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 FSF 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 (ALA) phase, and using our analysis that 83 percent
of ALAs 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
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.
Conclusion
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.
