Armchair air crash investigator – Asiana 214
At 18:26 UTC on Saturday July 6th, 2013 – Asiana Airlines Flight 214, a Boeing 777-200ER, crash landed at San Francisco International Airport. At the time of this writing, there have been 2 fatalities confirmed. This post is nothing more than speculation as to what occurred during the crash of Asiana Airlines Flight 214, and what caused the crash. I am not an air crash investigator and not associated with any aviation accident investigation agencies. This post is about what I think happened and not necessarily what will be discovered that actually happened.
Listening to LiveATC recordings, everything appears to be normal – there have been no indications of any mechanical or human issues and weather conditions were excellent; Asiana 214 heavy was cleared to land on runway 28 left. Moments later, ATC can be heard scrambling to divert and hold all aircraft coming in for landing or about to take off, the voice of the pilot of Asiana 214 can be heard but is unintelligible; ATC informs the pilot that emergency crews are on the way.
Various witness statements that I have heard throughout the day:
- The plane appeared to be too low.
- The landing gear struck the seawall that lies right before the blast-pad on runway 28L.
- A passenger aboard the aircraft said that the approach seemed very low and right before impact, the engines were heard to power up.
- Just before impact, the nose could be seen being raised.
- After impact, there was a large brief fireball that came from underneath the aircraft.
- Lots of reports that the aircraft “flipped over” – however, the term fishtailed is more accurate, as the aircraft skidded and
turned about 180-degreesswerved off the runway during its short ground roll. Update: I also found this image – a photo of the Asiana 214 actually crashing. If you look closely you could see the starboard (right) wing is raised up. This could also be a reason why people reported that the aircraft had flipped over.
The very first thing I did after I heard that there has been a major crash at SFO was check Twitter. Sure enough Flightradar21 had confirmed that there had been a crash and they had the log of the final moments.
Asiana Airlines flight OZ214, Boeing 777, HL7742 has crashed during laning in San Francisco http://t.co/bSgoVeggrU
— Flightradar24.com (@flightradar24) July 6, 2013
I also took a look at Flighaware’s tracklog for Asiana 214.
After looking at both sets of information, here are some things I thought about:
- At 2:24PM, the aircraft would be well considered on final approach.
- For the majority of the approach, the aircraft is descending at a rate of +1200 feet per minute, with a speed of
130+160+kts. Not a nose dive by any means, but a more steep and fast approach than usual I would say. While approach speeds and descent rates vary depending on conditions, the “normal” final approach descent rate would be between 600-800 feet per minute.
- The aircraft slows both its speed and its descent quite a bit in the last part of the flight – having a vertical speed of -120 feet per minute (which is typically what you touchdown with) at an altitude of 100 feet. A look at the aircraft’s position at the time shows its position was right before the seawall.
- The final bit of information is interesting. It shows a positive rate of climb – 120 feet per minute to be exact, which for a jet airliner, is very slow. For reference, at takeoff the vertical speed is most often +1500 feet per minute. Another interesting bit of information is the aircraft’s airspeed; 85 kts – for a Boeing 777 that is (and I’m guessing here) probably way below stall speed.
So we have a Boeing 777 coming in a bit hot and high on final approach – perhaps descending quicker than usual to get on the correct glide slope, but at the last moment we have a sudden change of attitude as the aircraft attempts to ascend with a low rate of climb and low airspeed.
So… what happened next? Well…
Eyewitness accounts and this image, show that the Asiana Boeing 777 struck the seawall before skidding across and off the runway. Further images of the aircraft and of the runway show major pieces of the tail section, namely the horizontal and vertical stabilizers, as well as the landing gear and one engine had been torn from the aircraft and came to rest on the runway.
Right away, it is apparent that the aircraft landed not even close to where it should be. That strip of pavement right after the water is not the runway, but the blast pad, a section unusable to aircraft. This section is followed by the displaced threshold, which is used for take off, taxi, but not landing. Only after you pass these two sections do you get to the actual usable runway for landing which is indicated by the vertical white stripes and the runway numbers. On runway 28L, the distance from the beginning of the blast pad to the runway threshold (the vertical white lines) is some 300 feet.
Putting it all together
- Asiana Flight 214 failed to maintain the correct glide slope and as a result struck the seawall. This is shown through the debris field which starts right at the seawall.
- This impact is what most likely caused the tail section and landing gear of the aircraft to be torn off. Eyewitness accounts and the final track log of the aircraft (which show a significant slow in both descent rate and airpseed) both tell that the aircraft had a significant nose up attitude. An over-flare of the aircraft an easily cause a tail strike event and if violent enough, such as that in a landing, it can be easily imagined that the tail would not be able to withstand such forces.
- The aircraft then skidded on to and off the runway
where it made a near 180 degree turn, like a car swerving out of control. It then came to rest where it promptly caught fire. Photos of the aircraft’s final resting place and position verify that the aircraft made a near 180 degree turn during its ground roll. Update: Aerial photos of the crash site, show that the aircraft skidded off the runway but it did not do a 180-degree turn – but rather swerved about 30-40 degrees before coming to a rest on the grass. The loss of the tail section, one engine, and likely other control surfaces on the wings would significantly contribute to the loss of control.
Four reds – you’re dead
So how did an advanced jet airliner with an experienced crew, on a clear day, land short of the runway? To answer that, you need to understand a few things about how airline pilots land aircraft.
Most, if not all landings done by airline pilots are landings using the ILS. ILS stands for Instrument Landing System and its purpose is to provide landing guidance to the aircraft and crew should they not be able to see the runway visually. Different airports have different types of ILS systems, some only provide vertical guidance others only horizontal guidance. But most major international airports provide extremely precise ILS systems that both provide vertical (glide slope) and horizontal (localizer) guidance and could work in sync with the aircraft’s autopilot to land the aircraft automatically. Now its important to understand that the majority of time, landings, at least in the final stage are done manually, even when the ILS is operational.
At the time of the crash of Asiana 214, it is my understanding the ILS systems on 28L/28R were inoperative, a look at the NOTAMs confirms this and the cause of this inoperation was because the guidance systems had to be adjusted due to a change in touchdown location because of some work being done on the runway.
With the ILS inoperative the only other way the pilot could land would be visually – that is, looking out the window and using your eyes (looking at the instruments to cross check would be a good idea too) to gauge distance and height. Landing visually requires obviously, the ability to see the runway out the window without any obstructions from clouds or other weather, this is known as VMC (visual meteorological conditions). VMC prevailed at the time of Asiana 214′s landing. With a visual approach and landing you still have some tools you can use, namely the PAPI/VASI.
PAPI (and VASI), which stand for Precision Approach Path Indicator and Visual Approach Slope Indicator, respectively, are a set of lights that assist the pilot when landing visually. A runway either has PAPI or VASI lights, not both. The lights change color depending on your angle relative to a specific point on the runway.
And the cause of the crash was…
I don’t know – everyone is just speculating at the moment. At the time of this writing, the NTSB is in possession of the flight data recorder and cockpit voice recorder – once those are analysed we will have a much better understanding into the final moments of the flight.
Nevertheless, I am an armchair air crash investigator so I will offer up some theories starting from most unlikely to most likely:
- Icing in the FOHE – Another crash of a Boeing 777, British Airways Flight 38 had a similar setting. The plane crashed short of the runway on final approach, the plane declared an emergency only seconds before hitting the ground. The cause of the crash was the inability of the engines to produce increased thrust due to icing in the fuel oil heat exchanger (FOHE). The aircraft, which was already slow and low due it being on short final, could not maintain its airspeed and was at risk of stalling – only by lowering the nose and increasing the descent rate to make up lost energy was the crew able to avoid a stall – at a cost of landing short of the runway. The reason why a repeat of this accident is extremely unlikely is because it was found that only Rolls Royce engines had this design defect. The Asiana Boeing 777 did not have Rolls Royce engines.
- Unawareness of inoperable ILS – The ILS on runway 28L/28R was not functioning due to runway repairs. More specifically the runway repairs had to do with changing the touchdown position. Whether or not they could pick up the ILS frequency is another question. But if they could pickup the ILS frequency and proceeded with an ILS landing, either using the autopilot or just following their instruments, it is a somewhat plausible theory that the crew proceeded with an ILS approach using erroneous information from an inoperable ILS which caused them to come up short of the runway. The problems with this theory is that: it suggests that they could even pickup the signals from the ILS, that the crew did not review airport documentation and NOTAMs, and that they simply did not look at all out the window and see that they were too low, either by their own visual perception or through the red lights they would most likely see from the PAPI lights.
- Confliction with autopilot/auto-throttle – The Boeing 777 is an advanced aircraft, and advanced aircraft typically have an auto-throttle (A/T) feature. As the name suggests, A/T allows the pilots to select a speed they wish to hold and the engines will automatically adjust thrust based on the aircraft’s attitude. Assuming (and this is mostly guessing) that the A/T was on and they selected a slow speed (so they could descend quick as they did) and that the throttles when physically moved have no effect when the A/T is engaged, it is possible that when the crew realized that they were too low, began a go-around by pitching the nose up and putting the throttles forward. However if the A/T negates any physical inputs from the throttles, it is possible that the crew forgot that the A/T was engaged, realized that they were too low, attempted to push the throttles forward, got no response, realized that the A/T was engaged, disengaged the A/T, pushed the the throttles forward, but at that point it was too late – the engines would not spool up in time and the aircraft impacted the seawall.
- Just a bad approach – Looking at the track log, the pilots were in a rush to get down. As I wrote before, not a nosedive by any means, but they did come in hot and high. They also bled off quite a bit of speed on their way down too. I believe the pilot flying, in an attempt to “catch” the glide slope, descended too quickly and inadvertently cut through the glide slope – going from too high to too low. From the pilot’s point of view, he would see the PAPI lights quickly go from four whites to four reds. At this point, I believe the aircraft was dangerously low to the water and the pilots realized this. However, this realization came too late – the aircraft was too slow to be able to climb. The pilot attempts to initiate a go-around, the nose goes up and the descent rate decreases. The pilot unsuccessfully attempts to climb, as the engines did not have time to spool up and provide the neccessary thrust to raise the airspeed. The aircraft then strikes the seawall – tearing off the landing gear, tail section, one engine, and most likely causes damage to other control surfaces. The aircraft climbs a bit however, the final nose up input followed by the impact most likely caused an aerodynamic stall which, combined with a loss of directional control from the loss of the tail section and engine, causes the aircraft to crash into the ground. The aircraft skids onto and off the runway where it
makes a near 180 degree turnswerves about 40 degrees to the left on the grass and comes to rest.
Again, I am no air crash investigator. These are just my conclusions reached through all the data available and my own aviation knowledge. My conclusions rely on data from sources like FlightRadar24 and Flightaware, which are not 100% accurate. The true answers will most likely be found in the black boxes that the NTSB has in their possession at the moment.
NTSB press conference:
Target speed for the approach was 137 knots. #Asiana 214
— NTSB (@NTSB) July 7, 2013
The throttles were advanced a few seconds prior to impact and the engines appear to respond normally. #Asiana 214
— NTSB (@NTSB) July 7, 2013
Sound of stick shaker began approx. 4-sec prior to impact. #Asiana 214
— NTSB (@NTSB) July 7, 2013
Call to go around made approx. 1.5-sec prior to impact. #Asiana 214
— NTSB (@NTSB) July 7, 2013