2
Varieties 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. In this situation, the pilot's commands are more or less continuous and depend, at least partly, upon pilot-perceived "errors" or differences between desired and actual aircraft responses. Near the opposite extreme is the "open-loop" control system, in which the pilot operates as a forcing function, generating commands to the effective aircraft that are not directly related to the pilot's perception of aircraft motion. In either case, the PVS operations involve "aircraft-pilot interactions" that constitute an all-inclusive set.

The interactions may result in motions that are desirable and "benign" or "undesirable." For this study, the interactions of interest are primarily closed-loop in character. They can result in favorable PVS responses that converge to provide the desired PVS performance, or they can result in undesired responses, either oscillatory or divergent. The focus here is on unfavorable, closed-loop PVS responses, both oscillatory and divergent. Unfavorable responses need to be understood in the context of the all-inclusive set of all PVS operations. The hierarchical structure shown in Figure 2-1 provides a taxonomy that is useful for classifying, discussing, and analyzing APC phenomena.



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--> 2 Varieties 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. In this situation, the pilot's commands are more or less continuous and depend, at least partly, upon pilot-perceived "errors" or differences between desired and actual aircraft responses. Near the opposite extreme is the "open-loop" control system, in which the pilot operates as a forcing function, generating commands to the effective aircraft that are not directly related to the pilot's perception of aircraft motion. In either case, the PVS operations involve "aircraft-pilot interactions" that constitute an all-inclusive set. The interactions may result in motions that are desirable and "benign" or "undesirable." For this study, the interactions of interest are primarily closed-loop in character. They can result in favorable PVS responses that converge to provide the desired PVS performance, or they can result in undesired responses, either oscillatory or divergent. The focus here is on unfavorable, closed-loop PVS responses, both oscillatory and divergent. Unfavorable responses need to be understood in the context of the all-inclusive set of all PVS operations. The hierarchical structure shown in Figure 2-1 provides a taxonomy that is useful for classifying, discussing, and analyzing APC phenomena.

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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) class. Routine piloting includes all well-accomplished piloting tasks as well as two kinds of PVS oscillations. The first, which arises from incomplete pilot adaptation to the effective aircraft dynamics, is very common and, fortunately, usually benign. These oscillations usually occur when the pilot is adapting to the aircraft dynamics and performing high-gain, precision-control tasks. For example, 15 oscillation incidents occurred during testing of the SAAB J-35 in 1960; 7 of these occurred when a pilot was flying the J-35 for the first time. From time to time in this learning process, the pilot's gain is momentarily high enough to create a closed-loop oscillation. The usual initial "cure" is simply for the pilot to get out of the loop by releasing the inceptor and relying on the stability of the effective aircraft dynamics to handle the recovery. Because this is basically a learning experience, the ultimate cure is practice. The other kind of closed-loop PVS oscillations that can be considered normal is a low-amplitude, damped oscillation, which is often referred to as a "bobble." Bobbles are associated with short-duration, excessive pilot gain. They are, at worst, short-term, mild PIOs that do not cause difficulties in controlling the aircraft. The next class of favorable and benign interactions includes oscillations deliberately introduced by the pilot to generate a periodic forcing function. The outstanding example of this is "stick pumping," when the pilot applies an oscillatory input to the aircraft either to "feel out" its effective dynamics or to counter large control-system nonlinearities (as a kind of "dithering control"). The pilot's input constitutes an open-loop forcing function, and the pilot's action and the resultant aircraft oscillation frequency are not directly conditioned by the aircraft's response. The undesirable APC events that are the subject of this study appear in the right half of Figure 2-1. Within this group, PIOs are distinguished from non-oscillatory APC events like divergences. The oscillations are akin to the benign "learning experience" variety, but they are not associated with pilot maladaptation. In fact, the pilot may be very experienced with the aircraft and with the task in general. Sometimes, however, the task specifics suddenly become unusually severe, requiring a highly aggressive pilot response to exert precise control and regulation of the aircraft. In this situation, getting out of the closed loop is not always feasible, so the demands for recovery focus on the PVS rather than just on the effective aircraft. Forced by circumstances to retain some level of control while attempting to recover, the pilot's gain may be too high but cannot be relaxed. The result can be a severe or even catastrophic PIO even with the very best, most well adapted pilot. The pilot-vehicle closed-loop system is simply not up to the demands imposed on it.

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--> Figure 2-1 Taxonomy of APC phenomena.

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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.) For Category I PIOs, the dynamics are essentially linear; Category II and III PIOs involve nonlinearities in the effective aircraft dynamics. In Category II PIOs, the nonlinearities result from rate or position limits (the rest of the effective aircraft dynamics are essentially linear). The nonlinear features in Category III PIOs are more complex. The nonlinear properties in both Category II and III PIOs can cause sudden changes in the effective aircraft dynamics that result in the abrupt (sometimes referred to as "cliff-like") onset of PIOs. The class in Figure 2-1 furthest to the right comprises non-oscillatory APC events. These Category III events can stem from several causes and tend to be highly idiosyncratic. Only a very few incidents have been identified to date, but the advent of FBW technology has introduced some new dimensions by permitting control mechanizations that can be troublesome, especially in highly limiting conditions. The next section, Categories of Aircraft-Pilot Coupling Oscillations, begins with a description of the three categories of oscillatory APCs. This description is followed by discussions of nonlinear, "cliff-like" PIOs and of non-oscillatory APC events. The next major section, Triggers, presents a fuller description of the underlying conditions and the kinds of triggers thought to be involved in initiating adverse APC events of all classes. Finally, several varieties of PIOs are illustrated by case studies. Four detailed examples are presented, along with a separate discussion of APC issues related to rotorcraft. The case studies are typical incidents and accidents encountered in the development phases of recent FBW systems. They are particularly instructive in that each exhibits a PIO in a concrete and specific context. Taken together, they provide a broad picture of a variety of potential triggers, patterns of behavior, PIO frequencies, and so on. Categories Of Oscillatory Aircraft-Pilot Coupling Events Because of the diversity in control axes, frequency ranges, and other important characteristics of PIOs, several kinds of classification schemes could be used. In the discussion of historical antecedents in Chapter 1, some notable PIOs were grouped by primary control axis and PIO frequency. Analytical studies such as McRuer42 rely on pilot behavioral models and closed-loop analysis procedures. These studies are used to elicit understanding and explain the phenomena and their associations as well as to develop and assess system modifications to reduce the potential for PIOs. The pilot models and analysis procedures are not specific to any one group in Tables 1-1a through 1-1d. This suggests that a desirable classification scheme should accommodate existing

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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. The three categories organize PIOs into classes according to whether they are essentially linear, characterized by one or two common nonlinearities, or characterized by more complex and extensive nonlinear features. Category I: Linear Pilot-Vehicle System Oscillations In Category I PIO phenomena, the effective aircraft characteristics are essentially linear, and the pilot behavior is "quasi-linear" and "time-stationary." Quasi-linearity means that many nonlinear elements have specific input-response pairs that appear to be similar to the input-response pairs for linear systems. This similarity leads to the notion that the pilot's output response to certain inputs can be divided into two parts: (1) the response of a linear element (known as a "describing function") that is driven by the particular input; and (2) an additional quantity (called the "remnant") that is added to this response. In the PIO situation, the input is sinusoidal (or nearly so), and the pilot's output is a periodic function that constitutes the sum of (1) a sinusoid at the same frequency and (2) a remnant composed of higher harmonics. These harmonics will ordinarily be significantly attenuated as they proceed around the PVS loop, so they do not usually materially affect the input to the pilot. The causally significant part of the pilot's dynamics in the PIO is then the pilot's sinusoidal input describing function, which for a particular input amplitude acts like a linear transfer characteristic. The time-stationary aspect of the Category I PIO means simply that the effective aircraft dynamics and the pilot's dynamics do not change during the PIO. In Category I PIOs, no significant frequency-variant nonlinearities28 operate in the controlled element dynamics. Simple amplitude-dependent series gain changes either in the pilot gain or the controlled-element gain can be considered special cases, so such things as nonlinear stick sensitivity or shifts in pilot attention may be admissible as features consistent with a Category I event. PIOs in this category may be deliberately induced by the pilot increasing his gain, in which case the situation is easily repeatable, readily eliminated by relaxing control (lowering pilot gain), and generally not threatening. In other circumstances (for example, when there are tight flight-path constraints and major triggering events or disturbances), the pilot may not have the option of reducing gain. Those cases may produce severe Category I PIOs. For a given pilot cue structure, analyses of Category I PIOs can reveal pilot-vehicle, closed-loop system dynamics, bandwidths,* resonance properties, etc., for nominal and PIO-based pilot gain levels, estimated pilot

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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) and the degree to which the higher-frequency dynamics of the pilot's neuromuscular system* may be involved. (Behavior patterns are well known in human-machine systems studies.42,45) A cross section of frequencies that have been observed appears in Table 2-1. In some PIOs, the pilot's behavior may initially be compensatory but may change to synchronous as the oscillation develops. The two key effective-aircraft factors associated with susceptibility to an essentially linear PIO are those that unduly restrict the pilot's ability to close the PVS loop for a broad range of gains or to achieve adequate closed-loop system performance. Much of the existing data on PIOs and poor flying qualities could be used to exemplify these factors and define them more quantitatively. Some aircraft configurations and associated analytical studies are particularly well suited to detailing these factors5,42 Such analyses can provide a more quantitative understanding of the effects of various effective-aircraft dynamics. Appendix C compares PVS dynamic properties for two configurations with almost identical effective aircraft dynamics except for high-frequency phase lags.* Excessive phase (or time) lag is one of the two most important aircraft-associated factors in Category I PIOs because it limits both the possible range of pilot gain adjustments and the attainable crossover frequency. These limitations directly affect the closed-loop PVS bandwidth and performance. The criteria for Category I PIOs, which are examined in detail in Chapter 6, give a quantitative answer to the question of just how much lag is "excessive." TABLE 2-1 Cross Section of Frequencies PVS Characteristic Typical PIO Frequencies Compensatory (pilot closes PVS loop to minimize error)   Extended rigid body effective aircraft and low-frequency pilot-neuromuscular system 2 to 5 rad/sec (0.3 to 0.8 Hz) Extended rigid body effective aircraft and high-frequency pilot-neuromuscular system 10 to 20 rad/sec (1.5 to 3 Hz); (sometimes referred to as "ratchet") Synchronous (pure gain pilot dynamics)   Extended rigid body effective aircraft and low-frequency pilot-neuromuscular system 4 to 10 rad/sec (0.6 to 1.5 Hz) Flexible mode effective aircraft and high-frequency pilot-neuromuscular system 6 to 20 rad/sec (1.0 to 3.0 Hz)   Source: McRuer.42

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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) or too low (aircraft is too sluggish). Too-high aircraft gain is a more important factor in Category I PIOs. Finally, it is important to reiterate that essentially linear (Category I) PIOs are not always severe. Linear PIOs are likely to occur whenever the pilot's dynamic adaptation is faulty. These PIOs can be commonplace learning experiences that disappear as the pilot becomes familiar with the aircraft's characteristics. However, linear PIOs that occur because of excessive time lag, inadequate available gain range, or both, do not disappear and can often be severe. They are likely to be encountered whenever the PVS is confronted with extreme demands, either for high-precision control or for control of large upsets or other unexpected events. Excessive time lag and inadequate available gain range are design flaws that should be eliminated as a matter of flight safety. Category II: Quasi-Linear Pilot-Vehicle System Oscillations with Rate or Position Limiting Category II PIOs are severe oscillations with amplitudes well into the range where rate and/or position limits become dominant. Rate limiting goes beyond the Category I scenario by adding an amplitude-dependent phase shift and by setting the amplitude of the limit cycle.* Category II events appear to be the most common jump-resonant, limit-cycle, oscillatory APC events. (An example of jump-resonance appears below in the section on Rate Limits.) The characteristics of typical Category II PIOs are described in the discussion of rate limiting in the next section and in more detail in Appendix C. These events are classified as a separate category primarily because rate limiting is present in a large proportion of severe PIOs. Rate limiting can be analyzed readily, and it is, perhaps, the most easily identifiable cause of a flying qualities cliff. Category II is a transitional category between Category I PIOs and the most general, nonlinear Category III PIOs. Category III: Nonlinear Pilot-Vehicle System Oscillations with Transitions Category III PIOs depend fundamentally on nonlinear transitions in either the controlled element or the pilot's behavioral dynamics. Shifts in the controlled element can be associated with the magnitude of the pilot's commands (akin to the rate limiting onset property in Category II). Category III PIOs may also result from a change of mode, from other internal changes

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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. Thus, a minimum of two sets of effective PVS characteristics are involved in Category III PIOs—pre-transition characteristics and post-transition characteristics. If these differ greatly, as they did in the T-38 and YF-12 incidents, very severe PIOs can occur. Nonlinear, Cliff-Like, Pilot-Involved Oscillations For years, the test pilot community has recited a litany of anecdotal observations such as the following: Severe PIOs are sudden and unexpected. Sometimes, just moments before the explosive onset of a severe PIO, the aircraft is docile and easily controlled. Flying qualities cliffs are "out there" awaiting the right circumstances to appear and create havoc. The validity of these observations is demonstrated by the historical events described in Chapter 1 and the case studies at the end of this chapter. The "cliff" metaphor is used to convey a sense of unexpected, dramatic, and excessively large motions of the aircraft. When cliff-like changes result from an incremental increase in the amplitude of the pilot's output, the PVS is not behaving like a linear system. Instead, this indicates the presence of significant nonlinearities either in the dynamics of the effective aircraft or in the pilot's behavior. The resulting PIOs are severe and exhibit rate-limited responses or other limit-based response patterns. Many, if not all, Category II and III PIOs exhibit cliff-like behavior. An interesting and instructive example of cliff-like APC events was encountered during flight tests of an F-14 backup flight control module with significantly restricted rate limits. Of particular interest to fleet operators was the feasibility of inflight refueling and shipboard landing. Given the decrease in available stabilator rate from 35 to 10 degrees per second, the test team recognized the potential for APC due to rate limiting. An incremental build-up was designed,…progressively sampling the flying qualities at decreasing ranges from the tanker aircraft, and culminating in basket engagement. Throughout the approach to approximately 5 feet from the basket, the team was delighted to observe solid Level I handling qualities. They then confidently

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--> proceeded to engagement. Immediately upon probe contact, a longitudinal APC event initiated. Though the pilot immediately selected idle power and extended the speedbrakes, the ensuing departure was so violent that his aircraft was above the top of the vertical tail of the tanker and in 90-degree angle-of-bank prior to the probe separating from the basket. The photo/safety chase [aircraft] 500 feet abeam had to aggressively maneuver to preclude being struck by the test aircraft, and the refueling store was badly wrenched from its position on the tanker's wing pylon. The test team's naive reliance on incrementalism had badly failed them.55 The results of a later flight test during which a similar APC event occurred gave more detailed information about the sudden shift in PVS behavior, including why the buildup did not reveal the severe handling qualities cliff. Obvious from this second departure was a significant stab for the center of the basket after the probe had passed the lip of the basket. …the instrumentation revealed a three-fold increase in the magnitude of the pilot's longitudinal inputs in the seconds immediately prior to basket contact. In retrospect, this was attributed to a tanking technique in which the pilot flew formation off of the tanker fuselage up to within 2–3 feet of the basket. At that point, the pilot's point of reference shifted to the basket itself as he maneuvered the aircraft to seat the probe directly in the basket coupling. …In shifting the reference to the basket the control [precision demanded] abruptly tightened to inches [from feet], with a consequent abrupt increase in gain over that which had been required to maintain even very tight formation.55 These incidents reveal several features of cliff-like phenomena—sudden changes in the "architecture" of the closed-loop PVS as "constructed" or set up by the pilot and dramatic changes in the effective aircraft dynamics in response to changes in the pilot's commands. The consequence is the sudden onset of highly dangerous, closed-loop system behavior. The flight test doctrine of "incrementalism," in which potentially dangerous conditions are approached carefully and gradually can be a "cruel deceiver in obscuring PIO perils" in situations where the sudden onset of a highly nonlinear gain or phase lag can trigger an APC event.55 It is essential that reliable test procedures be developed for discovering and exploring the nature of sudden shifts in the PVS that may contribute to severe APCs.

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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. In conventional manual control systems, the most common nonlinearities are rate and position limits in surface actuators and various design features (such as preloads, thresholds, and detents) of cockpit manipulators (inceptors) that are designed to offset unavoidable frictional and other unfavorable effects. APC problems that can arise from these characteristics are well known among the cognoscenti, and major attention is invariably paid to them in design and flight testing. The actuator rate and position limits are central matters in design; conditions under which rate limiting is likely to be encountered, as well as pilot techniques for coping with it, are well understood. On some older aircraft, rate limiting in surface actuation occurs when mechanical stops in hydraulic control valves (e.g., servo valve bottoming) limit continued movement of cockpit manipulators so the pilot may have a direct cue that rate limiting is present. Such features are not present on more modern, mechanically signaled aircraft, where valve over-travel is provided, and the cockpit crew is not aware when actuators are operating at the rate limit. In FBW designs, the crew has no physical connection at all to the actuators, so surface actuator rate limits are not directly apparent to the pilot. However, it is possible to design FBW systems that synthesize direct-control feel to the pilot, including inceptor motions that reflect automatic system commands or even the current position of the control surfaces. In contrast to classical aircraft, FBW FCSs offer a broad range of possibilities for nonlinearities that can be easily implemented. The greater variety of system mode possibilities requires a fairly large number of nonlinear elements just to cope with shifts in FCS mode and aircraft configuration with changes in various interfaces, etc. The easy-to-mechanize aspects of digital control also provide a fertile field for the introduction of special situation-sensitive features intended to offset events that designers perceive as unfavorable. Thus, limiters are deliberately inserted after command signal integrators; and elaborate nonlinear features are used to reduce the undesirable time lags caused by integrators (e.g., integrator windup). Limiters are also used to set relative degrees of command authority for various functions to keep the rate limiting intrinsic in actuators from destabilizing the SAS (stability augmentation system). In other words, there may be good reasons to introduce nonlinear features into the FCS using FBW technology. Unfortunately, designers do not always have a comprehensive understanding and appreciation of the accompanying side effects, not the least of which can be an enhanced susceptibility to adverse APCs. To illustrate how these nonlinear features can affect PIO potential, two

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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) that affect closed-loop operations are command-path gain shaping and rate limiting. These are introduced by the FCS rather than the aerodynamics of the aircraft. Figure 2-2 shows a simplified view of these nonlinearities in a FCS-aircraft combination. In this system, rate limiters are present in several different locations. In the primary manual control systems of yesteryear, the major source of rate limiting was fully powered, surface actuating subsystems. These are still present although they are sometimes ''protected" from becoming active by pre-actuator rate limiters. Because these nonlinear features are present by design, they are adjustable, in principle. Any unintended harm they may do, such as contributing to a severe APC event, should be viewed as a design flaw. Rate Limiting Extensive control-surface rate limiting has been observed in most recorded severe oscillatory APC events, but the initiation of these events has often been attributed to other causes, usually excessive time lags. It is assumed that these time lags build up to a rate-limited oscillatory amplitude. This thesis is based on analogies with linear systems. Such excessive lags have been shown to result in poor flying qualities and to be major contributors to PVS oscillations. The excessive time lag thesis can be further supported by flight test demonstrations indicating there is some merit in "alternate control schemes" designed to offset the effects of time lag caused by rate limiting.1,10,41 Detailed analyses also support the notion that rate limiting can exacerbate the effects of time lags.3,14,15,31,39,46 Figure 2-2 Most common FCS locations of command gain shaping, rate limiters, and position limiters.

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--> Conclusions The incident described above was a classic Category II PIO, with large-amplitude pilot inputs and both rate- and position-limited elevator activity (as indicated by curve 2 of Figure 2-9). Because of the air-to-ground transition aspects of these incidents (involving airplane, control laws, and pilot), a case could also be made that this was a Category III event. Landing and derotation is a time of high pilot urgency and gain. For this reason it was assumed during control law development that fixed-base piloted simulations would not be adequate for realistic evaluation in this regime. One lesson from this event is that pilot urgency can be replaced to a significant extent by artificially boosting pilot gain via a suitable tight-tracking task. For example, on-runway attitude tracking showed clear trends in the time history and associated frequency response data. Derotation is a key flight phase and deserves special attention in preflight evaluation. The 777 simulator had the same characteristic as the airplane but was not evaluated as effectively prior to flight test. Also, none of the first five flight test pilots experienced any difficulty during landing, thus illustrating the need for carefully designed flight tests by as many different pilots as possible. Case 3. McDonnell-Douglas C-17 The C-17 is a four-engine military transport aircraft with a quadruply redundant FBW control system. The aircraft can deliver cargo to austere airfields and land on unpaved runways. Description of Event On June 22, 1993, during mission number 176 of the C-17 flight test program, test aircraft T1 experienced a lateral APC event. The test was an approach to landing with hydraulic system #2 inoperative. On final approach, as the pilot corrected for crosswinds using rudders (at about 2 seconds, curve 2, Figure 2-17), he experienced a wing rock. The pilot initiated a lateral command to correct for the wing rock and entered a cycle of oscillatory lateral commands (3 to 12 seconds, curve 1, Figure 2-17). When the aircraft neared 10 degrees of roll attitude, right wing down (at about 8 seconds, curve 1, Figure 2-18) at approximately 15 feet from the ground (curve 3, Figure 2-18) the co-pilot initiated corrective action and attempted commands opposite to the pilot for two cycles (curve 1, Figure 2-17). The aircraft was finally stabilized and a go-around was initiated. The APC frequency (pilot) was about 0.5 Hz (3.14 rad/Hz) (curve 1, Figure 2-17). During this event, the maximum aileron command was +26

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--> degrees/−39 degrees, as was the actual maximum position of the right aileron (curves 3 and 4, Figure 2-17). The ailerons were rate limited at ±37 degrees/sec (curves 3 and 4, Figure 2-17). Maximum roll attitudes were +10 degrees/−6 degrees (curve 1, Figure 2-18), and maximum roll rates, were +14 degrees/sec /−16 degrees/sec (curve 2, Figure 2-18). Analysis Analysis showed that the APC event was caused by rate limiting of the ailerons. The rate limiting was caused by overcommanding the surfaces. The overcommand was caused by high gains on both the pilot command path and the feedback paths. Corrective Action The total lateral loop gain was reduced, thereby reducing the magnitude of aileron commands for the same stick movement. The overall phase lag of the system was also reduced by optimizing the existing structural filters and removing unnecessary filtering. The use of ailerons was reduced by using spoilers for manual commands only and continuing the use of ailerons for both commands and automated stability augmentation. Case 4. Airbus A 320 The Airbus A 320 is a twin-engine narrow-body commercial transport with a typical seating capacity of 150. The A 320 entered service in 1988, and approximately 560 A 320s and A 321s are currently in service. (The A 321 is a stretched version of the A 320.) Description of Event On April 27, 1995, at about 5:30 p.m. local time, an Airbus A 320 operated by Northwest Airlines was approaching runway 18 at Washington National Airport. Winds were from 220 degrees at 17 knots, gusting to 25 knots. At an altitude of 140 feet, the airplane began a series of roll oscillations that persisted for 30 seconds, reaching a maximum roll of about ±15 degrees (see Figure 2-19, curve 1). Approximately 12 seconds after the start of the roll oscillations, at an altitude of less than 50 feet, the crew initiated a missed approach procedure. The aircraft subsequently made a successful landing. No injuries were reported, and the aircraft was not damaged.

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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

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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

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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). The ailerons achieved maximum deflection of approximately +24 degrees/−20 degrees, and they achieved maximum deflection rates of 35 to 40 degrees/sec. The aircraft experienced maximum rolls of approximately +15 degrees (right wing down) and −16 degrees (left wing down). (The National Transportation Safety Board [NTSB] reported a maximum roll of +12.3 degrees/−15.3 degrees.) The aircraft experienced maximum roll rates of +23 and −24 degrees/sec. During the maximum rolls, the phase difference between stick position (in roll) and aileron position was approximately 216 degrees. During the maximum rolls, the phase difference between aileron position and roll was approximately 144 degrees. Analysis Data from the flight data recorder (FDR) indicate that, after performing the final turn to align with the runway, the captain made a series of 12 large, rapid, cyclic deflections on his sidestick controller. Most of the deflections were to the maximum values allowed by the mechanical stops on the controller (±20 degrees) (see Figure 2-19, curve 2). Although the pilot had reported experiencing an uncommanded roll of 30 degrees, data from the FDR indicated that aircraft control surfaces operated normally. The NTSB subsequently concluded that this incident was consistent with a PIO and that it was not the result of an uncommanded roll.52 During the approach, the flaps were deflected to the 20-degree position, (which is referred to as the CONF 3 position) as part of a noise abatement procedure. Prior to this incident, there had been approximately 10 similar incidents involving other A 320s. In each case, aircraft were landing in gusty wind conditions with flaps in CONF 3, and some pilots experienced difficulty maintaining lateral control. Airbus initially responded to these incidents by issuing a temporary revision to its flight crew operating manual recommending that flaps be set at full deflection (35 degrees, which is referred to as CONF FULL) whenever possible during turbulent landing conditions, to reduce the workload when flying manually. Airbus then developed a flight control software modification to improve the PIO characteristics of the A 320 in CONF 3. This modification reduced the sensitivity of the aircraft to lateral sidestick inputs.

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--> Figure 2-19 A 320 incident time history. Source: NTSB.51

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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) nor the FAA made it mandatory. As a result, various A 320 operators handled the matter differently. Some airlines, especially European airlines, disseminated the recommendations and incorporated the modification. Others, including Northwest Airlines, did not. Consequently the aircraft involved in this incident had not been modified, and the pilots were unaware of the Airbus recommendation to use CONF FULL rather than CONF 3 in turbulent conditions. After the event, Northwest Airlines voluntarily installed the modification on all of its A 320s, and subsequently installation of the modification was made mandatory. Operators have reported no problems since incorporating these changes. Conclusions Although this problem was corrected by procedural changes and software modifications, the committee concludes that this PIO was probably associated with the lateral flying qualities of the A 320 with flaps in the CONF 3 position. It was probably triggered by a wind gust as the pilot was completing his final turn prior to landing. This incident illustrates that information on APC problems (and solutions) is not always effectively disseminated to the pilots who need it. In fact, incidents such as this one that do not involve injuries or equipment damage often escape scrutiny by government agencies. As a result, unless there are multiple incidents or a serious accident occurs, relevant issues may not be fully resolved. Case 5. Special Considerations for Rotorcraft Rotorcraft (i.e., helicopters and tilt rotors such as the V-22 Osprey) have several characteristics that make them prone to PIO: limited stability significant delays in control effectors because of the time required for rotor response (typically 70 msec) and power actuation (20 to 30 msec) coupling of rigid body modes with rotor and transmission modes significant inherent cross-coupling of control that is highly nonlinear potential coupling with external slung loads

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--> FBW technology has only recently been incorporated into rotorcraft (e.g., V-22, RAH-66 Comanche, and NH-90). Thus, there has been relatively little opportunity to encounter FBW-related PIOs in rotorcraft. However, experience with research helicopters, which is described below, shows that there is reason for caution if not concern. FBW on rotorcraft can add delays to the FCS response time because of stick filtering and control law computation. For example, one FBW technology demonstrator aircraft (the Advanced Digital Optical Control System, ADOCS) exhibited PIOs in several high gain tasks, including vertical landing, dart-quickstop, and slope landing. End-to-end delays occurred as shown in Figure 2-20. A time history for a landing task is shown in Figure 2-21.67 A second example demonstrating the potential for rotorcraft PIOs occurred in an in-flight simulator.7 The command model was attitude command for pitch and roll; the yaw axis had heading hold. The pilot's inceptors consisted of a spring-loaded force feel system with very little damping, linear stick forces, and relatively low breakout forces. Another test used a lateral-position tracking task. A hover board mounted on a target vehicle was used to guide the helicopter into a hover over a given point at a given altitude (Figure 2-22). The lateral hover tolerance was ±3 m, Figure 2-20 Response time analysis for the advanced digital optical control system demonstrator. Source: Hamel.30 Figure 2-21 Sample time history for a rotorcraft vertical landing task. Source: Hamel.30

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--> Figure 2-22 Schematic drawing of a helicopter tracking a vehicle-mounted hover board. Source: Ockier.56 Figure 2-23 Helicopter lateral-position tracking task, velocity profile for the lateral vehicle displacement. Ockier.56 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. At the end of the maneuver, a stabilized hover was to be regained. Figure 2-24 shows the lateral stick input and the bank angle response for the lateral-position tracking task. With no time delay added to the inherent helicopter dynamics (τ = 90 msec), the attitude command model gave a bandwidth of 2.6 rad/sec and a phase delay of 0.1 sec. Although the response is not free of oscillations, there is no PIO tendency and the task was rated as having a CH PR (Cooper-Harper Pilot Rating)* of 5. Figure 2-25 shows the lateral stick input and bank angle for the same attitude command model with an added time delay of 100 msec (so that τ = 190 msec), resulting in a bandwidth of 2.2 rad/sec and a phase delay of 0.17 sec. A very clear PIO tendency can now be recognized, and the configuration was rated as having a CH PR of 7. Although the time delay is a partial reason for the PIO, there may also be a more important contributor—the biomechanical coupling between aircraft and stick/pilot. Figure 2-26 shows the two command systems versus the ADS-33D requirement for hover and low speed aggressive maneuvering.68 The second (PIO-prone) configuration is incorrectly predicted to have Level 2 handling qualities (''adequate to accomplish the mission flight phase, but some increase

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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. Source: Ockier.56 in pilot workload … exists"71). This discrepancy underlines the fact that other effects, such as the (biomechanical) aircraft-stick/pilot coupling may have an impact on the introduction of this particular PIO. Such effects are not included in any of the current criteria and would certainly be difficult to predict. It also

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--> Figure 2-26 Small-amplitude handling qualities criterion (target acquisition and tracking) from ADS-33D. Source: Ockier.56 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 A comprehensive review of rotorcraft-pilot coupling potential and experience, including the two cases outlined above, has recently been published.30