7
Findings and Recommendations

The findings and recommendations of the Committee on the Effects of Aircraft-Pilot Coupling on Flight Safety were developed during deliberations that included consideration of all information collected by the committee. The findings and recommendations reflect current levels of understanding about APC and the processes currently used to mitigate the risks posed by adverse APC events. Implementation of the recommendations would improve aviation safety now and in the future by improving the effectiveness of APC-related design and test procedures, specifications, certification standards, training, and research. The rationale for each of the committee's findings and recommendations appears in the chapter indicated by the chapter heading.

Chapter 1 Aircraft-Pilot Coupling Problems: Definitions, Descriptions, And History

Finding 1-1. Adverse APC events are rare, unintended, and unexpected oscillations or divergences of the pilot-aircraft system. Adverse 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.



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--> 7 Findings and Recommendations The findings and recommendations of the Committee on the Effects of Aircraft-Pilot Coupling on Flight Safety were developed during deliberations that included consideration of all information collected by the committee. The findings and recommendations reflect current levels of understanding about APC and the processes currently used to mitigate the risks posed by adverse APC events. Implementation of the recommendations would improve aviation safety now and in the future by improving the effectiveness of APC-related design and test procedures, specifications, certification standards, training, and research. The rationale for each of the committee's findings and recommendations appears in the chapter indicated by the chapter heading. Chapter 1 Aircraft-Pilot Coupling Problems: Definitions, Descriptions, And History Finding 1-1. Adverse APC events are rare, unintended, and unexpected oscillations or divergences of the pilot-aircraft system. Adverse 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.

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--> Finding 1-2. APC problems are often associated with the introduction of new designs, technologies, functions, or complexities. APC problems can also arise when existing aircraft are tasked with new operational missions for which APC susceptibility has not been assessed during development testing. (This can occur when commercial aircraft are converted to military use.) New technologies, such as FBW and fly-by-light FCSs, 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. Finding 1-3. APC problems have occurred more often in military and experimental aircraft, which have traditionally introduced advanced technologies, than in civil aircraft. Finding 1-4. Recently, civil and military transport FBW aircraft have experienced APC problems during development and testing, and some APC events have occurred in recent commercial aircraft service, although they may not always have been recognized as such. Finding 1-5. A recent trend in APC is that events have been associated with the introduction of FBW and aircraft automation systems. Chapter 2 Varieties Of Aircraft-Pilot Coupling Experience Finding 2-1. There are two major types of severe APC events—PIOs and non-oscillatory APC events. Finding 2-2. From the pilot's perspective, there are three varieties of PIOs: relatively benign, initial or early encounters that occur when the pilot is learning to adapt to the effective aircraft dynamics severe, potentially dangerous oscillations stemming from a combination of extreme task demands, which require very high gain in the PVS, and deficiencies in the effective aircraft dynamics, such as excessive time lag severe, potentially dangerous oscillations occasioned by pilot commands that are usually motivated by task demands and are large enough to cause a major nonlinear change (flying qualities cliff) in the effective aircraft dynamics

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--> Finding 2-3. Conflicting priorities between different control authorities acting on the same effector can cause a severe safety risk or flying qualities cliff when the system is operating at or near the limiting conditions of the effector's positions or rates, thereby creating a latent non-linearity in the effective aircraft dynamics for an unsuspecting pilot. Finding 2-4. Non-oscillatory APC events are also likely to occur or to be triggered when the aircraft trim is inconsistent with the pilot's expectations. Recommendation 2-1. An active and aggressive search for APC tendencies, as contrasted with an incremental approach, should be included in efforts to discover cliff-like APC tendencies. Recommendation 2-2. Reliable test procedures should be developed to discover and explore in detail sudden shifts in the PVS. Chapter 3 Aircraft-Pilot Coupling As A Current Problem In Aviation Finding 3-1. With current test data recording and instrumentation equipment, APC events discovered in flight testing have almost always been defined well enough to permit detailed analysis and the development of fixes for the specific cause or causes. Finding 3-2. Operational aircraft are not usually equipped with flight data collection systems that can provide investigators with enough data to discern whether APC was a causal factor in an accident or incident. Finding 3-3. New generation flight data recorders provide enough data to analyze flight events encountered by civil transport aircraft in great detail. However, the proposed sampling rates may be inadequate for determining APC triggering events. Finding 3-4. APC accidents and incidents have occurred when the pilot suddenly and unexpectedly was required to take manual control, often when the autopilot was disengaged while the aircraft was in a grossly out-of-trim condition of which the pilot was unaware. Finding 3-5. Operational line pilots have little or no exposure to APC potential and are not trained to recognize the initial symptoms or to understand that APC does not imply poor airmanship. This may limit reporting of APC events.

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--> Recommendation 3-1. A system should be developed whereby pilots are enabled and encouraged to report unusual events, including events that result from their inadvertent actions, without fear of punitive action. In particular, renewed efforts should be made to improve reporting of APC events by pilots to existing safety reporting systems, such as the Aviation Safety Reporting System. Recommendation 3-2. Airlines should analyze flight data recorders for adverse events to detect trends and head off incidents and accidents before they occur. Recommendation 3-3. The parameters recorded by flight data recorders and the sampling rates used should be selected to enable identification of APC events and causes. Chapter 4 Precluding Adverse Aircraft-Pilot Coupling Events Finding 4-1. The approaches used to address APC risk are inconsistent throughout the civil and military aviation communities. The incidence of APC events could be reduced through more effective and consistent use of existing tools and capabilities during design, analysis, simulation, and testing. Finding 4-2. Currently, the FAA has no structured criteria for assessing adverse APC events during the certification process. Finding 4-3. Over the years, the results of a great many separate development efforts, exemplified in this study by the Boeing 777 and the F-14 backup flight control module (see Chapter 2), have independently arrived at the conclusion that testing with high-gain pilot tasks oriented toward discovering APC tendencies is necessary for adequately exploring the APC characteristics of modern aircraft. Finding 4-4. There are no widely accepted analysis and test guidelines for APC tendencies. As a result, even when APC-related tests are authorized and funded, test procedures are sometimes based on the personal experiences and preferences of the test personnel. Current practices do not systematically integrate design-team efforts to address APC issues early on, nor do they consistently make the best use of early indications that a problem may exist. Recommendation 4-1. Insufficient attention to APC phenomena generally seems to be associated with a lack of understanding and relevant experience.

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--> This shortcoming should be addressed through improved education about APC phenomena for pilots and other personnel involved in aircraft design, simulation, testing, certification, operation, and accident investigation. Recommendation 4-2. A disciplined and structured approach should be taken in the design, development, testing, and certification stages to maximize the effectiveness of existing techniques for mitigating the risk of adverse APC tendencies and for expediting the incorporation of new techniques as they become available. This is especially important in areas where effective procedures and standards do not currently exist (e.g., FAA certification standards). Recommendation 4-3. Organizations should adopt and implement risk minimization techniques in design and development policies, processes, and procedures. These techniques should be tailored and routinely updated to accommodate applications of newly developed technologies. Recommendation 4-4. Appropriate analysis and simulation should be conducted throughout all program phases. Highly demanding tasks with known and suspected triggering events should be included in simulation, flight test, and certification; this is critical to mitigating APC risk. Recommendation 4-5. In the interest of aviation safety, the free exchange of APC-related information on design and manufacturing processes and on aircraft performance characteristics should be encouraged throughout the military and civil aviation communities, nationally and internationally. Chapter 5 Simulation And Analysis Of The Pilot-Vehicle System Simulation Finding 5-1. Non-real-time, fixed-base, moving-base, and in-flight simulation tools can all play effective, complementary roles in discovering and understanding APC tendencies, as well as aiding in the assessment and partial validation of possible solutions. During simulations, APC potential is often indicated by subtle factors, such as increased pilot workload or sensitivity of the PVS to changes in aggressiveness. Actual PIOs or non-oscillatory APC events may not be found in all piloted simulations.

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--> Finding 5-2. Situations that appear to be susceptible to APC events have sometimes been ignored on the basis that ''pilots will not (or do not) fly like that." Finding 5-3. Pilots who have a range of experience and who have been sensitized to look for APC events are crucial to the effective use of piloted simulations and development testing. More than two or three pilots must be involved for a thorough examination of marginal conditions. Finding 5-4. Incremental expansion of a task or function envelop may not be effective for discovering Category II and III PIOs and some other types of APC events. Analysis Finding 5-5. When state-of-the-art PIO analysis tools and procedures are properly used, they are helpful for making a first cut in the APC discovery process, uncovering conditions likely to produce APC events, guiding more detailed and focused piloted-simulations, and generalizing experimental results via interpolation and extrapolation. Finding 5-6. The weakest points in pilot-vehicle analysis for APC situations are pilot models that describe transient conditions in PIO onsets associated with changes in the controlled element. Not enough fundamental experimental data are available to build adequate models for these transient phases. Finding 5-7. Although analytical approaches are available to address Category III situations, they have not yet been validated experimentally. Recommendations Recommendation 5-1. Existing simulation and analysis tools, including their joint use as complementary procedures, should be refined to be more specific and selective. Validating simulation details, protocols, and tasks and collecting and correlating them with flight test results should be given high priority. Recommendation 5-2. A high priority should also be assigned to collecting data that can be used to validate existing analytic tools and to provide the empirical bases for new ones.

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--> Recommendation 5-3. Tasks should be selected not only to be representative of nominal flight conditions, but also to explore the boundaries and extreme situations that may lead to APC events. Situations that cause APC events should not be eliminated because "pilots will not (or do not) fly like that." Recommendation 5-4. A "discovery search" stage that encourages exploratory behavior by the pilot in search of PIOs and non-oscillatory APC events should be part of piloted simulation. This should include carefree flying as well as deliberate attempts to induce and explore APC tendencies (e.g., control reversals at PIO frequencies). Chapter 6 Criteria For Assessing Aircraft-Pilot Coupling Potential Finding 6-1. The measures and metrics used in Aircraft-Bandwidth/Phase Delay, Gain/Phase Template and Average Phase Rate, and Smith-Geddes Attitude-Dominant Type III criteria offer relevant and valuable insights for assessing and understanding attitude-dominant Category I PIO potentials. The Dropback and modified Neal-Smith criteria can also play important supplementary roles. Thus, each has something to offer in providing insights, pinpointing troublesome areas, and enhancing understanding. However, none is sufficient to predict with absolute accuracy the presence or absence of Category I PIO potential in either the pitch or lateral axis. Finding 6-2. There are no validated metrics or criteria applicable to Category II and III PIO phenomena or non-oscillatory APC events. Such criteria are critical to a full assessment of the APC potential of new commercial and military aircraft. Recommendation 6-1. An eclectic approach that applies a mix of criteria should be used for design assessment. Recommendation 6-2. The current boundaries used to predict Category I PIO tendencies should be fine tuned to reduce known shortcomings. Boundaries should be adjusted from time to time to accommodate new data. Recommendation 6-3. Research to develop design assessment criteria and analysis tools should focus on Category II and III PIOs and non-oscillatory APC events. Additional research is also needed to extend the application of existing criteria to the lateral axis. This research should combine experiments

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--> with the development of effective mathematical analysis methods capable of rationalizing and emulating the experimental results. Recommendation 6-4. Existing specification and certification standards for military and commercial aircraft should be updated periodically to reflect advances in APC assessment criteria and testing techniques.