Johnathan Council M4 Complexity
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Running head: COMPELXITY OF THE AIRBUS 320 AUTOTHRUST SYSTEM
Complexity of the Airbus 320 Autothrust System
Johnathan Council
Embry-Riddle Worldwide
12 November 2023
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COMPELXITY OF THE AIRBUS 320 AUTOTHRUST SYSTEM
Complexity of the Airbus 320 Autothrust System
For a system to be complex, it may be considered relatively unpredictable, requires a considerable amount of effort to understand, and is defined by more than the sum of its parts (Mobus, & Kalton, 2014). It is not just a property of the system, it is an amalgamation of human perception and interaction with the system (Schottl, & Lindemann, 2015). Complexity in and of itself is not a clearly defined attribute of a system nor does system complexity inherently mean that a system is good. As pointed out by Mobus and Kalton, systems can be too complex and with increased complexity comes the potential for small system failures that can produce cascading effects causing a collapse of an entire system (2014). Research by Brown concluded similar risks with his complexity creep theory, which suggests increasing levels of complexity create a greater potential for risks (2016). Increased complexity may solve certain problems while creating new ones. Modern commercial aircraft are an example of complex sociotechnical systems comprised of smaller, equally complex subsystems. Over time, aircraft automation has increased in complexity in many ways. One complex subsystem that is misunderstood and requires considerable time to fully understand is the autothrust (ATHR) system of an Airbus 320 (A320). The A320 ATHR system is a complex system often misunderstood by trainees and underestimated by even experienced pilots transiting from other aircraft. The result has been an increase in undesirable aircraft states and a push by the industry to revisit the intricacies and complexities of the system that is tied to many other subsystems inside the A320.
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COMPELXITY OF THE AIRBUS 320 AUTOTHRUST SYSTEM
Figure 1
An Airbus 320 Throttle Quadrant.
Note
: This is the standard throttle quadrant found in all Airbus 320 variants. Autothrust systems are not new to commercial aviation, however, they have evolved into much more complex systems in modern aircraft. While the ATHR system is not the most complex system on an A320, it is complex enough that not fully understanding the system can and has resulted in issues in-flight. As can be seen in the image above, the throttle quadrant has four detents along the side (i.e. 0, CL, FLX/MCT, and TOGA). On the ground, the system is automatically armed during normal run-up procedures and remains armed under normal operating conditions. The ATHR becomes active when the levers are placed between the 0 and
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COMPELXITY OF THE AIRBUS 320 AUTOTHRUST SYSTEM
CL detents. Typically, after receiving clearance for takeoff, aircrews place the levers in either the
FLX/MCT or TOGA detent depending on thrust needs. After reaching 1000 feet above the ground, the aircraft system commands the levers be moved back to the CL detent. Upon placing the levers in the CL detent, the aircraft automation assumes control of the aircraft throttles and the ATHR becomes active. Even though the aircraft is making adjustment to the thrust, the levers
on the throttle quadrant will not physically move, unlike Boeing aircraft (this can become precarious for pilots switching from Boeing aircraft to Airbus). When active, the ATHR system has the ability to adjust thrust output from idle up to “climb” thrust. The ATHR system does not simply control thrust output from the engines though.
The autopilot and ATHR are separate systems. If the autopilot is engaged as well, the systems will work in concert to determine the appropriate climb airspeed, vertical rate, or attitude depending upon the pilots’ specific request or constraints programed into the system. It does the same thing while descending. The information is shared between the two systems to determine the appropriate amount of thrust for a given mode of flight. Under certain circumstances, it will disregard speed barriers depending on what the pilot is asking it to do and if the pilot is unaware of what the system will ignore in order to meet their specific request, the system may overspeed or worse, fly to slow and risk a stall event. The system is also tied to the flight director (FD) that gives pitch and roll commands to the pilot via an attitude display. Figure 2 below shows an example of an attitude display. The FD is the green crosshair overlayed on top of the attitude indicator. The ATHR system will adjust power to allow the pilot or aircraft to follow FD commands whether the autopilot is engaged or not.
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COMPELXITY OF THE AIRBUS 320 AUTOTHRUST SYSTEM
Figure 2: A320 Attitude Indicator and Flight Director (FD)
Note
: The green crosshair represents the FD which is giving commands based on the programmed computer or specific pilot requests made via a control panel. What has gotten some pilots in trouble recently in the commercial industry is not understanding that when the autopilot is off and the ATHR on, the system will attempt to follow FD commands even though the pilot may be trying to do something else. This means that the system is giving commands to the engines that may not correspond with the attitude inputs from the pilot. Not understanding what the ATHR will do under specific circumstances and modes can
have less than desirable results. Pilots transitioning from other aircraft can experience a negative
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COMPELXITY OF THE AIRBUS 320 AUTOTHRUST SYSTEM
transfer of training, where previous training interferes with and disrupts performance of a new task (Mussgens, & Ullen, 2015). A lack of understanding or familiarity of the ATHR system can place the aircraft in a dangerous state. In stressful environments such as busy airspace or modes of flight that are especially close to the ground, misuse and misinterpretation of this system can exacerbate stress and increase the likelihood of mistakes and errors. The ATHR concept may seem simple, but this system is much sophisticated than it first appears. Careful consideration by new and experienced pilots must be given and special care taken by training facilities to ensure that this complex system is adequately understood prior to operating in the national airspace environment.
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COMPELXITY OF THE AIRBUS 320 AUTOTHRUST SYSTEM
References
Brown, J. P. (2016). The effect of automation on human factors in aviation.
The Journal of Instrumentation, Automation and Systems
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(2), 31–46. https://doi.org/10.21535/jias.v3i2.916
Mobus, G. E., & Kalton, M. C. (2014).
Principles of Systems Science
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Müssgens, D. M., & Ullén, F. (2015). Transfer in motor sequence learning: Effects of practice schedule and sequence context.
Frontiers in Human Neuroscience
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. https://doi.org/10.3389/fnhum.2015.00642
PM Flight. (2016, January 5).
A320 primary flight display
. PMFlight. https://pmflight.co.uk/a320-primary-flight-display/
Schöttl, F., & Lindemann, U. (2015). Quantifying the complexity of socio-technical systems – a generic, interdisciplinary approach.
Procedia Computer Science
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, 1–10. https://doi.org/10.1016/j.procs.2015.03.019