Jul

21

While it is impossible to state the exact cause of the crash of Asiana flight 214 at San Fransisco International Airport on July 6th, 2013, and catastrophic accidents always have more than one contributing factor, one can speculate a bit.

The facts so far indicate that the aircraft was below its target approach speed or Vref of 137 knots for the last minute or so of the flight and the speed was decreasing all the way to impact. See flight data from Flight Aware here.

Why is this important? Pilots need to maintain control of the energy state of the aircraft. Too much airspeed on approach will have the aircraft generating too much lift when it approaches the touch down zone and the aircraft will glide down the runway and land too late to stop before the end of the runway. Or, the aircraft will simply be too fast to stop prior to the runway end. Escape from this situation is simple - since the aircraft is generating so much lift and is fast already, the pilot can execute a maneuver called a "go around" and simply apply thrust and take off again. So an excess of energy is usually only dangerous if the pilots insist on continuing with the landing. But a lack of sufficient energy is incredibly dangerous and must be avoided to ensure a safe landing - this is managed by keeping the aircraft at its target approach speed. A lack of speed is deadly because it leaves the pilots with no buffer - no extra energy to use to escape from a tricky situation or a mistake of some sort.

The target approach speed supplies the aircraft with some key abilities, first and foremost is the ability to maneuver. The aircraft needs sufficient speed for the control surfaces to be effective - the ailerons and rudder need a certain amount of airflow over them to work. Speeds significantly below the reference speed may place the aircraft in a nose high pitch attitude that may result in a tail strike, even if everything else is done correctly. But the key in this incident is that a proper speed gives the aircraft the ability to generate sufficient lift to arrest or reduce its sink rate simply by applying back pressure on the elevator controls - pulling back on the stick. But if the aircraft is too slow, pulling back on the stick will not stop the descent and will eventually result in an aerodynamic stall - the wings will cease generating lift and the aircraft will begin sinking even more rapidly than before.

Whether the aircraft actually stalled is not important, what is key here is the inability to arrest the descent. The pilots of Asiana 214 attempted to recover from this situation by applying thrust. The problem with thrust is that it takes too much time for the engines to spool up to speed and then transfer that energy into the airframe - many seconds sometimes and in this situation, so close to the ground, a couple of seconds is not enough, you need an immediate response. Another thing about the target approach speed is that the engines are not idle, they apply thrust all the way to touchdown and are more readily available to provide lift if needed.

The news has made a great deal of noise about the absence of a key piece of instrumentation - the Instrument Landing System or ILS  for runway 28 Left. The truth is that while having the Glide Slope portion of the ILS available is very helpful, it is not necessary AND, the glide slope for 28 Right may have been available, which is for purposes of vertical guidance, identical to 28 Left. See one of the published visual approach procedures to 28L and 28R here to see the dual ILS localizer beams projected east into the approach flight path:  Personally I doubt that any pilot on approach to 28 Left would use the glide slope for 28 Right as the ILS would display lateral flight path information (Localizer) that is misleading and incorrect.

Glide path information can be obtained in a number of ways, including the shape that the visual appearance of the runway takes in the pilots field of view — this is something that pilots acquire with experience and an experienced pilot gets a real feeling for when something isn't right. This is a major focus in one of the best books about flying ever written: Stick and Rudder: An Explanation of the Art of Flying by Wolfgang Langewiesche. Another, and this was almost certainly available to the pilots of Asiana 214 and I suspect that they were using it, is something called a PAPI or Precision Approach Path Indicator which is a system of lights that appear to change color depending on your position on, above or below the prescribed glide path to the runway. In any case, it appears that the aircraft was established on an appropriate glide path for most of the approach, however as its airspeed bled off, it was unable to maintain the proper descent rate and in the last few moments it descended below the glide path and then into sea wall prior to the runway threshold.

Okay. Every professional pilot knows all of this and much, much more. So what happened? The news has highlighted the fact that the pilot at the controls is new to the Boeing 777 and was being supervised by an instructor pilot. I have heard that this was the instructor pilots first trip as an instructor in this aircraft. The flight is a long one and fatigue may have played a role. It has been said that culture might have played a role — an unwillingness to point out something important and perhaps obvious to another pilot for fear of insulting him. There may have been an over-reliance on technology — trusting that the auto-throttles were maintaining the target airspeed — highly possible. But I'm not so sure. Even if auto-throttles are utilized, it remains a CRITICAL duty to maintain awareness of airspeed — airspeed has to be monitored constantly, in all phases of flight.

So how do pilots monitor airspeed? In this picture of a primary flight display, on the left you can see the gray speed tape. As the speed changes, the tape scrolls up or down and the current speed can be viewed in the black window in the center. In this example, notice the magenta number 250 above the tape and the magenta pointer to the right of the current speed window. (The pointer is commonly referred to as a bug.) This means that the pilots have selected a target speed of 250 KIAS (Knots Indicated Air Speed) AND since the bug is aligned with the current speed window, the aircraft is AT that speed. When the speed of the aircraft moves away from the selected target speed, the speed bug will move up or down with the tape — away from the current speed window. So this gives a very good visual indication to the pilots of where the aircrafts speed is in relation to the selected target speed. Also, the airspeed tape will display an amber band that signifies an area of unsafe speed. (In the example picture, an amber or red broken band is visible, displayed on the right edge of the tape at 265 KIAS and above — indicating an unsafe speed that is, in this case, too fast). Okay, so the pilots were flying a visual approach. In this case, the pilot flying may be mostly "Heads Up" — with his eyes looking out the windshield. But the pilot not flying is required to monitor the approach and will spend a lot of time "Head Down" — with his eyes directed at the instruments and is required to alert the pilot flying of any deviations from target airspeed or flight path.

So what happened? Why weren't the pilots aware of the low speed situation? In my opinion, there was probably something going on in the cockpit that distracted the pilots from monitoring instruments for key pieces of information, specifically, airspeed. Distractions on the flight deck are not new to aviation, they have been a major cause of concern for years and years.

So much so that in 1981 the FAA enacted FAR 121.542 and FAR 135.100 to help curb the number of these accidents. Commonly known as the "sterile cockpit rule," these regulations specifically prohibit crew member performance of non-essential duties or activities while the aircraft is involved in taxi, takeoff, landing, and all other flight operations conducted below 10,000 feet MSL, except cruise flight. (Click here to go to FAR 121.542 and 135.100.)

(above quote from here)

And from the same NASA ASRS Directline article cited above:

The most habitually cited offense was extraneous conversation between cockpit crew members.

It is therefore my guess that a distraction of this type may very well have been the root cause of the accident. Time and cockpit voice recorder tapes will tell.

Additional Notes:

1. Excellent article about Lessons Learned and a slightly technical discussion on some of the pitfalls of using the auto-throttles in the Boeing 777 at Flying Magazine:

2. Another mildly technical and well written article about the accident at Wired.com.

3. Discussion of cognitive factors associated with distractions (the side bar titled "Task Management" is of particular interest.)

4. Technical data about SFO airport (including links to published approaches)


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