Aviation Safety and Pilot Control Understanding and Preventing Unfavorable Pilot-Vehicle Interactions (1997) / Chapter Skim
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Pages 106-125
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5Simulation and Analysis of the Pilot-Vehicle System The most important design tools for avoiding, discovering, understanding, and correcting APC events are simulation and analysis. Although the emphasis is on avoidance, discovery (or lack thereof)
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evaluated to declare an aircraft design free of APC tendencies. Joint simulation and PVS analysis may be used to select configurations and piloting tasks that appear to be prone to APC problems.
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In-Flight Simulators. Unless an in-flight simulator is specifically designed for a particular cockpit and inceptors, the pilot-related fidelity may be limited.
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and used at an office or laboratory computer work station and involves none of the "virtual reality" requirements of a simulation with an actual pilot in the loop. The model of the effective vehicle dynamics can be quite complex and can include many system nonlinearities attributable to either vehicle aerodynamics or control system implementations.
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In the investigations of the famous T-38 PIO recorded in Figure 1-1, fixedbase simulations were also useful for making some qualitative evaluations but were not successful in duplicating the PIO.35 This is not surprising, because the bobweight effect, which depends upon cockpit motion for its existence, was ultimately demonstrated to be an important factor. It is noteworthy that the Lockheed Martin YF-22 incident described in Chapter 2 could not be reproduced in a fixed-base simulator, despite the fact that numerous pilots flew the accident profile and that the precise conditions surrounding this incident were known.11 Based on these and similar experiences, many experts question using fixedbase simulation to discover and eliminate adverse APC tendencies.
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action with its fundamental apparently locked to the rate peaks.23 (emphasis added) Gibson also describes an example in which an aircraft had PIOs in pitch, roll, or both in each of its first five flights.25 On the sixth flight, a divergent pitch PIO occurred in what was initially a routine landing.
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process, which often results in APC events, such as the YF-22 accident, that have a high degree of visibility. The exaggerated criticism -- that the simulation process is not effective -- rather than less than 100 percent effective -- is unfortunate because it tends to impugn the overall efficacy of simulation.
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A general conclusion about these simulators is that, once an APC tendency has been observed in flight, it is possible to construct a piloting task that will exhibit the same tendencies in ground-based simulation. In addition, as simulator fidelity increases (e.g., moving versus fixed-base, in-flight versus ground-based)
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Figure 5-2 A PIO (APC) rating scale.
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Figure 5-4 A comparison of PIO ratings for formation-flying by the NASA Flight Simulator for Advanced Aircraft (FSAA)
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his actions to avoid the flying qualities cliff, demonstrating the possibility of avoiding an APC problem once the potential for the problem is known. Demonstrations like this are necessary to train test pilots to determine the APC potential of aircraft and to inform line pilots that APC events may be associated with FCS deficiencies rather than with poor piloting.
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simulators) might produce APC events attributable to the limitations of the simulator rather than to deficiencies in the aircraft being simulated.
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OVERVIEW OF HUMAN PILOT CHARACTERISTICS Modern Piloting Tasks Modern flight control and navigation systems are characterized by two features: (1) they employ FBW controls, and (2)
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deviations and responds accordingly, he effectively closes a feedback control loop. The flight control computer may receive signals from vehicle motion as well as commands generated by the pilot.
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engaged in landing a large commercial aircraft is constrained by the task and environment to the extent that the pilot's behavior and performance can often be adequately described by what is referred to as a "describing function model."34,44 In its complete incarnation, such a model describes both the pilot's "linear behavior" (i.e., behavior that could be replicated by a relatively simple inanimate control device with linear characteristics) , and the pilot's "nonlinear behavior" (i.e., behavior that can be attributed to the complex, nonlinear sensing and actuation capabilities of a human being)
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and the line directed away from the block represents an "effect," or output. The block itself represents the transfer function of the system in question (e.g., the human pilot)
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synchronous with, the vehicle oscillation. This suggests that, in analyzing the PIO, the pilot model can be reduced to a simple gain, with a magnitude that produces incipient instability from the standpoint of control system design.
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Optimal Control Model One successful example of pilot modeling using the modern approach is the optimal control model (OCM) of the human pilot.34 The primary hypothesis behind the OCM is that the trained, motivated human pilot behaves in an optimal manner.
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command generated by the flight control computer and displayed electronically to the pilot in the cockpit. The pilot's task is to force the aircraft to follow this command with minimum error.
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sequenced outputs (i.e., movements of the cockpit inceptor) that result in aircraft responses that are almost exactly the ones desired.
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