While the NTSB investigation into the July 6 crash of an Asiana Airlines Boeing 777-200ER at San Francisco focuses in on the actions of the crew, safety experts are already learning valuable new design lessons from the violent crash landing.
Structurally the key early lesson appears to be that the aircraft’s fuselage did extremely well, and remained substantially intact throughout the initial impact and subsequent ground loop. Although the forward two-thirds of the fuselage was gutted by the post-crash fire, the overall structural integrity of the fuselage was not compromised by the impact with significant buckling only evident in two zones, forward of the wing root and aft by Section 47/48 where the empennage was ripped from the rest of the airframe.
NTSB investigators examine the ruptured aft pressure bulkhead and exposed aft cabin section (NTSB)
While it may be more difficult for investigators to assess the post-crash condition of the fire-damaged forward and mid-cabin sections, there is much to be learned from the condition of the aft cabin which bore the brunt of the crash landing. Interior shots from the aft cabin, close to the buckled section by doors L4 and R4, show how the seating and cabin floor were evidently affected by the initial impact (and resulting loss of belly skin and structure below Section 47/48), as well as the subsequent ground loop. Much of the aft cargo hold and lower lobe structure appears to have been either crushed or ripped away by the impact and slide down the runway.
Interior of the aft cabin section (NTSB)
Nevertheless, despite massive damage, the interior represents a very survivable picture with the internal trim, ceiling panels and sidewalls apparently largely intact and mostly attached. Boeing originally designed 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) to transfer loads above 46,000lb (21,800kg) and withstand loads of up to 9g. Tie rods absorb up and down loads while truss-type sway bracing structure supports 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.
In these two views note how the fuselage (above) is still essentially structurally intact after the ground-loop and, after the fire (below), no longer supports its own weight and rests on the ground.
From a systems perspective the investigators will be focusing on the performance of the safety systems, door operation and emergency inflatable slide deployment. Early reports appear to indicate cabin staff had issues with activating at least one of the slides, and that it partially inflated inside the cabin trapping one of the flight attendants. The structure also appeared to have resisted the main effects of the post-crash fire for long enough to allow for all the survivors to evacuate.
A close up of the detached fin and rudder, still attached at its root to part of the fuselage (NTSB)
From the flight deck perspective, the NTSB’s preliminary analysis of the cockpit voice recorder and flight data recorder reveals that pilots had issues with speed control shortly before impact. My colleague John Croft reports that NTSB chairman Deborah Hersman, during the NTSB’s first on-scene press conference on July 7, says pilots had discussed a target approach speed of 137 kt, but airspeed just before the crash was “significantly below” that value, “and we’re not talking about a few knots,” she says.
Based on the cockpit voice recorder, Hersman says at 7 seconds before impact, one crewmember called for an increase in speed, and at 4 seconds before hitting the sea wall, the stick shaker is heard. At 1.5 seconds before impact, Hersman says a crewmember called for a go-around. Earlier in the approach, the crew verified that the 777’s landing gear was down and the flaps set to 30 degrees.
In the 777, go-around is engaged by pushing either of the two the TO/GA (take-off/go-around) switches on the throttles. The mode is designed to remain engaged even if the aircraft touches down while executing the go-around, and is automatically armed when the flaps are not up, or the glideslope is captured. With the first push of either TOGA switch the auto-throttle is designed to engage in thrust mode for a 2,000 ft per minute climb, and with second push the auto-throttle engages for full go-around thrust. There is no confirmation yet from the NTSB if, in the brief 1.5 seconds between calling for go-around power and impact, whether the TOGA function was actually activated.
Asiana meanwhile says the pilot in command for the landing of flight 214 was making his first landing at San Francisco in the 777, and was still in training on the type. Although the pilot, Lee Kang-Kook, has reportedly amassed 9,793 hours of flight time on various aircraft (presumably mostly with Asiana which he joined in 1994), only 43 hours have been logged on the 777. The other pilot, Lee Jeong-min, has 3,220 hours of flying experience with the 777 and a total of 12,387 hours of flight time says Asiana.