Federal investigators are asking several key questions as they continue their probe into Saturday’s Asiana Airlines crash landing at San Francisco International Airport (SFO) that killed two passengers and injured dozens. The National Transportation Safety Board (NTSB) is hoping that its interviews Tuesday with the four pilots of Flight 214 will shed some light on the aircraft’s erratic approach and the cockpit’s last-minute attempt to abort the landing before the Boeing 777’s landing gear and tail struck a seawall at the end of the runway.

The NTSB will also examine how safety systems and procedures onboard enabled all but two of the 307 passengers and crew to escape with their lives despite severe damage to the aircraft’s fuselage.

Air traffic controllers at SFO had cleared Flight 214 for a visual approach to its runway late Saturday morning. Despite good visibility around the airport and no reports of onboard mechanical failure during the descent, the aircraft flight data recorder indicates that the 777 approached about 64 kilometers per hour slower than the target landing speed of 250 kph, NTSB Chairman Deborah Hersman said in a press conference Sunday.

Seven seconds before impact one of the crew members called for the aircraft to increase its speed, according to information obtained from the cockpit recorder. Three seconds later, the aircraft’s "stick shaker”—a feature that vibrates the pilot’s control yoke—warned of an incorrect approach angle. Finally, a crewmember initiated a “go around”—an attempt to abort the landing and regain altitude—1.5 seconds before impact.

During an NTSB press conference Tuesday afternoon, Hersman indicated that her agency had interviewed three of the four flight crew members, with the fourth being interviewed as the press briefing was underway. She relayed that the pilot at the helm of the 777 during the crash landing was in the process of certifying on that aircraft (although he is an experienced pilot on other aircraft models) and was accompanied by a flight instructor (an experienced 777 pilot) in the co-pilot’s seat.

Although information provided by the flight crew still needs to be verified against the voice and flight recorders, the instructor told NTSB investigators that the crew had set the aircraft’s speed at 250 kph and thought that the 777’s automatic throttles, or autothrottles, were maintaining that speed during the approach. At about 60 meters from the runway, he realized the autothrottles were not maintaining the correct speed and the aircraft was coming in too low, hence the belated attempt at a go around.

Hersman said that the autothrottles recovered from the wreckage were in the “armed” position, meaning that they “were available to be engaged.” The NTSB will be able to determine whether these were set properly only after checking the flight data recorder, she added.

Runway safety tech
The NTSB’s investigation encompasses not only on the onboard factors that may have led to the crash but also the performance of safety features on the runway, which is about 3,500 meters long and sits four meters above sea level. The Federal Aviation Administration requested a flight test of the runway’s instrument landing system, which guides pilots to a safe approach, shortly after the crash. The system’s localizer, which orients incoming aircraft with respect to the runway’s centerline, functioned properly during the test.

But investigators are also examining the relative importance of the airport landing system’s other key component—the glide-slope indicator that provides pilots with a steady path of descent—which at SFO has been out of commission since June 1. Hersman said in her Sunday press conference that pilots had been notified in advance of the outage.

The glide-scope indicator gives pilots a 3-D arc to follow from a predetermined destination in the sky, says Jason Day, an instructor in Arizona State University’s Aviation Program and a former commercial airline pilot. Although the system is not infallible, he adds, “if you follow that, your main wheels should touch down on what are called the 1,000-foot markers, those giant white rectangular blocks painted on the runway.” Commercial airline pilots use the glide-slope indicator during more than 99 percent of landings, Day says. Still, he adds, “on a clear day, it should be no challenge at all for a pilot to land without the glide-path system.”

Day’s first reaction on seeing the accident was that the aircraft “landed short, meaning they were trying to utilize as little of the runway as possible, and they just came in too low and struck the tail.” Pilots often try to minimize runway use to keep airport traffic moving, according to Day: “The air traffic controllers want you to get on the ground and get off the runway as soon as possible, and most pilots try to accommodate that.”

Switch to manual
The pilot at the helm of Flight 214, identified as Lee Kang-kuk, turned off the autopilot—which regulates airspeed, among other things—at 500 meters above the ground, 82 seconds before the crash, according to Hersman.

If the pilot had kept the autothrottle engaged, the aircraft would have made adjustments to keep the power up and maintain the proper speed, Day says. But it’s not uncommon for pilots to switch to manual control for a landing. “A lot of pilots don’t like the automation when they get that close to the ground,” he says. “They don’t trust those systems.” Both localizer and glide-slope systems use complex antennas that are sensitive to runway obstructions—such as parked vehicles or uneven terrain—that might interfere with their signal and provide an incorrect reading.

Investigators will want to know exactly how events unfolded as soon as the pilot switched to manual controls, because that coincided with the dramatic reduction in speed, says Dave Esser, professor of aeronautical science at Embry–Riddle Aeronautical University. Once the aircraft lost so much speed, it was less likely to respond to attempts to abort the landing, he adds. “When you have an aircraft of that size, inertia is working against you,” he says. “You’ve got that much mass, and you get slow, and then when you do put the throttles up they don’t respond immediately. If they had initiated the full takeoff and go around sooner, I’m sure it would have worked.”

Safety improvements
The final NTSB report should also provide insights into how nearly all passengers and crew survived the crash landing and fiery skid down the runway. Passengers can attribute their improved chances of survival today to a series of safety improvements that have evolved over the past half century, says Bill Waldock, professor of safety science and director of Embry–Riddle’s Robertson Aircraft Accident Investigation Laboratory. Airlines began replacing flammable plastics in the interiors of their aircraft with fire-hardened materials following two catastrophic accidents in the 1980s—one in Cincinnati and the other in Manchester, England—in which fires broke out before the planes even left the ground, claiming the lives of dozens of passengers.

Aviation engineers also took cues from the automotive industry and have designed stronger fuselages over the years built to withstand greater impacts without breaking apart. Repositioning fuel tanks helped keep fires from spreading too quickly to the cabin, giving passengers and crew precious minutes to escape. Evacuation slides, which used to be bright yellow for greater visibility in sea rescues, now have an aluminized coating to repel heat and resist bursting during evacuations, Waldock says.

Flight 214’s well-trained team of flight attendants may have also saved lives, Waldock says. Two slides that were supposed to inflate outward instead inflated toward the inside of the plane, hurting two flight attendants, according to Hersman. Still, the pilots and attendants were able to get most of the passengers to safety even as the cabin’s roof burned overhead. “It’s the post-impact fire that kills more people,” Esser adds. “That’s where the flight attendants come in and do their job and get the people out of the plane before the smoke and flames are an issue.”