Central to the question of just what happened in the cockpit of Asiana Flight
214 in the moments before it slammed into a sea wall at San Francisco
International Airport is why the crew allowed the airspeed to fall so far below
the targeted airspeed needed to bring the Boeing 777 in safely.
National Transportation Safety Board Chairman Deborah Hersman told reporters
in her final briefing Thursday that a preliminary review of flight data
indicates “no anomalous behavior” of the autopilot, auto-throttle system or
flight director — three systems regularly used by pilots during various phases
of a flight, including landing.
Saturday’s crash killed three people and injured dozens more. But even as
crews finish clearing the wreckage from Runway 28L, the charred and broken
airplane serves as a reminder of the remarkable engineering of modern airliners
and safety advancements that allowed so many people to survive an impact that
sheared the plane’s tail, snapped off an engine and scattered debris across a
wide swath of runway.
Flight 214 came in under clear skies, with low wind and excellent visibility.
Because of clear weather, and an instrument landing system that was offline at
the time, the pilots were cleared to make a visual approach. Hersman says
interviews with the pilots indicate they believed the auto-throttle system was
engaged during final approach, meaning it would automatically control engine
thrust, maintaining the reference airspeed requested by the pilots. The
reference airspeed is based on the weight and configuration of the airplane, and
provides a safe airspeed for the pilots to fly during the approach.
The pilots disconnected the autopilot 82 seconds before hitting the sea wall,
at 1,600 feet. This is normal during a visual approach, when pilots “hand fly”
the airplane. On modern airliners, however, it is possible for the pilots to
leave the thrust control — controlled by a pair of levers between the pilots in
the 777 — in an automated mode. Though retired captain and veteran airline
instructor Gene Fish says pilots are often taught to disconnect the
auto-throttle system when hand flying because it could end up “chasing” the
control inputs made by the pilot. With more than 39,000 hours, Fish has been
flying since 1954 and has flown the 727, 737, 747, 757, 767 and 777. He says as
a pilot makes adjustments to the descent profile, the auto-throttles make the
appropriate changes, but because they are acting based on the control inputs of
the pilot, a new change from the pilot could happen just as the auto-throttles
are providing the thrust from the previous input.
“When the autopilot works with the auto throttle, it’s making minute changes,
maybe a tenth of degree of a pitch change,” Fish says of the close coupling
between autopilot and the auto-throttle system. “With hand flying the airplane,
we’re making two, three, four, maybe five degree pitch changes.”
He says most pilots find it easier to keep the proper thrust setting and
pitch attitude when hand flying an approach, by using their hand to control the
thrust levers (the larger set of levers on the right in the center console
picture below) since the pilot can anticipate their own needs as they adjust
their descent.
If a pilot does want to use the auto-throttle system, they must first “arm”
the system so it can be used, something Fish says this done at the beginning of
a flight and usually left on. On the glare shield above the instrument panel in
front of the pilot are a pair of switches used to arm the system for the right
and left engines. Immediately to the right of the switches is a digital display
showing the selected airspeed chosen by the pilot. It is the left most display
on the glare shield, displaying 142 knots in the photo above. A dial beneath it
allows a pilot to select the desired airspeed.
In order for the autopilot or the auto-throttle system to fly at the assigned
airspeed, the pilot must engage the system(s) with the push of a button. If the
autopilot and auto-throttle system is engaged, it can use the speed selected by
the pilot if in speed mode, and can also fly the airplane on a selected heading,
or to a selected waypoint while holding or adjusting altitude. The autopilot
system in a 777 also can land the aircraft on runways equipped with the
necessary approach.
We recently visited a Boeing 777 simulator and were able to make two landing
approaches. On the first, the auto-land system was used in which the autopilot
guided the airplane to touchdown on the center line of the runway. On the
second, we hand-flew the airplane, with the auto-throttle system engaged. We
could see and feel even small changes in the levers as they moved about while
the system adjusted the thrust needed to maintain the requested airspeed of 136
knots.
Hersman said the agency will look carefully at what the flight crew was doing
and what it’s thinking was during the final minute of the flight, and closely
examine its use of the auto-throttle system.
“There are expectations that the crew is monitoring speed as they’re on
approach,” she told reporters.
During a normal approach, one of the two pilots in an airliner typically
keeps an eye on airspeed during descent to ensure the airplane is flying at a
safe speed. One question raised by many pilots is why the Asiana 214 crew did
not notice the decaying airspeed until seconds before impact, by which time the
plane already was some 30 knots (~35 mph) below the target speed of 137 knots
(~157 mph). Hersman said there is no mention of airspeed on the cockpit voice
recorder as the plane descended through 500 feet at 134 knots, until after a
callout was made at 100 feet — by which time the airspeed was less than 112
knots. Falling just a few knots below the target speed usually is sufficient to
prompt a response from a pilot. It wasn’t until after the 100 foot announcement
— roughly nine seconds before impact — that there were two calls in the cockpit
for a go-around.
Another question raised by pilots is what, if any, role might have been
played by the difference in auto-throttle system designs between Boeing and
Airbus aircraft. In a Boeing airliner, the levers move when the system is
engaged, providing tactile feedback. In the Airbus system, the levers are placed
in different positions for different phases of the flight, but do not move as
the system adjusts the thrust. The two systems also differ in how much absolute
control the fly-by-wire systems allow the pilots to have during flight.
The pilot at the controls during the crash was in the final stage of his
transition training in the Boeing 777. He had spent the past several years
flying an Airbus A320 and had nearly 10,000 hours’ experience. The instructor
pilot in the right seat was a veteran 777 captain with more than 12,000 hours of
flying experience.
Despite the eerie scene of wreckage scattered across the runway, Hersman said
several things about the debris field highlight the safety of a modern airliner.
One common photo of the scene shows the main landing gear, which hit the seawall
and broke off. Although that may look alarming to the lay person, Hersman says
preliminary findings indicate the landing gear did exactly what it is supposed
to.
“Landing gear is designed to be frangible, so that it can separate from the
aircraft,” she noted. “It performed as deigned.”
The landing gear is designed to snap so there is less chance it might
puncture the fuel tanks, located in the wings just above the gear, possibly
creating a fire. Hersman said Asiana 214's tanks were “not breached” and the
fire was started by engine oil that ignited, slowly spreading flames to other
parts of the airplane.
Beyond determining what caused the crash, federal investigators and a team
from Boeing and others in the airline industry will carefully analyze the
airplane. What they learn of the crash, and how the airplane performed during
and after it, could lead to improvements in future aircraft or retrofits to
existing airplanes.
With the on-sight investigation nearing completion, Hersman said “this is
just the tip of the iceberg,” with regards to the work that needs to be done.
Investigators will continue to analyze the flight data and voice recorders,
along with the pilot interviews and aircraft wreckage to try and understand what
might have led to the pilots not maintaining the necessary airspeed before the
crash. Hersman says it typically takes 12 to 18 months to produce a final
report.
