The recent unfortunate accident involving Flight 214, a Boeing-777 aircraft, of Asiana Airlines at San Francisco is a case of a fully serviceable aircraft flying in to the ground in VMC conditions; due to an approach that became increasingly unstabilized with height. Could this accident be averted, had the crew taken a timely decision to go-around? Conditions being VMC, as per the recommendations enumerated below, the approach should have stabilized latest by 500 ft and should have remained so below 500’, to continue with an approach to land. As per reports, the airplane was configured for landing with 30 degrees of flaps and gear down with a target threshold speed of 137 knots. The aircraft descended through an altitude of 1400 ft at 170 kts and slowed down to 149 kts at 1000 feet. The throttles were reportedly at idle and the auto throttle was armed. At 500 feet altitude, 34 seconds prior to impact, the speed dropped to 134 kts, which was just below the target threshold speed. Any speed below the target speed, the approach should have been considered unstabilized, especially below 500 ft in VMC. The situation was allowed to worsen further when the airspeed dropped significantly, reaching 118 knots at 200 feet altitude. Eight seconds prior to impact, the throttles were moved forward. Airspeed reduced further to 112 knots at an altitude of 125 feet. Seven seconds prior to impact, one of the crew members made a call to increase speed. The stick shaker sounded 4 seconds prior to impact. One second later the speed was 103 knots, the lowest recorded by the FDR. One of the crew members made a call for go-around at 1.5 seconds before impact. This was too late to prevent an accident off an approach that was unstabilized. Accidents during the approach and landing (ALA) phase account for a major percentage of all accidents.
Analysis of data collected by a go-around study being conducted by the Flight Safety Foundation’s (FSF) international and European aviation committees has shown that potentially 54 percent of all aircraft accidents in year 2011 could have been prevented by a timely go-around decision by the flight deck crew. Clarifying on the figure of 54%, the director of global programs is quoted to have said that, “this is based on 65 percent of that year’s accidents being in the approach and landing () phase, and using our analysis that 83 percent of s could be prevented by a go-around decision”. The study has also elaborated that “the majority of accidents over the last 10 years have occurred during the approach, landing and go-around flight phases. The study has also highlighted the fact that the lack of a go-around decision is the leading risk factor in approach and landing accidents and is the primary cause of runway excursions during landing. Yet, less than 5% of unstabilized approaches lead to a go-around.”
Unstabilized approaches have been attributed to various factors, which include company policies, human factors, weather, crew resource management, ATC and automation. The Asiana case can also be attributed to a number of these factors. The crew felt that the auto throttle should have maintained the speed at 137 kts; the PF and PM were not effectively flying and monitoring the flight path and parameters, which permitted the IAS to drop well below the target speed, and the throttles to remain at idle; company culture may have also come into play in this scenario; fatigue after a long flight due to improper work load assignment – there were four pilots on this flight; three of them in the cockpit during the approach. All of these factors that led to the unstabilized approach would have been taken care of, if the crew had made the decision to go around in time. When things are not as planned in aviation, it is always better to have height and speed in hand. In this particular case there were many issues that pointed towards an unstabilized approach. To understand this it is best to study the elements that constitute a stabilized approach.
Stabilized approach concept is all about maintaining a stable speed, descent rate, and vertical/ lateral flight path in the landing configuration. It is felt that a stabilized approach would generally lead to a safe landing, as the crew’s awareness of the horizontal/ vertical flight path; the IAS and the energy-condition of the aircraft would lead to improved overall situational awareness during the approach. Also, flying a stabilized approach permits the crew to devote adequate time and limited human ‘attention resources’ to flying, monitoring, communications, weather conditions, systems check, and most importantly to decision making. As has been brought out earlier, 95% of unstabilized approaches do not lead to a go-around, and this has contributed to 54% of the total accidents in 2011, and the lack of go-around decision is a major risk factor in ALA. It is now an accepted fact that the decision to execute a go-around is no indication of poor performance.
Recommended Elements of a Stabilized Approach
The following recommendations are developed by the Flight Safety Foundation. All approaches should be stabilized by 1,000 feet above airfield elevation (AFE) in instrument meteorological conditions (IMC) and by 500 feet AFE in visual meteorological conditions (VMC). An approach is considered stabilized when all of the following criteria are met:
· The airplane is on the correct flight path
· Only small changes in heading and pitch are required to maintain the correct flight path
· The airplane speed is not more than VREF + 20 knots indicated airspeed and not less than Vref
· The airplane is in the correct landing configuration
· Sink rate is no greater than 1,000 fpm; if an approach requires a sink rate greater than 1,000 fpm, a special briefing should be conducted
· Thrust setting is appropriate for the airplane configuration, and not below the minimum power on approach as defined by the aircraft operating manual.
· All briefings and checklists have been conducted.
· Specific types of approaches are stabilized if they also fulfill the following:
- ILS approaches should be flown within one dot of the glide slope and localizer
- During a circling approach, wings should be level on finals when the airplane reaches 300 feet AFE.
- Unique approach procedures or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing.
For safety reasons, an approach that becomes un-stabilized below 1,000 feet AFE in IMC, or below 500 feet AFE in VMC should be discontinued and a go-around executed. Also, stabilized conditions should be maintained throughout the rest of the approach for it to be considered a stabilized approach. If the above criteria cannot be established and maintained at and below 500 feet AFE, initiate a go-around.
Considering the statistics and the facts enumerated above, it is felt that all efforts should be made to fly stabilized approaches, and if due to some reason an approach becomes unstabilized then the decision to go-around should be taken well in time. A well considered, and executed, go-around will go a long way in ensuring safe operations, and enhance safety in aviation.