Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions - Softcover

Committee On The Effects Of Aircraft-Pilot Coupling On Flight Safety; National Research Council; National Academy Of Sciences

Softcover

ISBN 10: 0309056888 ISBN 13: 9780309056885

Publisher: National Academies Press, 1997

Synopsis

Adverse aircraft-pilot coupling (APC) events include a broad set of undesirable and sometimes hazardous phenomena that originate in anomalous interactions between pilots and aircraft. As civil and military aircraft technologies advance, interactions between pilots and aircraft are becoming more complex. Recent accidents and other incidents have been attributed to adverse APC in military aircraft. In addition, APC has been implicated in some civilian incidents. This book evaluates the current state of knowledge about adverse APC and processes that may be used to eliminate it from military and commercial aircraft. It was written for technical, government, and administrative decisionmakers and their technical and administrative support staffs; key technical managers in the aircraft manufacturing and operational industries; stability and control engineers; aircraft flight control system designers; research specialists in flight control, flying qualities, human factors; and technically knowledgeable lay readers.

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Committee on the Effects of Aircraft-Pilot Coupling on Flight Safety, National Research Council

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AVIATION SAFETY AND PILOT CONTROL

Understanding and Preventing Unfavorable Pilot-Vehicle Interactions

NATIONAL ACADEMIES PRESS

Copyright © 1997 National Academy of Sciences
All right reserved.
ISBN: 978-0-309-05688-5

Chapter One

Executive Summary

Unfavorable aircraft-pilot coupling (APC) events are rare, unexpected, and unintended excursions in aircraft attitude and flight path caused by anomalous interactions between the aircraft and the pilot. The temporal pattern of these pilot-vehicle system (PVS) excursions can be oscillatory or divergent (non-oscillatory). The pilot's interactions with the aircraft can form either a closed-loop or open-loop system, depending on whether or not the pilot's responses are tightly coupled to the aircraft response. When the dynamics of the aircraft (including the flight control system [FCS]) and the dynamics of the pilot combine to produce an unstable PVS, the result is called an APC event.

Although it is often difficult to pinpoint the cause of specific APC events, a majority of severe APC events result from deficiencies in the design of the aircraft (especially with regard to the FCS) that result in adverse coupling of the pilot with the aircraft. In certain circumstances, this adverse coupling produces unintended oscillations or divergences when the pilot attempts to precisely maneuver the aircraft. If the PVS instability takes the form of an oscillation, the APC event is called a "pilot-involved oscillation" (PIO). PIOs differ from aircraft oscillations caused by deliberate, pilot-imposed periodic control motions, such as "stick-pumping," that are open-loop in character. An open-loop, forced oscillation does not constitute a PIO. If the unstable motions of the closed-loop PVS are divergent rather than oscillatory in nature, they are referred to as either APC events or as non-oscillatory APC events.

APC events can result if the pilot is operating with a behavioral mode that is inappropriate for the task at hand, and such events are properly ascribed to pilot error. However, the committee believes that most severe APC events attributed to pilot error are the result of adverse APC that misleads the pilot into taking actions that contribute to the severity of the event. It is often possible, after the fact, to carefully analyze an event and identify a sequence of actions that the pilot could have taken to overcome the aircraft design deficiencies. However, it is typically not feasible for the pilot to identify and execute the required actions in real time.

PIO phenomena comprise a complete spectrum. At one end of the spectrum is a momentary, easily corrected, low-amplitude bobble, a type of oscillation often encountered by pilots getting used to new configurations-basically a learning experience. This type of oscillation can happen on any aircraft and has been experienced by most pilots at one time or another. At the other end of the spectrum is a fully-developed, large amplitude PIO, a chilling and terrifying event that jeopardizes the safety of the aircraft, crew, and passengers. Fortunately, severe PIOs are rare.

Other severe APC events have been noted in which the excursions in aircraft motion diverge over time rather than oscillate. The few events of this nature that have been positively identified have had serious consequences. Large amplitude, dangerous PIOs and non-oscillatory APC events are the particular concerns of this report.

Recently, there have been several highly visible APC-related accidents involving military aircraft, as well as a number of incidents involving civil aircraft. At the same time, there has been widespread introduction of new fly-by-wire (FBW) FCSs into commercial transports. Almost all new FBW-equipped aircraft have exhibited APC events at some time during development, and these untoward coincidences have captured the attention of policymakers, test pilots, technical managers, and engineers. Although FBW systems are not inherently more or less susceptible to severe APC events, the flurry of incidents in aircraft development programs suggests that some side effects have not been fully explored or anticipated. Thus, as a matter of prudence the National Aeronautics and Space Administration asked the Aeronautics and Space Engineering Board of the National Research Council to conduct a study to assess APC-related aspects of recent incidents and accidents, aircraft development processes, the introduction of FBW and fly-by-light technology into FCSs, and national and international efforts devoted to APC research. This report is the result of that study, and it recommends steps that could be taken to improve aviation safety by reducing the kinds of APC problems seen recently and countering new types of APC problems that may arise.

The following high-level conclusions of the study committee are worth highlighting. (Subsequent sections include the committee's key findings and recommendations, and all findings and recommendations are listed in Chapter 7.)

There are many varieties of oscillatory and non-oscillatory APC events. Although none of these is welcome, only a rare subset is dangerous. Among the dangerous ones are events that exhibit "cliff-like" characteristics, which means that a PVS may fly superbly up to the sudden onset of a dramatic and potentially catastrophic APC event. What these severe APCs are, when they are likely to occur, and how to find (and fix) them are key issues. Most of the severe PIOs for which flight recordings exist have exhibited oscillations characterized by rate limited responses in control surface actuators or effectors. (Control surface actuators and effectors are rate or position limited when commanded movement exceeds limits imposed by design intent or physical structure on the rate of movement or extreme position of the control surface.) In most cases the pilots indicated that the onset of the PIO was sudden, unexpected, and cliff-like.

Piloted simulations have proved to be useful for investigating APC tendencies. However, neither piloted simulations nor available design and testing criteria can guarantee that a new aircraft will not be involved in an APC event.

Severe APC events are invariably new "discoveries" that often occur in transient and highly unusual circumstances. To avoid their discovery by operational pilots under unfavorable circumstances, test pilots must be allowed some freedom to search for APC tendencies in simulations and flight tests.

Data on recent APC events indicate that they are not uncommon in development testing where data recording and pilot reports are sufficient for causes to be determined and solutions developed. There are only a few reports of severe APC events in operational aircraft, but because there are no mandatory reporting requirements and recordings are often inadequate, the danger cannot be assessed adequately.

The committee was disturbed by the lack of awareness of severe APC events among pilots, engineers, regulatory authorities, and accident investigators.

THE AIRCRAFT-PILOT COUPLING EXPERIENCE

APC events usually occur when the pilot is engaged in a highly demanding, closed-loop control task. For example, many of the reported APC events have taken place during air-to-air refueling operations or approaches and landings, especially if the pilot is concerned about low fuel, adverse weather, emergencies, or other circumstances. Under these conditions, the pilot's involvement in closed-loop control is intense, and rapid response and precise performance of the PVS are necessary. Even so, these operations usually occur routinely without APC problems. APC events do not occur unless there is a transient triggering event that interrupts the already highly-demanding PVS operations or requires an even higher level of precision. Typical triggers include shifts in the dynamics of the effective aircraft (the combination of the aircraft and FCS) caused by increases in the amplitude of pilot commands, FCS changes, minor mechanical malfunctions, or severe atmospheric disturbances. Other triggers can stem from mismatches between the pilot's expectations and reality.

PIOs have been part of aviation history since the beginning of manned flight, and severe PIOs persist in spite of major efforts to eliminate them. When one kind of PIO occurs, usually unexpectedly, it stirs corrective actions. The experience is generally useful, in that the conditions thought to underlie that type of PIO tend to be avoided in designing new aircraft. As other PIOs occur under different circumstances, the cycle is repeated. With time, understanding improves and some causes are circumvented, but the occurrence of closed-loop oscillations remains a constant; only the details change with the aircraft and FCS technology.

From the pilot's perspective, there are three varieties of PIO experiences, ranging from benign learning experiences to severe and potentially dangerous oscillations. The benign "bobbles" are easily countered by the pilot's exit from the closed-loop PVS. By contrast, in many severe PIOs the pilot becomes locked into behavior that sustains the oscillation, even though the pilot often feels totally disconnected from the system. If the deficiencies in effective aircraft dynamics are essentially linear in nature, such as excessive time lag in response to a pilot input, a Category I PIO may result. If the effective aircraft dynamics change as a function of pilot-command amplitude or of FCS mode shifts, thereby creating a nonlinear sudden-onset change (a "cliff") in the effective aircraft dynamics, the resulting PIO is assigned either to Category II (when the dominant nonlinearities are associated with rate or position limiting of the control surfaces) or Category III (when the nonlinear changes are more complex). The Category II and III PIOs are particularly insidious because the effective aircraft dynamics and the associated flying qualities can be good right up to the instant the PIO begins. Identifying the potential for these PIOs, which almost always occur under unusual conditions when the PVS is operating near the margins, is a major challenge to test pilots and engineers. An extensive search process with a "discovery" mentality is needed to ensure that Category II or III tendencies are not overlooked.

Non-oscillatory APC events are not as well defined or understood as PIOs. Even if the pilot is extremely active and initiates many control reversals, the aircraft does not necessarily respond in an oscillatory fashion. Instead, a buildup of lags in the response of the aircraft's control effectors to the pilot's commands may ultimately lead to a divergence from the intended aircraft movement. As in the case of severe PIOs, pilots in these cases often report a sense of feeling detached from the aircraft behavior in terms of both awareness of what is happening and in terms of the temporal connections between pilot command and aircraft response.

Finding. Adverse APC events are rare, unintended, and unexpected oscillations or divergences of the pilot-aircraft system. APC events are fundamentally interactive and occur during highly demanding tasks when environmental, pilot, or aircraft dynamic changes create or trigger mismatches between actual and expected aircraft responses.

IMPACT OF NEW TECHNOLOGY

As phenomena in aviation history, APC problems have often been associated with the introduction of new technologies, functionalities, or complexities. There is a time lapse before flight experience with a new technology reveals the subtle changes in effective aircraft dynamics that may increase the susceptibility of a new aircraft to APC events. This partly explains why APC problems are more prevalent in military aircraft, which have traditionally introduced advanced technologies, and less common in civil aircraft, which have tended to adopt new technologies only after they have been proven in military aircraft. The prevalence of APC problems in military rather than commercial aircraft may also be associated with the nature of military operations, which frequently include maneuvers that require higher pilot gains than are commonly used on commercial aircraft.

FBW technology, which for this report includes fly-by-light technology, is a recent example of a new technology that has migrated from military to civil aircraft. The application of FBW technology has created FCSs that confer important overall system advantages in terms of performance, weight reduction, stability and control, operational flexibility, and maintenance requirements. FBW also offers opportunities for novel approaches to solving all kinds of problems with aircraft stability and control (including correcting APC tendencies). Yet, the flexibility inherent in FBW technology has the potential for creating unwanted new side effects and unanticipated problems.

In an aircraft equipped with a FBW FCS, information is transmitted from the cockpit to the control surfaces entirely by electrical means. The cockpit control device may not indicate to the pilot when the control surfaces are rate or position limited. The result may be a mismatch between the pilot's expectations and the aircraft's actual response, which can directly contribute to an APC event. In addition, FBW technology allows aircraft designers to design an FCS that features an elaborate set of system modes intended to enhance aircraft performance for a variety of missions under all expected flight conditions. When properly implemented, shifts between these system modes are smooth and unobtrusive and do not interfere with the pilot's operation of the aircraft. However, the complexity inherent in an advanced multiredundant FBW FCS makes it difficult for the designers, much less the pilots, to anticipate all of the possible interactions between the FCS and the pilot. The FCS may operate in ways that the pilot does not expect and does not recognize, thereby increasing the potential of encountering an APC event. As the potential for untoward events expands with the introduction of new technologies, increased vigilance is necessary to ensure that new systems do not inadvertently increase the susceptibility of new aircraft to APC events.

Finding. APC problems are often associated with the introduction of new designs, technologies, functions, or complexities. New technologies, such as FBW and fly-by-light flight control systems, are constantly being incorporated into aircraft. As a result, opportunities for APC are likely to persist or even increase, and greater vigilance is necessary to ensure that new technologies do not inadvertently increase the susceptibility of new aircraft to APC events.

AIRCRAFT-PILOT COUPLING EVENTS AS A CURRENT PROBLEM IN AVIATION

A major task of the committee was to assess the current status of APC events as a safety problem in aviation. In the context of aircraft development and testing, the record clearly shows that although adverse APC events are rare, they can pose a major safety concern. The same record also provides an extraordinary set of recent examples that should alert project and engineering managers, design engineers, test pilots, and aircraft operators to the need to address concerns about APC events as a central flying qualities and safety issue. These concerns can be addressed through detailed test plans, elaborate flight-test data recorders, and highly trained pilots like the ones who participate in the developmental stages of new aviation technology. Addressing these concerns will ensure that APC events that occur during development become matters of record.

When an aircraft enters operational service, the elaborate flight data recorders are routinely removed. The flight data recorders that are installed on many commercial aircraft employ a limited number of channels and sample rates; many military aircraft have no flight data recorders at all. For these and other reasons, confirmed APC-related incidents or accidents on operational FBW aircraft are quite rare.

(Continues...)

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Edition details

• Publisher: National Academies Press
• Publication date: 1997
• Language: English
• ISBN 10: 0309056888
• ISBN 13: 9780309056885
• Binding: Paperback
• Number of pages: 220