impact and loss of belly skin and structure below Section 47/48, the bulk of the aft cargo hold and lower lobe structure beneath the floor of the aft cabin appears to have been either ripped away by the slide along the runway or crushed by the vertical impact that ended the ground loop.
Nevertheless, despite massive damage, investigators say the surprisingly small number of fatalities and relatively intact interior present a very survivable picture, with much of the internal trim, ceiling panels and sidewalls still in place. This is partly thought to be due to Boeing's internal design concept, in which the tie rods supporting the arch of the secondary support structure (which holds the interior of the cabin ceiling panels and overhead bins to the fuselage monocoque) transfer loads above 46,000 lb. and withstand loads of up to 9g. Tie rods were built to absorb up and down loads, while truss-type sway bracing structure support the ceiling laterally. The seats are designed to meet the 16g crash load certification standard, while the seat tracks were originally designed to cope with stresses of 9g.