Aviation Safety and Pilot Control Understanding and Preventing Unfavorable Pilot-Vehicle Interactions (1997) / Chapter Skim
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From page 30... ...
2Varieties of Aircraft-Pilot Coupling Experience INTRODUCTION From the pilot's perspective, aircraft-pilot interactions fall somewhere between two extremes -- the pilot may be fully interactive, or the pilot may be effectively detached. In the fully interactive extreme, the pilot is said to be "in the loop," and the PVS operates as a closed-loop feedback control system.
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At the highest level, aircraft-pilot interactions are divided into benign and undesirable. Routine piloting, which is the most prevalent form of aircraft-pilot interaction and which can involve both open-loop and closed-loop operations of the PVS, is shown on the far left of Figure 2-1 as the most benign (and desirable)
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Fi gu re 2 -1 T ax on om y of A PC p he no m en a.
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Depending on the effective aircraft dynamics, three categories of unfavorable PIOs can be distinguished. (Each of these categories is described in more detail in the following section.)
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pilot behavior models, be consistent with procedures for analyzing appropriate feedback control systems, and have direct connections with the varieties of PIO as these are reflected in experimental databases for pilot and PVS dynamics, PIO experiments, etc. To fulfill these objectives, the categories described below have been adopted.
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ratings and commentaries, and the sensitivity of closed-loop system properties to changes in the effective aircraft characteristics. The easiest feature to estimate for Category I events is the frequency range, which depends primarily on the pilot's behavior pattern (compensatory or synchronous)
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The other major factor in Category I PIOs is inappropriate effective aircraft gain. This can be either too high (aircraft is too sensitive to control)
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in the FCS, or from changes in the aerodynamic or propulsion configuration of the aircraft. Category III PIOs can be much more complicated than Category I or II PIOs because they necessarily involve transitions in the dynamics of either the pilot or the effective aircraft.
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proceeded to engagement. Immediately upon probe contact, a longitudinal APC event initiated.
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Common Cliff Producers The cliff metaphor evokes a picture of sudden, large changes in aircraft motions associated with relatively slight changes in pilot activity. Such changes can only occur if there are significant nonlinearities in the PVS dynamics.
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examples of nonlinear features capable of producing cliff-like behavior in FBW systems are described below. The two most common and significant nonlinear characteristics within the effective aircraft (see Figure 1-2)
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Although it is clear that rate limiting phenomena are important factors in fully-developed, severe PIOs encountered in operational situations, the actual process by which rate limiting causes severe PIOs is neither well documented nor well understood. The possibility that rate limiting phenomena are primary initiating factors in the development of some severe PIOs has not received enough attention despite compelling evidence.
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actuation system. Figure 2-3b illustrates that, for small amplitude commands, the actuator follows the command input with a small time lag (defined by the inverse of the bandwidth of the actuator as a linear system)
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Figure 2-3b Surface actuator rate limiting effects for various input amplitudes showing linear system response times. Source: Klyde.39 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 43
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Figure 2-3c Surface actuator rate limiting effects for various input amplitudes showing near saturation response times. Source: Klyde.39 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 44
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Figure 2-3d Surface actuator rate limiting effects for various input amplitudes showing highly saturated response times. Source: Klyde.39 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 45
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A typical scenario begins with a pilot who is well adapted to an essentially linear, pilot-aircraft closed-loop system operating at high gain to satisfy task requirements for precision control (akin to the initial phases of the F-14 attempt to line up the refueling probe with the tanker drogue)
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Command Path Gain Shaping Almost all FBW FCSs incorporate gain shaping in the pilot's command path. Gain shaping adjusts the gain of the effective aircraft dynamics as a function of the pilot's command signal.
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Figure 2-4 Example of command gain shaping for a nonlinear element. Source: Kullberg and Elgcrona.40 Under limiting conditions, the FCS can sometimes remove the pilot from direct access to the control effectors in order to execute these functions.
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leads to major problems, including some of the so-called "three-dimensional PIOs" of Table 1-1. The following APC events illustrate the kinds of problems that can occur.
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Figure 2-5 JAS 39 accident time history. Source: Kullberg and Elgcrona.40 TRIGGERS In most cases, severe PIOs are initiated by one or more stimuli acting as triggering events.
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in pilot gain. The external situation may also demand precision control that requires high-gain piloting, such as pitch control during landing.
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properties as a function of the aircraft configuration. An example is the unanticipated level of the auto-speedbrake deployment in the Boeing 777 event (described in the Case Studies section of this chapter)
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unusual situations where the control strategy is inconsistent with the pilot's intentions. The lack of some inceptor-motion or other proprioceptive feedback in some FBW aircraft deprives the pilot of some "display" cues, and this lack of cues can, perhaps, trigger a Category II or III APC event.
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sequence started with the pilot manually flying the aircraft, although the flight director guidance system and autothrottles were engaged to maintain speed. During the approach, the pilot inadvertently activated the autopilot takeoff/goaround switch.
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inappropriate subset of the relevant control variables or may simply close the loop on the wrong variable. As the complexity of modern FBW FCSs increases, the underlying effective aircraft dynamics can be task-tailored.
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the aircraft began a series of pitch oscillations at an altitude of approximately 40 feet. After four or five oscillations, the aircraft impacted the runway.
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Note that the large oscillations in pitch rate (curve 2) , pitch attitude (curve 3)
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Figure 2-7 YF-22 accident time history. Source: Harris.32 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 58
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Figure 2-8 YF-22 pitch rate command stick gradients. Source: Harris.32 Post-Event Simulations The accident review team attempted to recreate the YF-22 event using offline and fixed-base pilot-in-the-loop simulators, but piloted simulations were unable to recreate the APC event.32 Case 2.
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generated a vertical plunge and pitch down (see Figure 2-9, curve 3) that was checked by a rapid airplane-nose-up column input (Figure 2-9, curve 1)
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Figure 2-9 Time history for 777 landing derotation, baseline control law. Source: McWha.47 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 61
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Fi gu re 2 -1 0 N or m al m od e el ev at or c on tro l l aw .
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Figure 2-11 Time history for 777 attitude tracking on runway, baseline control law. Source: McWha.47 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 63
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Figure 2-12 Time history for 777 attitude tracking on runway, secondary mode. Source: McWha.47 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 64
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Category II APC event. Normal acceleration cues sensed by the pilot may have aggravated the problem.
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Figure 2-13 Time history for 777 attitude tracking on runway, revised control law. Source: McWha.47 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 66
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Figure 2-14 Time history for 777 attitude tracking on runway, revised control law plus command filter. Source: McWha.47 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 67
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Figure 2-15 Bandwidth criteria applied to landing derotation, effect of 777 control law changes on pitch attitude/column position frequency response. Source: McWha.47 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 68
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Figure 2-16 Elevator/column gain and phase, effect of 777 control law changes on landing derotation. Source: McWha.47 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 69
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Conclusions The incident described above was a classic Category II PIO, with largeamplitude pilot inputs and both rate- and position-limited elevator activity (as indicated by curve 2 of Figure 2-9)
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degrees/−39 degrees, as was the actual maximum position of the right aileron (curves 3 and 4, Figure 2-17)
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Figure 2-17 C-17 test aircraft lateral oscillations during approach to landing with hydraulic system #2 inoperative. Source: Kendall.38 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 72
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Figure 2-18 C-17 test aircraft lateral oscillations during approach to landing with hydraulic system #2 inoperative, continued. Source: Kendall.38 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 73
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As this is an operational airplane, the flight data presented in Figure 2-19 suffer from sampling limitations associated with the data recorder. However, the approximate estimates that can be made indicate the following: • The PIO frequency was approximately 0.31 Hz (2.5 rad/sec)
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Figure 2-19 A 320 incident time history. Source: NTSB.51 VARIETIES OF AIRCRAFT-PILOT COUPLING EXPERIENCE 75
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Although the Airbus service bulletin did not clearly indicate that the modification made important improvements in the handling qualities of the A 320 in CONF 3, Airbus promulgated the information widely. However, neither the French certificating authority (Direction Generale de l'Aviation Civile)
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FBW technology has only recently been incorporated into rotorcraft (e.g., V-22, RAH-66 Comanche, and NH-90)
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and the horizontal tolerance was ± 1.5 m. The task was to maintain the hover position relative to the hover board while the target vehicle moved a distance of 100 m in 20 seconds using the velocity pattern shown in Figure 2-23.
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Figure 2-24 Time history of the helicopter lateral-position tracking task with no added time delay. Source: Ockier.56 Figure 2-25 Time history of the helicopter lateral-position tracking task with 100 msec of added time delay.
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illustrates the importance of appropriate force-feel systems for helicopter handling qualities and for the onset of PIOs. For helicopters flying with an attitude command system, additional damping, rapid follow-up trim, or even active, non-linear controllers may be necessary.56 Figure 2-26 Small-amplitude handling qualities criterion (target acquisition and tracking)
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