The structure of the Boeing 777-200ER performed mostly as designed in the double-impact with the runway during the 6 July Asiana Airlines crash in San Francisco, says Deborah Hersman, chairman of the US National Transportation Safety Board (NTSB).
The incident that claimed the lives of two passengers onboard put the first blemish on the 777 family's 18-year service history without a fatal accident, but so far the NTSB has found no reason to fault the performance of the structural design of the twin-engined widebody.
In particular the NTSB's on-site team has verified that the landing gear and the fuel tanks performed as their designers intended in such a crash, Hersman says.
The main landing gear struck the lip of the sea wall as the pilots of Flight 214 came in too low and slow on the approach to Runway 28 Left. Instead of getting jammed into the fuselage, the gear broke cleanly off the aircraft.
"It's important that the landing gear separate, so it doesn't create a safety hazard," Hersman says.
The fuel tanks, meanwhile, were never breached despite the fire that burned through the upper section of the forward fuselage well after the passengers de-planed from the aircraft. The fire was instead ignited by a ruptured oil tank near the engine, perhaps sparing the aircraft an even more dangerous fuel source for the blaze.
"We don't see a fuel-fed fire," Hersman says. "The fire that we saw was not as a result of punctured fuel tanks."
In addition, the 777's propulsion and automated systems appeared to function as designed. The Pratt & Whitney PW4090 engines and flight control surfaces were performing normally in the seconds before the crash, according to the NTSB's initial review of the flight data recorder. Despite apparent confusion by the pilot monitoring of the status of the autothrottles, the flight data recorder showed no anomalies in the functionality of the autopilot, flight director and autothrottle systems, Hersman says.
As the NTSB prepares to wrap up the on-scene investigation in San Francisco, many questions still remain about why the 777 crashed on an otherwise routine approach.
Investigators have focused on the actions and working dynamic of the flight crew, as well as how the crew operated and managed airspeed, altitude and automated systems. The NTSB also has questioned the US Federal Aviation Administration about the approach patterns to Runway 28L since 1 June, when the airport deactivated a glidescope indicator that denied the crew the use of a stabilized approach.
However, Hersman was very clear that the manual flying by the crew was well within the routine given the weather conditions, visibility and functioning systems on that day.
"The systems don't show any anomalous behaviour based on our preliminary FDR traces," Hersman says. "We talked about the glideslope [indicator] being out on Runway 28 Left, so this crew coming in was flying manually. They intended to fly manufally. That's a normal and expected operation for a flight crew coming in to an airport like San Francisco on a clear, 10mi visibility day."