Monday, January 25, 2021

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  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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