Comments on the Asiana Airlines Flight 214 Accident at SFO: Could this be an “unstabilized approach” accident?
[Note: The following comments are offered by a retired TWA B-767 captain and former naval aviator. The author was based at both SFO and JFK in the course of a thirty-two year career at TWA.]
First, I would like to offer my sympathy to the passengers and crew of Asiana Flight 214, the uninjured as well as the injured. All aboard will be permanently affected by this experience, no matter the extent of their physical injuries. I would also like to extend condolences to the families of the two young passengers from China who lost their lives in this tragic accident.
Second, I would like to comment that the media coverage I have seen has generally been fairly good in terms of accuracy, and correct usage and understanding of aviation terminology. One particularly annoying pattern is the repeated reference to jet fuel as “gas”, as in “gas tanks”.
The nature of this accident is such that many of the usual causal factors can be eliminated immediately, especially those relating to weather, Air Traffic Control, and navigation aids. The remaining areas for investigation concern potential mechanical and/or fuel issues, and pilot performance. If no mechanical or fuel issues are identified, the focus will shift almost exclusively to pilot performance issues.
Assuming that the investigation uncovers no mechanical or fuel issues, the question becomes:
How could this happen?
I certainly don’t propose to answer that question, but I can offer some perspective as an experienced airline pilot regarding the procedures used at TWA as well as at all U.S. airlines to preclude an accident resulting from what pilots refer to as an unstabilized approach.
The many accidents which occurred in the early days of swept-wing jet aircraft operation in the military services demonstrated dramatically the dangers associated with unstabilized approaches.
The aerodynamic design of jet aircraft with swept wings is optimized for high speed, high altitude cruise operation. However, major aerodynamic configuration changes are necessary to operate the aircraft safely at the low speeds and low altitudes necessary for approach and landing, as well as in the take-off and departure regime. Of these two, the approach and landing phase of flight is most critical due to the lower engine power settings involved, higher drag from landing gear and high-lift devices (flaps, slats, and leading edge flaps), and the need to control precisely the return to a runway surface.
Early jet engines had several characteristics which introduced certain operational problems not familiar to the pilots of the reciprocating engine/propeller aircraft which preceded the jets.
- First, the early jet engines produced relatively low thrust, and that thrust was produced exclusively from the primary airflow through the compressor and turbine sections of the engines.
- Second, the early jet engines produced very little thrust at low per cent RPM, while the increase in thrust was non-linear as the per cent RPM increased. Most of the increase in thrust as the throttles were advanced occurred in the higher portion of the per cent RPM range of the engine.
- Third, the early jet engines were relatively slow to accelerate.
This engine behavior was distinctly different from the behavior of the reciprocating engines these early jet pilots had flown, which responded to throttle movement with immediate increase in power. Additionally, propeller aircraft without swept wings did not require the extensive use of high-lift devices, and hence did not experience the accompanying extraordinary increase in aerodynamic drag associated with high-lift devices employed extensively on jet aircraft.
Accordingly, throttles could be, and normally were, closed on propeller aircraft shortly before touchdown in order to avoid an undesirable aircraft response to the flare maneuver as the nose is raised to the landing attitude with back pressure on the elevator control. However, closing the throttles prematurely on a jet aircraft, in the extreme high drag configuration required for landing, can result in a sudden loss of airspeed and/or altitude, and a hard landing.
The point of this discussion, and the end result of the early experience with swept-wing jet aircraft, was recognition of the absolute necessity for a “stabilized approach” for safe operation of swept-wing jet aircraft in the landing phase of flight.
A “stabilized approach” is an approach to landing in which airspeed, power, and rate of descent (referred to as “sink rate” on final approach prior to landing) are stable, the aircraft is aligned with the centerline of the runway, and the aircraft is on a stable descent path (referred to as the “glide slope” of on Instrument Landing System, or ILS, approach) which will result in touchdown in the landing zone approximately 500 feet beyond the approach end of the runway.
The engine-related issues in swept-wing jet aircraft operation were significantly reduced as jet engine technology improved. Engine fans were added to the first several compressor stages of the engine by extending the length of the blades in these first few compressor stages beyond the engine case, providing an additional source of thrust in the secondary airflow outside the core of the engine. (Fan thrust acts much like propeller thrust, providing an immediate thrust response to increase in engine RPM.) Additionally, later jet engines became significantly more powerful and responsive to throttle movement.
Despite improvements in jet engine technology, little could be done to reduce aerodynamic drag during landing approach. In fact, development of leading edge high-lift devices and complicated trailing edge flap configurations actually increased aerodynamic drag in the “dirty” configuration in most advanced jet aircraft.
The solution to the safety issues associated with the high-drag characteristics of swept-wing jet aircraft in approach and landing configuration is the stabilized approach. Training of pilots constantly emphasizes this requirement, and standard in-flight operating procedures serve to insure that a stabilized approach is established before a landing is executed. If there is no stabilized approach, there should be no landing. If an unstabilized approach develops in the final approach phase, “go-around” is called for, and is required by virtually all airlines.
Insuring a stabilized approach involves both electronic warning systems and pilot procedures. Electronic systems have been developed called TCAS (for “Traffic Collision Avoidance System”) which provide aural cockpit warnings of traffic conflicts, as well as high sink rates at low altitude and certain other warnings. These warnings are automatic and impossible to ignore. If such a warning occurs, immediate pilot action is required. In the case of a high sink rate warning, abandonment of the landing approach and a go-around is mandatory.
With respect to standard pilot operating procedures, at TWA we had certain specific call-outs which were required on every approach.
By way of explanation, it is standard procedure to rotate the “pilot flying” (PF) and the “pilot not flying” (PNF) duties between the captain and the first officer (co-pilot) at the discretion of the captain. The duties of the PNF are primarily communications, and secondarily, reading checklists, and thirdly, monitoring the actions and performance of the PF. If the actions of the PF indicate a misunderstanding of an ATC instruction, or if an unsafe condition is recognized, the PNF is required to so advise the PF or take such other action as is necessary to maintain safety of flight.
On final approach the PNF is charged with monitoring all of the stabilized approach parameters as follows:
- At 500 feet above the surface (above ground level or AGL), as registered precisely on the radar altimeter, the PNF is required to call out airspeed and sink rate. He also monitors power, glide path, and runway alignment, and makes appropriate call-outs if there are significant deviations from the stabilized approach parameters. Normal sink rate at 500 feet AGL is 600-700 feet per minute (FPM). Any sink rate in excess of 1000 FPM or any airspeed deviation in excess of 5 knots above or below the predetermined final approach airspeed (as indicated by a preset manual bug on the airspeed indicator) is cause for a go-around. Although some discretion is allowed in unusual circumstances, particularly in extremely gusty conditions, a pilot knows instinctively when the approach is unstabilized such that safety of flight dictates a go-round.
- At 100 feet AGL the PNF calls out “100 feet”. At this point if a safe landing is not assured, a go around is mandatory.
- On wide-body aircraft, starting at 50 feet AGL, the PNF calls out altitude every 10 feet until touchdown.
If there were procedures like this in effect at Asiana Airlines, and if the required procedures were followed, it is almost impossible to imagine how the accident we saw at SFO could have occurred.
I would also add that the approach to runway 28L at SFO is a long straight-in approach to a long runway. I have flown hundreds of approaches to this runway, in many types of aircraft, both propeller and jet. In my opinion, the approach to runway 28L at SFO is one of the safest approaches in the United States.
(Note: I flew twin engine propeller aircraft into SFO as an air ambulance/charter pilot during a TWA furlough in the mid-70s. Among my passengers were presidential candidate Jimmy Carter along with his Secret Service protection, and Rev. Jim Jones of the People’s Temple.)
My recollection is that runway 28L has a Visual Approach Slope Indicator (VASI), near the touchdown point on the left side of the runway, which indicates by means of an array of red and white lights the vertical position of the approaching aircraft relative to the desired visual glide path. This ground-based indicator is easy to use and interpret. It becomes second-nature to a pilot to be aware of this indication in order to confirm the visual glide path and avoid a land-short incident.
Absent adverse weather conditions, which were not present in the case of Asiana Flight 214, there should have been no difficulty in establishing a stabilized approach to this runway.
The pilots of Asiana Flight 214 have been described as “veteran pilots”. If this is true, it is almost unimaginable that these pilots could have allowed an unstabilized approach to develop on runway 28L at SFO, or that they would deviate from standard operating procedures to the extent that they would continue an unstabilized approach to an attempted landing. To have done this would have required them to ignore loud aural warnings from the TCAS equipment, as well as to deviate from the most basic principles of piloting a swept-wing jet aircraft to a safe landing.
For this reason, I cannot exclude the possibility of some kind of mechanical malfunction as the cause for this accident until it is absolutely ruled out.
If mechanical malfunctions are ruled out, then it is impossible not to conclude that there is something seriously deficient with either the pilot training system or the flight operations procedures at Asiana Airlines, or both.
David F. LaRocque
Captain TWA (ret)
CDR USNR (ret)
