On a seemingly routine visual approach on clear late morning over San Francisco, a Boeing 777-200ER flown by Asiana Airlines started an approach that began too high, descended too low, swerved off the centreline and, in the final, crucial moments, slowed to nearly a stall.
The resulting crash on runway 28L that killed two passengers and sent more than 180 to the hospital triggered a US safety investigation that promises to reopen sensitive questions over crew resources management, automation, air traffic management and culture.
The experience and working dynamic between the pilot flying, captain Lee Gang-guk, and the pilot monitoring (but still in command), Lee Jeong-min, is already a major focus for investigators. Lee Gang-guk had recently transitioned to the 777 from the Airbus A320, while Lee Jeong-min was flying as an instructor in the 777 for the first time.
Perhaps reviving distant memories of South Korean pilots and a culture of deference to authority, the US National Transportation Safety Board (NTSB) also wants to know whether Lee Jeong-min and the relief first officer present on the flightdeck suppressed voicing any concerns to Lee Gang-guk.
"We are certainly interested to see if there are any issues where are any challenges to crew communication," says Deborah Hersman, NTSB chairman. "We will be looking at those relationships as we move forward."
Investigators will also look at a series of complications with the automated systems designed to make the pilots' job easier, but appeared, in this case, to flummox and confuse the crew.
First, a deactivated glideslope indicator - due to construction at the airport - forced the crew to fly a non-instrumented approach, which appeared to make them uncomfortable. According to Hersman, the pilot monitoring - and not the pilot flying - realised they were coming in too high on the glideslope as the aircraft passed through 4,000ft (1,220m).
In an attempt to correct the mistake, the crew - working in vertical speed mode - set a descent rate of 1,500ft in the flight director, which corresponds to an unusually steep descent rate.
The next automation problem appeared after the steep descent from 4,000ft put the aircraft too low on the glidepath. Neither the pilot flying nor the pilot monitoring noticed that they were too low until passing through 500ft and approximately 35s away from a normal landing. The pilot flying said in interviews with the NTSB that he was temporarily blinded by a flash of light from the ground, which Hersman says is being investigated.
"We really don't know what it could have been," she adds.
The pilots however were already flirting with disaster. Many airlines require pilots to be in final landing configuration and on the glideslope as the aircraft passes through 1,000ft. The Asiana 777 had already passed through 500ft and was low on the glideslope, but the crew did not break off the approach.
As the crew attempted to pull the nose up and regain altitude, the aircraft also yawed off the centerline of the runway, forcing the pilot flying to make quick lateral corrections at the same time as he was trying to ascend.
At this point, another automated tool caught the crew off-guard. As Lee Gang-guk, the pilot flying, raised the nose up, the pilot monitoring later told the NTSB that he assumed the auto-throttles were still engaged.
The auto-throttles were either inhibited as a result of the selected operating mode, turned off unknowingly by one of the crew members or had somehow failed. Neither pilot advanced the throttles to compensate for the vertical corrections, which appeared to cause the aircraft to bleed off speed from 137kt (254km/h) down to a low of 103kt within 3s from impact with the seawall.
The NTSB is still trying to understand the complexities involving how the autothrottle operates in the different operating modes of the 777.
"In the last 2.5 minutes in the flight, we see multiple autopilot modes, and we see multiple autothrottle modes," Hersman says. "We need to understand what those modes were - if they were commanded by the pilots, if they were activated inadvertently, if the pilots understood what the mode was doing."
The status of the autothrottle system, however, may not relieve the pilots of blame for losing track of the airspeed as it departed from the reference of 137kt. Moreover, some 777 pilots are trained in simulators to catch inoperative auto-throttles on approach and successfully execute the approach or go-around.
The lack of a glideslope indicator to enable a stabilised approach is also not likely to be a probable cause, as airline pilots are expected to be able to hand-fly a routine visual approach to an 11,000ft runway.
Like the automation issue with the auto-throttles, however, the NTSB wants to understand what role - if any - the air traffic management tools played in the crew's decisions that morning. The NTSB has asked the US Federal Aviation Administration to provide data on all of the missed approaches to runway 28L since 1 June, when the glidescope indicator was deactivated to begin the three-month construction project.
It is still early in the investigation, but so far, neither the Asiana crew nor the NTSB has found any reason to blame the Pratt & Whitney PW4090 engines on the seven-year-old aircraft for failing to maintain the aircraft at the reference airspeed. The crew had plenty of power available, but it was not summoned until it was already too late.
Unless a fault is detected upon further analysis, that likely means that the incident is not a repeat of the British Airways Flight 38, which crashed landed short of the runway at London Heathrow airport in 2008. In that non-fatal incident, an unusually large number of ice crystals that formed in the fuel clogged a heat-exchanger on a Rolls-Royce Trent 800 engine.
As the Asiana crew faces questions about actions before the crash, they are also under scrutiny for the decisions made immediately afterwards. Despite a crash that ripped off the tail of the aircraft and spun it in a violent 360 degree circle that cracked a rib of the reserve first officer in the jump seat, the flightdeck initially told the lead flight attendant to keep passengers seated rather than immediately evacuate, Hersman says.
The order from the flightdeck was rescinded only after a flight attendant came forward from the middle section of the cabin to report that the No. 2 engine was on fire. But the passengers and crew lost 90s of evacuation time before video reviewed by the NTSB showed the cabin doors finally opening and the slides deploying.
"The pilots indicated [to the lead cabin attendant] that they were working with air traffic control," Hersman says, adding that the flightcrew was not fully aware of the fuselage damage when they gave the order to keep the passengers seated.
Another key subject of the investigation will be the crashworthiness of the seats and the integrity of the escape slides. Although the fuselage sustained two massive impacts and stayed mostly intact, two of the escape slides somehow deployed inside the cabin. One of the slides pinned down a flight attendant, who was the last person on the aircraft to be rescued as flames began to consume the interior of the fuselage sidewall, Hersman says.