The
National Transportation and Safety Board of the United States, NTSB, recently
released it’s findings on the crash of Asiana’sFlight 214, which crashed on July 6, 2013 at San Francisco International
Airport (SFO). All professional aviators
and their instructors as well as air traffic control specialists should read
and study the report. The information in
it describes the enormous potential for and realization of unimagined failures
that always exist in the complex relationship between humans and the machines
they control.
There
is no question that the crew of Asiana 214 (humans) failed to effectively
manage their Boeing 777 in the final moments of the flight. The NTSB detailed the operational errors of
the crew, the design flaws of the flight guidance system and some relevant
issues related to their training. The report
also highlights numerous other contributory factors, but uncharacteristically
for the NTSB, missed a very obvious one, air traffic control. The NTSB has been the world’s
“gold standard” in aviation accident investigation for decades. Unfortunately, it seems the gold might have
lost some of its glitter.
The
probable cause of Asiana 214 focused on human failures, primarily by the crew
and secondarily by designers and trainers, which were thoroughly examined in
the investigation. However, there are
other human components of this story that should also be examined for their
role in this fatal accident. Specifically overlooked was the environment in
which the flight operated. The machine
was a Boeing 777. The environment was the airspace near SFO. The humans are the
engineers, instructors, air traffic controllers and pilots who design, train
and operate them. The NTSB report on Asiana 214 surprisingly describes the
contributory role of the aircraft and each of these other elements except
air traffic control. How can this report
have such an obvious omission? This is
very perplexing. Was it just a huge
oversight or an intentional exclusion?
In light of the fact that there was speculation by the Board on other
contributing factors, this oversight is even more astonishing.
Currently,
at large metropolitan airports where the air traffic volume is heavy, a flight
crews’ role has evolved into that of a facilitator
between air traffic control instructions and the airliner’s
flight guidance system rather than that of aviator. Like all pilots flying modern airplanes into
highly controlled airports, including SFO, the crew of Asiana 214 was tasked
with converting the commands (not suggestions) of ATC into a desired flight
path and energy state by programming the aircraft’s
flight guidance system. The Boeing 777 is
both designed and its pilots trained to operate it with reference to the flight
guidance system. Desired parameters are
entered and the computed information is shown to the pilots by reference to the
flight directors on the primary flight display (PFD) or heads up display
(HUD). The pilots follow the flight
director commands manually or by engaging the autopilot. The flight directors are turned off in flight
on only very rare occasions.
Because the Asiana crew was
ineffective applying their approach clearance to the flight guidance system,
their workload increased dramatically.
ATC’s instruction to maintain a high speed longer than normal increased task saturation as well.
A late landing clearance by SFO tower provided an additional distraction
diverting the crew’s attention away from subtle flight guidance information in
the final seconds before the crash.
Although
there are many aspects to the air traffic control handling of Asiana 214, I
will limit the discussion of ATC’s contributory role to the
time period after the crew received their approach clearance.
From
the NTSB history of the flight (italics):
"According to recorded
information, NORCAL stated “Asiana 214 heavy, San Francisco airport 9 to 10 o’clock, one seven miles, do you have it
in sight?” The flight responded they had the field in sight and NORCAL cleared
them to fly a visual approach to runway 28L on an assigned heading of 310° to
intercept final approach. At a point approximately 14 NM5 from the airport NORCAL instructed the
flight to maintain 180 kts until 5 NM from the airport."
This command by ATC, as well as the inoperative
electronic glide slope, set up a series of events that facilitated the
crash. It is not certain, but clearly
reasonable to infer that if the crew had been allowed to slow the aircraft at
their discretion, or if there had been an operable electronic glide slope, the
unintended consequences of autothrottle “HOLD” would have occurred at a higher
altitude or not at all. How can the NTSB
omit these relevant facts?
The NTSB found that the flight could have been
completed successfully without the electronic glide slope. That statement is not particularly probative
since it is made without context. As the Asiana pilots pointed out in their
submission, a Flight Safety Foundation (FSF) report on approach and landing
accidents (ALA) stated that 75% of flights did not use or have a precision
approach aid available. The FSF further
states that the accident risk is 5 times greater for commercial aircraft flying
non-precision approaches.
In the hearings the NTSB conducted on the human
factors of this accident, Dr.s Sarter and Abbott and Captain McKenney all
testified to the interrelationships between parts of a safety system. As the
“swiss cheese metaphor” for safety systems shows, the slices are only effective
when used in combination. Removal of one
slice may or may not be significant depending on the make up of the other
slices. Why does one plane crash in the
same conditions that allow many others to be successful? That is why the role of ATC in this accident
cannot just be brushed as inconsequential.
These are exactly the issues the NTSB is tasked with considering.
“The trainee captain stated
that after arming and intercepting the localizer he set the DUYET intersection
crossing altitude of 1,800 ft. MSL in the mode control panel (MCP) altitude
window and began using the AFDS vertical speed (VS) mode to descend at 1000
feet per minute (FPM). The IP stated the arc on the navigation display (ND)
showed they would be high at DUYET. The observer stated he thought they were a
little higher than the normal profile when they were cleared for the approach,
and they extended the landing gear earlier than normal because they needed to
get down. The trainee captain set the command airspeed bug to 172 kts, and
acknowledged when the observer commented this was below the assigned 180 kts.”
This emphasizes the fact that the 180 knot
restriction was increasing the crew's task saturation by complicating energy management solutions.
At the prompting of the IP, the
trainee captain increased the command vertical speed to 1,500 FPM. The trainee
captain commanded the flaps be set to 20° and requested the IP to reset the
command altitude from 1,800 ft. to 3,000 ft., the missed approach altitude, as
the flight approached 2,000 ft. MSL.
According to recorded data, at
1,600 ft. MSL the AFDS pitch mode changed to FLCH SPD, the throttles began to
increase power, and the airplane pitch attitude began to increase; this was
followed by autopilot disconnect and shortly thereafter a reduction in the
thrust levers to the idle position, followed by a change of the autothrottle
mode from THRUST to HOLD. The trainee captain stated he considered pressing the
FLCH pushbutton to obtain a higher descent rate but he could not recall what he
did for sure. He disconnected the autopilot and called out “manual flight.”
None of the three pilots could recall the autothrottle status displayed on the
flight mode annunciator (FMA). The IP stated he set the command airspeed to the
approach speed of 137 kts. and turned both flight director (FD) switches off
and then turned the right FD switch back on. Recorded data showed the left FD
switch was turned off but the right FD switch remained on.
Pre selecting the missed approach altitude is
common practice by B777 crews. However,
setting an altitude above the aircraft without the flight guidance capturing
and maintaining either an altitude or glide slope caused the autopilot to
initiate a climb. The pilot naturally and appropriately disconnected the
autopilot and manually flew the aircraft. Unfortunately, that unusual set of
circumstances caused the auto throttle mode to remain in "HOLD" and
not respond to the commanded speed as the crew expected.
“The trainee captain called for
the flaps to be set to 30°, and after a delay due to the airspeed being in
excess of the flap limit speed of 170 kts. the IP placed the flaps to 30°.”
Because ATC instructed Asiana 214 to maintain 180
knots until 5 miles, they would be 10 knots above the limit speed for flaps 30°
and 43 knots above their target approach airspeed at approximately 1500’ AGL (3 degree glide path). Without
an electronic glideslope or a VNAV PATH, this complicated programming of the
flight guidance system. At 1000’ AGL,
their mindset would likely have been slowing the airplane down quickly as they
descended and closed on the runway. The
power needed to be reduced to idle to get below the limit speed for flaps 30
(170 knots) as well as slowing to the target approach speed of 137 knots. This maneuver, set up by ATC requirements was
clearly contributory to the accident.
Flight 214 was handed off to, but not
acknowledged by, San Francisco Tower. In
combination with the ATC imposed energy management complication, lack of
electronic glide slope and flight guidance mode confusion, this was another
significant distraction.
“The instructor pilot stated he
saw 2 red and 2 white precision approach path indicator (PAPI) lights at 1,000
ft. MSL and speed was a little high. The observer saw the descent rate on the
vertical speed indicator was in excess of 1,000 FPM as the flight descended
through 1,000 ft. MSL, and he called out “sink rate” several times. The IP
stated he heard this callout but they were still high and the descent rate was
only 1,000 to 1,100 FPM.”
Slightly above the glide path and slowing, the
trainee captain manually flew the aircraft toward what he thought would be a
stable approach at 500’. Unaware of the
significance of the autothrottle “HOLD” mode, the aircraft slowed toward target
speed. Still not cleared to land and
below 1000’ the
crew once again queried San Francisco Tower and received landing clearance 9
seconds later. The late prompted landing
clearance was an ATC distraction that diverted the crew’s attention at a very critical time.
The aircraft was on glide path with the idle
power as it descended toward 500’. At
about 500’ the
aircraft began to drift below the PAPI (glidepath). As the pilot began to adjust the descent
with pitch, the aircraft quickly slowed well below target speed. By the time the crew was alerted, noticed the
low speed and initiated a go around it was too late.
In any accident, there are many factors that
intersect to create the causal conditions.
Failure to consider all of them results in an incomplete and ineffective
investigation. The relevance air traffic control resources, procedures and
practices might have had on this accident is not the depth of analysis we have
come to expect from the NTSB.
It would be extremely naive to believe
ineffective or improper use of the flight guidance system to comply with air
traffic control instructions only affects a few crews. Recognition by the NTSB that air traffic
control practices have a profound impact on how crews operate the flight guidance
systems of modern airliners would be more consistent with their role in
aviation safety.
Since the report on Asiana 214 was prepared under
his leadership, I would hope that Acting NTSB Chairman Hart would be given the
opportunity to give his perspective on this controversy during his confirmation
process.
