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Formal Verification and Modeling in Human-Machine Systems: Papers from the AAAI Spring Symposium Learning the Task Management Space of an Aircraft Approach Model Joseph Krall and Tim Menzies Lane Department
Formal Verification and Modeling in Human-Machine Systems: Papers from the AAAI Spring Symposium Learning the Task Management Space of an Aircraft Approach Model Joseph Krall and Tim Menzies Lane Department of CS&EE West Virginia University, WV, USA Misty Davies Intelligent Systems Division NASA Ames Research Center, CA, USA Abstract Validating models of airspace operations is a particular challenge. These models are often aimed at finding and exploring safety violations, and aim to be accurate representations of real-world behavior. However, the rules governing the behavior are quite complex: nonlinear physics, operational modes, human behavior, and stochastic environmental concerns all determine the responses of the system. In this paper, we present a study on aircraft runway approaches as modeled in Georgia Tech s Work Models that Compute (WMC) simulation. We use a new learner, Genetic-Active Learning for Search-Based Software Engineering (GALE) to discover the Pareto frontiers defined by cognitive structures. These cognitive structures organize the prioritization and assignment of tasks of each pilot during approaches. We discuss the benefits of our approach, and also discuss future work necessary to enable uncertainty quantification. The Motivation Complexity in Aerospace Complexity that works is built of modules that work perfectly, layered one over the other. Kevin Kelly The National Airspace System (NAS) is complex. Each airplane is an intricate piece of machinery with both mechanical and electrical linkages between its many components. Engineers and operators must constantly decide which components and interactions within the airplane can be neglected. As one example, the algorithms that control the heading of aircraft are usually based on linearized versions of the actual (very nonlinear) dynamics of the aircraft in its environment. (Blakelock 1991) Each airplane must also interact with other airplanes and the environment. For instance, weather can cause simple disruptions to the flow of the airspace, or be a contributing factor to major disasters. (NTSB 2010) Major research efforts are currently focused on models and software to mitigate weather-based risks. (Le Ny and Balakrishnan 2010) The glue for these interacting airspace systems consists primarily of people. Pilots and air traffic controllers are the final arbiters and the primary adaptive elements; they are Copyright c 2014, Association for the Advancement of Artificial Intelligence (www.aaai.org). All rights reserved. expected to compensate for weather, for mechanical failures, and for others operational mistakes. They are also the scapegoats. Illustratively, examine the failure of a software system at the Los Angeles Air Route Traffic Control Center on September 14, (Geppert 2004) Voice communications ceased between the controllers and the 400 aircraft flying above 13,000 feet over Southern California and adjacent states. During the software malfunction, there were five near-misses between aircraft, with collisions prevented only by an on-board collision detection and resolution system (TCAS). At the time, the FAA was in the process of patching the systems. As often happens in software-intensive systems, the intermediate fix was to work around the problem in operations the software system was supposed to be rebooted every 30 days in order to prevent the occurrence of the bug. The human operators hadn t restarted the system, and they were blamed for the incident. If the current state of airspace complexity causes palpitations, experts considering what might happen in the planned next generation (NextGen) airspace can be excused for fullfledged anxiety attacks. The future of the NAS is more heterogeneity and more distribution of responsibility. We are seeing a switch to a best-equipped best-served model airlines who can afford to buy and operate equipment can get different treatment in the airspace. One example is the advent of Required Navigation Performance (RNP) routes, in which aircraft fly tightly-controlled four-dimensional trajectories by utilizing GPS data. With GPS, an aircraft can be cleared to land and decscend from altitude to the runway in a Continuous Descent Arrival (CDA); these approaches save fuel and allow for better-predicted arrival times. However, at airports with these approved routes, controllers must work with mixed traffic airplanes flying CDA routes and airplanes flying traditional approaches. In the future, the airspace will also include Unmanned Aerial Systems (fullyautonomous systems, and also systems in which a pilot flies multiple aircraft from the ground), and a wider performance band for civil aircraft. The overall traffic increase is leading to software-based decision support for pilots and controllers. There is an active (and sometimes heated) discussion about just how much authority and autonomy should remain with people versus being implemented in software. Decisions about where loci of control should reside in the airspace is an example of a 21 wicked design problem (Rittel 1984; Hooey and Foyle 2007) as evidenced by the following criterion: Stakeholders disagree on the problem to solve. There are no clear termination rules. There are better or worse solutions, but not right and wrong solutions. There is no objective measure of success. The comparison of design solutions requires iteration. Alternative solutions must be discovered. The level of abstraction that is appropriate for defining the problem requires complex judgments. It has strong moral, political, or professional dimensions that cannot be easily formalized. In this paper we will first discuss a simulation that is designed to study human-automation interaction for CDAs. We will then overview the current state-of-the-art for uncertainty quantification within this type of complex system, and focus on techniques for exploring the Pareto Frontier. In the next section, we will explain and demonstrate a new technique (GALE) for quickly finding the Pareto Frontier for wicked problems like those we study in our use case. We will conclude by overviewing our future plans. How Do Pilots and Air Traffic Controllers Land Planes? Our Test Case and Its Inputs In this paper, we use the CDA scenario within the Georgia Institute of Technology s Work Models that Compute (WMC). WMC is being used to study concepts of operation within the (NAS), including the work that must be performed, the cognitive models of the agents (both humans and computers) that will perform the work, and the underlying nonlinear dynamics of flight. (Kim 2011; Pritchett, Christmann, and Bigelow 2011; Feigh, Dorneich, and Hayes 2012) WMC and the NAS are hybrid systems, governed both by continuous dynamics (the underlying physics that allows flight) and also discrete events (the controllers choices and aircraft modes). (Pritchett, Lee, and Goldsman 2001) Hybrid systems are notoriously difficult to analyze, for reasons we will overview in the next section. WMC s cognitive models are multi-level and hierarchical, (Kim 2011) with: Mission Goals at the highest level, such as Fly and Land Safely, that are broken into Priority and Values functions such as Managing Interaction with the Air Traffic System. These functions can be decomposed into Generalized Functions such as Managing the Trajectory, that can still be broken down further into Temporal Functions such as Controlling Waypoints. In this paper, we present some preliminary results in which we have varied four input parameters to WMC in order to explore their effects on the simulation s behavior. We list these parameters in bold, and then describe them. The Scenario is a variable within the CDA simulation with four values. In the nominal scenario, the aircraft follows the ideal case of arrival and approach, exactly according to printed charts, with no wind. In the late descent scenario, the air traffic controller delays the initial descent, forcing the pilots to quickly descend in order to catch up to the ideal descent profile. In the third, unpredicted rerouting, scenario, the air traffic controller directs the pilot to a waypoint that is not on the arrival charts, and from there returns the pilot to the expected route. In the final scenario, the simulation creates a tailwind that the pilot and the flight deck automation must compensate for in order to maintain the correct trajectory. The late descent, unpredicted rerouting, and tailwind scenarios all have further variants, modifying the times at which the descent is cleared, the waypoint that the plane is routed to, and the strength of the tailwind, respectively. Function Allocation is a variable that describes different strategies for configuring the autoflight control mode, and has four different possible settings. A pilot may have access to guidance devices for the lateral parts (LNAV) and the vertical parts (VNAV) of the plane s approach path. Civil transport pilots are likely to have access to a Flight Management System (FMS), a computer that automates many aviation tasks. In the first Function Allocation setting, which is highly automated, the pilot uses LNAV/VNAV, and the flight deck automation is responsible for processing the air traffic instructions. In the second, mostly automated, setting, the pilot uses LNAV/VNAV, but the pilot is responsible for processing the air traffic instructions and for programming the autoflight system. In the third setting, the pilot receives and processes the air traffic instructions. The pilot updates the vertical autoflight targets; the FMS commands the lateral autoflight targets. This setting is the mixed-automated function allocation setting. In the final, mostly manual, setting, the pilot receives and processes air traffic instructions, and programs all of the autoflight targets. The third parameter we are varying in this paper is the pilots Cognitive Control Modes. There are three cognitive control modes implemented within WMC: opportunistic, tactical, and strategic. In the opportunistic cognitive control mode, the pilot does only the most critical temporal functions: the actions monitor altitude and monitor airspeed. The values returned from the altitude and the airspeed will create tasks (like deploying flaps) that the pilot will then perform. In the tactical cognitive control mode, the pilot cycles periodically through most of the available monitoring tasks within WMC, including the confirmation of some tasks assigned to the automation. Finally, in the strategic mode, the pilot monitors all of the tasks available within WMC and also tries to anticipate future states. This anticipation is implemented as an increase in the frequency of monitoring, and also a targeted calculation for future times of interest. Finally, the fourth variable we explore in this paper is Maximum Human Taskload: the maximum number of tasks that can be requested of a person at one time. In previous explorations using WMC (Kim 2011), the author chose three different levels: tight, in which the maximum number of tasks that can be requested of a person at one time is 3; moderate, in which that value is 7; and unlimited, in which a person is assumed to be able to handle up to 50 requested tasks at one time. WMC uses a task model in which tasks have priorities and can be active, delayed, or interrupted. (Feigh and Pritchett 2013) If a new task is 22 passed to a person and that person s maximum taskload has been reached, an active task will be delayed or interrupted, depending on the relative priorities of the tasks that have been assigned. Delayed and interrupted actions may be forgotten according to a probability function that grows in the time elapsed since the task was active. For the studies in this paper, we assume that people can handle between 1 and 7 tasks at maximum. (Miller 1956; Cowan 2000; Tarnow 2010) Our analysis seeks to explore the effects that each of the four variables above has on the following five outputs: the number of forgotten tasks in the simulation (Num- Forgotten Tasks), the number of delayed actions (NumDelayedActions), the number of interrupted actions (NumInterruptedActions), the total time of all of the delays (Delayed- Time), and the total time taken to deal with interruptions (InterruptedTime). In our results, we refer to these outputs as (f1.... f5) and average each of their values across the pilot and the copilot. In Kim s dissertation (Kim 2011), she primarily studies function allocation and its effect on eight different parameters, including workload and mission performance. In this sense, the WMC model by itself as Kim chose to use it (much less the airspace it is meant to simulate) is wicked. In particular, there is no single measure of success, and there is no agreement as to which of the measures is more important. Kim analyzed all of the combinations of the above four variables, and manually postprocessed the data in order to reach significant conclusions about how the level-ofautomation affects each of her eight metrics. An Overview of Uncertainty Quantification Within Hybrid, Wicked Systems Remember that all models are wrong; the practical question is how wrong do they have to be to not be useful. George E.P. Box and Norman R. Draper Validation is the process by which analysts answer Did we solve the right problem? Uncertainty (and risk) quantification is core to the validation of safety-critical systems, and is particularly difficult for wicked design problems. WMC is a tool that is aimed at validating concepts of operation in the airspace. It abstracts some components within the airspace, and approximates other components, and must itself be validated in order to understand its predictive strengths and limitations. Validation efforts can take as a given that WMC s predictions are useful, and be focused on discovering the risks in the concepts of operation (in which case the analysis is usually called risk quantification). Uncertainty quantification within the model is usually focused on comparing the predictions to those we get (or to those we expect to get) in reality. The questions we are asking in each of these two cases are different, but the underlying tools we use in order to analyze them is often the same. In the case of risk quantification, where we want to validate the concept of operation, we explore the input and output spaces of our models, looking for those that perform better or worse among the many metrics we ve chosen to examine. For simulations with long response times, or for which we hope to learn about a broad class of behaviors using relatively few trials, we build a secondary model that is easier to evaluate than the original simulation. Whichever surface we can evaluate, whether it is the original or a secondary model, is called a response surface. In the case of uncertainty quantification, where we want to validate our model, we again build a response surface for our model and compare this against the response surface built using real (or expected) behaviors. A common way of characterizing a response surface is by building a Pareto Frontier. A Pareto Frontier occurs when a system has competing goals and resources; it is the boundary where it is impossible to improve on one metric without decreasing another. (Lotov, Bushenkov, and Kamenev 2004) A Pareto Frontier is usually discovered using an optimization methodology. In rare cases, it may be possible to analytically discover the Pareto Frontier this is unlikely in wicked design problems like those we are studying here. More often, we use a learning technique to discover the Pareto Frontier given concrete trials of the system. Classical optimization techniques are often founded on the idea that the response surface and its first derivative are Lipschitz continuous everywhere (smooth). For smooth surfaces, it is possible to find a response surface that is arbitrarily close to our desired function using polynomial approximations by the Weierstrass Approximation Theorem. (Bartle 1976) For the hybrid, complex, non-linear problems we are studying here, no such guarantee of smoothness exists. Modal variables like the cognitive control modes in WMC usually require combinatorial approaches in order to explore. For other WMC inputs, such as the maximum human taskload, a domain expert might reasonably suspect that there is an underlying smooth behavior. For some WMC inputs we haven t modeled yet, such as flight characteristics of the aircraft or the magnitude of a tailwind, there is almost certainly a smooth relationship, but it may be nonlinear. Classical techniques handle the mix of discrete and continuous inputs by solving a combinatorial number (in the discrete inputs) of optimization problems over the continuous inputs, and then comparing the results across the optimizations in a post-processing step. (Gill, Murray, and Wright 1986) This technique can be computationally very expensive, especially when you consider that continuous optimization techniques are sensitive to local minima (in our nonlinear aerospace problems), and several different input trials should be performed. Statistical techniques such as Treed Gaussian Processes and Classification Treed Gaussian Processes, can be used to build statistical emulators as the response surfaces for simulators, and have the advantage that they can model discontinuities and locally smooth regions. (Gramacy 2007; He 2012) As a disadvantage, they are limited by computational complexity to relatively few inputs (10s but not 100s). More recent techniques, such as those based on particle filters, can handle significantly many more inputs. (He and Davies 2013) All of the above techniques have the limitation that they optimize for one single best value. To optimize across several criterion (such as the five we analyze for this paper or 23 the eight in Kim s thesis) using the above techniques, the analyst usually needs to build a penalty function, a formula that is strictly monotonic in improvement across the desired metrics and weights each metric according to its relative value. In this paper, we choose instead to explore the class of multiobjective response surface methods, as detailed in the next section. GALE: Active Learning for Wicked Problems Wicked problems have many features; the most important being that no objective measure of success exists. Designing solutions for wicked problems cannot aim to produce some perfectly correct answer since no such definition of correct exists. Hence, this approach to design tries to support effective debates by a community over a range of possible answers. For example, different stakeholders might first elaborate their own preferred version of the final product or what is important about the current problem. These preferred versions are then explored and assessed. The issue here is that there are very many preferred versions. For example, consider the models discussed in this paper. Just using the current models, as implemented by Kim et al. (Kim 2011), the input space can be divided 144 ways, each of which requires a separate simulation. In our exploration, we further subdivide the maximum human taskload to evaluate 252 combinations. Worse yet, a detailed reading of Kim s thesis shows that her 144 input sets actually explore only one variant each for three of her inputs. Other modes would need to be explored to handle: Unpredictable rerouting; Different tail wind conditions; Increasing levels of delay. If we give three what-if values to the above three items then, taken together, these 3*3*3*252 modes*inputs would require nearly 7000 different simulations 1. This is an issue since, using standard multi-objective optimizers such as NSGA-II (Deb et al. 2000), our models take seven hours to reach stable minima. Hence, using standard technology, these 7,000 runs would take 292 weeks to complete. In principle, such long simulations can be executed on modern CPU clusters. For example, usi
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