777 Asiana San Francisco : crash atterissage

curufinwe a écrit:PW4000

On Jul 7th the NTSB reported in a press conference at San Francisco Airport, the crew was cleared for a visual approach to runway 28L, the crew acknowledged, flaps were set at 30 degrees, gear was down, Vapp was 137 knots, a normal approach commenced, no anomalies or concerns were raised within the cockpit, 7 seconds prior to impact a crew member called for speed, 4 seconds prior to impact the stick shaker activated, a call to go-around happened 1.5 seconds prior to impact, this data based on a first read out of the cockpit voice recorder. According to flight data recorder the throttles were at idle, the speed significantly decayed below target of 137 knots - the exact value not yet determined -, the thrust levers were advanced and the engines appeared to respond normally. The NTSB confirmed the PAPIs runway 28L were available to the approaching aircraft before the accident, however were damaged in the accident and thus went out of service again. The localizer was available, the glideslope was out of service, according NOTAMs were in effect. There were no reports of windshear and no adverse weather conditions. The air traffic controller was operating normal, no anomaly was effective, until the controller noticed the aircraft had hit the sea wall. The controller declared emergency for the aircraft and initiated emergency response. ARAIB and Asiana personnel have arrived on scene and have joined the investigation. The Mayor of San Francisco reported runway 10L/28R was cleared for service.

 

Beochien a écrit:Mouais , qq pb d`evacuation, et 2 victimes, c`est regrettable !
On peut se demander comment la situation a pu echapper a un Cdt experimente en place gauche !
Mais le comportement du fuselage, apres l`impressionnant demi looping, donne finalement l`impression d`un avion sacrement costaud ...
Juste comparer avec le A330, Lybien, qui est arrive plus vite au sol quand meme !

Admin a écrit:La queue a pris une belle partie de l'impact
On est bien d'accord c'est un triste événement

Mais Beo, on verra quel est le rôle des pilotes... et les conclusions

The 777 can catch you out with with what is known as the "FLCH trap."

When you are above the glide slope and need to get down in a hurry Flight Level Change (FLCH) is a useful mode to use. Normally you transfer to another mode like glideslope or vertical speed, or you switch off the flight directors.

However in this situation the glideslope was off the air so the ILS would not have ben selected or armed. If the flight directors were left on and the plane was descending at a high rate in FLCH the autothrottle would have been inhibited and would not have put on power so the thrust levers would have stayed at idle.

If the Asiana was a bit high (quite normal for SFO) then regained the visual glideslope, the rate of descent would have decreased and the speed would have started slowly reducing but with the thrust levers staying at idle the 777 would now be in the same situation as the Turkish 737 at AMS, ie speed decreasing below Vref and not being noticed.

The 777 has autothrottle wake up, ie when the aircraft approaches a stall the power comes on automatically to almost full power. This gives pilots great confidence however autothrottle wake up is inhibited in FLCH.

So 777 pilots will be looking at this scenario and wondering if Asiana were in FLCH with flight directors on, too high, stabilised late and did not notice they were still in FLCH and that the autothrottle was not keeping the speed to Vref plus 5 untl too late.

Just a theory but I think it far more likely than engine failure, radalt failure or autothrottle failure and I suspect when the events are unravelled this will be what has happend.

Lee Kang-kuk, the pilot at the controls of Asiana's 214 flight to San Francisco, was training on Boeing 777s, and was making his first attempt to land the jet at San Francisco airport. His supervisor was making his first flight as a trainer

The pilot in charge of landing the plane on Saturday was in training on the 777 and was roughly halfway through the process, while seated next to him was a co-pilot on his first flight as an instructor. Both were experienced pilots, although they had not flown together before, Hersman said.

"At about 500 feet, he realized that they were low," Hersman told reporters, referring to the instructor pilot's account of the failed last-second attempts to avoid Saturday's disaster. "Between 500 and 200 feet (150 and 60 meters), they had a lateral deviation and they were low. They were trying to correct at that point."

Referring to the instructor pilot, she said it was not until 200 feet that "he recognized the auto-throttles were not maintaining speed" and tried to abort the landing. Hersman had previously said that the plane had been at an altitude of 200 feet 16 seconds before crashing.

Three of the four pilots on board were in the cabin during the landing, although only two could see the runway, Hersman said, citing the interviews by investigators with the crew.

Hersman said an examination of the wreckage showed that the auto-throttle was "armed," but it was not clear if it had been properly engaged or had somehow failed before the plane slowed to a near-stall and hit the ground. "We need to understand a little better" how the auto-throttle is used, she said.

"They had set speed at 137 knots (158 mph), and he assumed that the auto-throttles were maintaining speed," Hersman said of the instructor pilot.

She noted that the pilots were responsible for maintaining airspeed.

"We have a flying pilot and two other pilots in the cockpit and they have a monitoring function," she said. "One of the critical things that needs to be monitored on an approach to landing is speed. So we need to understand what was going on in the cockpit and also what was going on with the aircraft."

During interviews with the U.S. National Transportation Safety Board, Korean investigators and the airline on July 8 and 9, the instructor pilot told officials that the aircraft was “slightly high” when it descended through 4,000 ft. on the approach. He then set the aircraft’s vertical speed mode for a 1,500 ft./min. descent rate, NTSB chairman Deborah Hersman said at a July 9 report on the investigation. During a July 8 update, Hersman said the pilots disconnected the aircraft’s autopilot at 1,600 ft., presumably to hand-fly the approach.

The instructor pilot told investigators that at 500 ft. altitude, he realized the aircraft was below the visual glideslope provided by the precision approach path indicator (PAPI) lights on the airport, since the system showed three red and one white light. When on the proper glideslope, pilots see two white and two red lights. All red indicates a position significantly below the glideslope while all white lights indicate being well above the reference glideslope. The electronic glideslope generally used as part of an instrument landing system was not operating due to runway construction at the airport this summer.

“He told [the left-seat] pilot to pull back [on the control wheel],” says Hersman of the interview. “He had set the speed at 137 kt. and assumed the autothrottles were maintaining the speed.” Autothrottles, if armed and turned on, will automatically increase or decrease engine thrust to maintain a preset speed, in this case 137 kt., the reference landing speed for the 777-200ER that day. The NTSB is investigating why the autothrottle did not work as the instructor had expected, an issue that could include mode confusion related to the interaction of various auto-flight modes.

Hersman says investigators documenting switch positions is the cockpit after the crash noted that the autothrottles were armed. In that state, the system will automatically activate when speeds are low regardless of whether pilots have the autothrottle system turned on or off.

Preliminary data from the aircraft’s flight data recorder shows that the speed had already decayed to 134 kt. as the aircraft passed through 500 ft., and would ultimately drop as low as 103 kt. at 3 sec. before impact.

The 777 continued to slow as the pilots attempted to “correct a lateral deviation” as it descended from 500 ft. to 200 ft. “At 200 ft., the four PAPIs were red and the airspeed was in the hatched area,” says Hersman. The “hatched” markings on an airspeed tape warn pilots of an impending stall. The instructor pilot at that point recognized that the autothrottle was not maintaining speed and established a nose-high go-around attitude. He attempted to push the throttles forward for more power, but says the pilot-flying had already done so.

Information from the flight data recorder showed that the pilots first increased engine power from flight idle at 125 ft. altitude, reaching 50% thrust 3 sec. before impact.