Sriwijaya Air Flight 182 Investigation Focuses on Automatic Throttle


 

On Saturday, January 9 of this
year, Sriwijaya Air Flight 182 took off from Soekarno-Hatta International Airport
near Jakarta a little after 2 P. M. with 62 passengers and crew on board,
headed for Pontianak, Indonesia.  It was
raining heavily, and less than five minutes into the flight, shortly after the
plane crossed the shore of the Java Sea, flight controllers and radar lost
contact with it.  A fishing boat reported
hearing a loud explosion in the vicinity of the disappearance, and search
parties have since discovered debris spread over a 2-km area (over a
mile).  There were no survivors.

 

Although the flight-data
recorder was recovered, the cockpit voice recorder (CVR), a separate piece of
equipment, broke apart in the crash.  The
CVR’s memory unit is designed to survive even if the CVR is destroyed, but
understandably it is difficult to locate, and has not yet been found.

 

Even without the CVR, investigators
with the Indonesian National Transportation Safety Committee have begun to
focus their attention on the automatic thrust controller of the Boeing 737
plane, which entered into service in 1994. (This is an earlier version of the
737 than the later MAX series, which was grounded for more than a year over
concerns about flight software when two crashes were traced to software
malfunctions.) 

 

Manual aircraft throttles are
very similar to the gas pedal in a car: 
they simply control the rate of fuel supplied to the engines.  Depending on wind conditions, angle of
attack, and other factors, a given throttle setting can result in very
different amounts of either thrust or speed. 
To maintain constant speed, pilots would have to be constantly fiddling
with manual throttles.  So modern
airliners have autothrottle control systems that can be set either to maintain
a constant speed (within safety limits) or a constant thrust, which is
convenient for takeoff and landing.  In
this connection, many autothrottles have a takeoff/go-around switch that is
normally engaged only during takeoff or if the pilot decides to abort a
landing.  Pressing the go-around switch
provides a controlled amount of added thrust that will enable a smooth
transition from an aborted approach to going around for another approach.  But to engage the go-around switch before entering
an approach descent is a bad mistake.

 

This is apparently just what
happened in another crash involving a Boeing 767 cargo plane heading for
Houston’s Bush Airport in February of 2019. 
Investigators believe that while the plane was a good distance away from
the airport, the first officer’s wristwatch accidentally hit the
“go-around” button on the autothruster, causing a surge in power.  The plane was in the clouds, which obscured
all visible horizon references, and a sudden acceleration can feel like an
upward tilt to pilots who are not sufficiently trained to ignore such
sensations and pay attention to their instruments instead.  Evidently, the first officer, who was flying
the plane at the time, decided that the plane was stalling and pushed the
control stick forward, sending the aircraft into a dive from which it could not
recover.  It crashed into a swampy area
east of Houston with the loss of all three crew members. 

 

It’s not yet clear what happened
to Sriwijaya Flight 182, but the flight data recovered so far show that one of
the plane’s two engines was producing much more thrust than the other for the
final seconds prior to the crash.  News
reports indicate that there may have been problems with the autothrottle
computer on this particular plane in the weeks before the crash.

 

Unequal thrust on a plane, if
not corrected or compensated for, can cause severe rolling and even destabilize
its flight and send it into a dive. 
Normally, pilots are trained to notice such situations and to deal with
them promptly.  But a factor in the
Houston crash was inattention:  neither
the captain nor the first officer understood what was going on until it was too
late to do anything about it. 

 

In the weather conditions
under which Sriwijaya Flight 182 took off, the pilots had to rely on their
instruments during and after takeoff, and it’s possible that they failed to
figure out what was happening during the fifteen seconds or so that the
aircraft took to fall from 10,000 feet into the Java Sea. 

 

Fifteen seconds is not much
time to diagnose one specific problem from a complicated set of instrument
readings and take exactly the right evasive action to deal with it.  As aircraft have become more complex, there
are more things that can go wrong, and pilots have to be trained to deal with
each one of them in the appropriate way. 
Especially during takeoff and landing, where margins of error are small,
having the correct response to a sudden emergency can mean the difference
between life and death.

 

But 99.9% of the time,
piloting a modern aircraft is about as exciting as driving to the grocery
store—less, if you consider that in driving a car you don’t have some friendly
ground controller telling every other car exactly where to go so you have
plenty of room to drive in.  But during
every second of flight, pilots are expected to be in a state of high vigilance
with the procedures for dealing with dozens of different kinds of emergencies
at their mental fingertips.  While some pilots
do meet these standards, as for example Chesley Sullenberger did during the
2009 Flight 1549 ditching in the East River, others do not.  The ones that don’t can nevertheless fly
without incident for years, because the systems they operate behave themselves.

 

But when something goes wrong,
as it evidently did during Flight 182 out of Jakarta, the responses of the
pilots are critical.  While we will have
to await the full investigation results to be sure, it appears that both
maintenance and training procedures for Sriwijaya Air may need to be overhauled.  Better maintenance can prevent mechanical
malfunctions such as autothrottle failures from happening, and better training
can help pilots more closely approach the ideal of eternal vigilance that is
ready to deal with even unlikely emergencies such as autothrottle
failures. 

 

Sources:  I referred to
an article in Bloomberg News that describes what is currently known of the
accident investigation results for Sriwijaya Air Flight 182 at https://www.bloomberg.com/news/articles/2021-01-20/faulty-automatic-throttle-eyed-in-indonesia-jet-crash-probe.  I also referred to Wikipedia articles on the
crash as well as that source’s articles on autothrottles, Atlas Air Flight
3591, and takeoff/go-around switches. 



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