3
Aircraft-Pilot Coupling as a Current Problem in Aviation

As one aspect of the charge to evaluate the current state of knowledge about adverse APC, the committee was asked to "review and assess recent incidents and accidents in which adverse APC is known or suspected." For several reasons, this has not been a straightforward task for all stages of aviation, and unequivocal answers have been hard to find. For developmental aircraft, the use of elaborate flight test data recorders usually ensures that APC events become a matter of record. Plausible causes can usually be determined and corrective action taken. However, there are no requirements to actively seek out adverse APC tendencies during the development or certification process for either military or commercial aircraft. Thus, an aircraft being developed might not be exposed to PIO-prone situations.

Once an aircraft enters operational service, multichannel, high, fidelity high sample-rate flight-data recording equipment is no longer used to monitor flight performance. The FDRs installed in commercial transports have far less capability, and military aircraft may have none at all. Other factors that work against a concrete and unequivocal assessment of APC potential are discussed below. In an effort to address the task of reviewing and assessing recent incidents and accidents, the committee examined, more extensively than anticipated, a variety of information sources, including accident and incident investigations, flight data recordings, and pilots. The principal sources of information are discussed in separate sections of this chapter.



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--> 3 Aircraft-Pilot Coupling as a Current Problem in Aviation As one aspect of the charge to evaluate the current state of knowledge about adverse APC, the committee was asked to "review and assess recent incidents and accidents in which adverse APC is known or suspected." For several reasons, this has not been a straightforward task for all stages of aviation, and unequivocal answers have been hard to find. For developmental aircraft, the use of elaborate flight test data recorders usually ensures that APC events become a matter of record. Plausible causes can usually be determined and corrective action taken. However, there are no requirements to actively seek out adverse APC tendencies during the development or certification process for either military or commercial aircraft. Thus, an aircraft being developed might not be exposed to PIO-prone situations. Once an aircraft enters operational service, multichannel, high, fidelity high sample-rate flight-data recording equipment is no longer used to monitor flight performance. The FDRs installed in commercial transports have far less capability, and military aircraft may have none at all. Other factors that work against a concrete and unequivocal assessment of APC potential are discussed below. In an effort to address the task of reviewing and assessing recent incidents and accidents, the committee examined, more extensively than anticipated, a variety of information sources, including accident and incident investigations, flight data recordings, and pilots. The principal sources of information are discussed in separate sections of this chapter.

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--> Trends From A Review Of Accidents And Incidents To review and assess recent incidents and accidents, the committee approached the problem over a broad front. This review was the focus of two workshops held to gather the best information available (see Appendix C for participants). To supplement the workshop data, subgroups of the committee examined numerous potentially relevant databases, held lengthy discussions with a Russian colleague, and went to Europe to collect information available there. The following partial list indicates the scope of the sources examined: NTSB reports Aviation Safety Reporting System data (1990–1994) technical literature briefings by representatives of several airlines information in the public domain (e.g., articles in Aviation Week and Space Technology and information on the Internet) workshop briefings by parties involved in specific APC events workshop briefings by specialists from the U.S. Air Force Wright Laboratory and research contractors internal FAA incident data briefings by the Air Accidents Investigation Branch of the United Kingdom developmental experience from aircraft companies (Boeing, McDonnell Douglas, Lockheed Martin, British Aerospace, Airbus, and Saab) manufacturer safety publications U.S. military flight test experience After an extensive investigation and review, the committee was able to identify five features or trends of APC-related accidents. APC events almost always occur during the development of new classes of aircraft that operate in new flight regimes or employ new technologies, such as FBW. This is also apparent in Tables 1-1 and 1-2, which summarize adverse APC events of varying severity in the development of advanced aircraft, including almost all partial or total FBW aircraft for which data were available. These include high performance military aircraft, such as the F-16, Tornado, F-18, YF-22, and JAS 39; large bomber and transport aircraft, such as the B-2, A 320, C-17, and Boeing 777; and the Space Shuttle Orbiter. Even during development testing, PIOs and APC events are rare. Many pilots conduct extensive flight test operations with no difficulties until just the right combination of triggering event, pilot dynamics, and effective aircraft dynamics occurs.

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--> Once an APC event is discovered in the development stage, it becomes highly visible. Developers are motivated to uncover the sources and contributing factors and to correct deficiencies. In other words, APC susceptibility is often detected and corrected as a natural part of the development, flight testing, and certification process. Confirmed occurrences of APC-related incidents on operational FBW aircraft are rare, although some exist. Analysis of severe PIOs almost always show that control surfaces were rate limited during the event. (Rate limiting is indicated when a graph of control surface position versus time produces a triangular plot, as in curve 5 of Figure 2-7 and curve 2 of Figure 2-9). The contrast between the presence of PIOs and other APC events in nearly all FBW aircraft during development tests by highly skilled, focused test pilots and the near absence of APC events in operational stages with line pilots has been noted as a "curious disconnect." This disconnect can, perhaps, be interpreted in two ways, both of them speculative. First, all major PIO tendencies have been discovered in the course of development. This explanation is most applicable if the development process includes a dedicated, effective effort to discover the circumstances (e.g., maneuvers and aircraft and FCS configurations) in which APC-prone tendencies are the most severe (for instance, when processes such as those recommended in Chapter 4 are applied). For some aircraft, however, an active investigation of APC characteristics may not have been conducted throughout the development process (or the effort to discover APC problems may have been flawed). Although these aircraft may appear to be immune to PIOs, it may simply be that they did not happen to encounter the conditions that would lead to a PIO with that particular aircraft. If that is the case, an unexpected PIO could result when production aircraft do encounter the necessary conditions. Unanticipated APC events usually greatly focus the attention of the responsible engineers and make them true believers in the potential hazards associated with APC events. A second interpretation of why some operational aircraft have no reports of PIOs or other APC events is that there has been a detection or reporting oversight. This could be because of differences between test pilots and line pilots, who may not have an adequate understanding of PIOs or who may interpret APC events as signs of pilot error. Such factors could lead to nonreporting of PIOs that occur in operational aircraft. Flight safety demands that operational pilots avoid PIOs and other difficult situations rather than seek them out. Other reasons for the absence of reported incidents may be that accident investigators do not adequately consider the extent to which APC events contribute to incidents or accidents, and there may be inadequacies in recording capabilities and/or analytical procedures. In any event, the committee was not able to assess fully the existing exposure of APC in operational fleets because of limitations in the reporting

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--> systems. Most currently available FDR systems do not sample at rates sufficient to allow investigation of high-frequency, oscillatory APC events. Older FDRs do not sample enough of the relevant parameters to identify potential APC phenomena. Requirements for improved FDRs are being considered that would increase the likelihood of identifying APC tendencies before catastrophic events occur. However, the temporal resolution of improved FDRs may still not be sufficient to identify high-frequency APC events. Reporting of APC events by pilots to safety reporting systems, such as the Aviation Safety Reporting System, is also thought to be limited by cultural factors. Because there has been a historical association of PIOs with inexperience and poor airmanship, pilots are often reluctant to admit having been involved in a PIO or APC event. This problem is exacerbated by the lack of a clear boundary between a benign APC event (e.g., an oscillation experienced by a novice student or a turbulence-induced oscillation) and an adverse APC event that could result in catastrophe. Flight Data Recorders Depending upon the sophistication of the FDRs and the number of parameters being recorded, FDRs make possible accurate reconstruction of events associated with a particular flight. The first requirement for installing FDRs on commercial aircraft was issued by the Civil Aeronautics Administration, the predecessor to the FAA, on August 1, 1958. Similar requirements were subsequently issued by regulatory authorities in other nations. To facilitate investigations of serious incidents and accidents, crashworthy FDRs are now required on most commercial aircraft in airline service. The number of parameters that the U.S. Federal Aviation Regulations require crashworthy FDRs to measure on a particular aircraft varies from 6 to 34, depending upon the aircraft's date of manufacture and the date the FAA issued the type certification for that aircraft. Older FDRs only collect basic flight data: altitude, airspeed, heading, normal "g," microphone keying, and time. Newer FDRs also collect data such as pitch attitude, roll attitude, and either control-surface positions or control-column positions. Data sampling rates are generally once per second, although a few parameters are recorded at higher rates, eight per second being the highest. Current regulations allow some aircraft equipped with the old six-channel recorders to use them into the twenty-first century. However, a proposed change to the Federal Aviation Regulations would increase the number of monitored parameters on new aircraft to 88, including a requirement to monitor parameters such as cockpit flight-control input forces. Nevertheless, the proposed sampling rates would be similar to the ones now required of older recorders.

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--> In addition to FDRs, other recorders are also available, and many individual operators have installed them voluntarily in their aircraft. These recorders, known as quick access recorders (QARs), are usually not crashworthy, but they record many additional parameters besides the ones required by FAA regulations. For example, some QARs can record as many as 400 separate parameters. In addition, the high capacity memories of QARs means they can store data for many more flight hours than FDRs, and the data are stored on easily removable media, such as optical disks. Most QARs record data at the same rates as FDRs required by regulation. QARs are generally used to monitor the performance of aircraft and engine systems for operational and maintenance purposes. They can also be used to evaluate crew actions and performance. Flight Operational Quality Assurance Regular examination of FDR or QAR data can also be used to identify potential problems affecting flight safety. Such efforts, which are known in U.S. industry as Flight Operational Quality Assurance (FOQA) programs, may reveal adverse trends before they result in an accident or serious incident. FOQA programs use ground-based computers to analyze QAR data to verify that aircraft systems are operating normally, that aircraft are being operated in accordance with standard operating procedures, and that they are being flown within the safe flight envelope.* Airlines in many parts of the world outside the United States, especially in Europe, have had FOQA programs for many years. However, this has not been the normal practice in the United States. British Airways, which has been a leading proponent of FOQA programs, claims that its FOQA program has identified potential causes of accidents, which were then eliminated through appropriate corrective action. Modern FDRs and QARs have the potential to facilitate the investigation of APC-related accidents and incidents by allowing investigators to compare many relevant parameters, including pilot control-surface inputs, FCS command signals transmitted to control-surface actuators, control-surface position, and aircraft motion. In fact, Airbus has recommended using a FOQA process to search specifically for APC phenomena during routine operations. Safety analysis equipment would need to be programmed and personnel trained to detect specific traits; a FOQA program would also provide quantitative data on how frequently APC-related phenomena might occur during the routine operation of conventional and FBW commercial aircraft. However, it would be necessary to increase the data sampling rates of certain parameters because the current rates are too low to allow satisfactory APC or PIO analysis. Generally, rate adjustments can be made by changes in software, but the possibilities may be limited by the design of recorders and their interfaces with particular

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--> aircraft. Depending upon the limitations, the costs could be quite high for an entire fleet. Proper searches for and examination of PIO phenomena would require a sampling rate for some parameters of at least 10 per second. Analysis of flight data from commercial aircraft on a routine basis in the United States has been limited by several factors. Airline pilots have been concerned about possible punitive actions by management or regulatory agencies in the event that FOQA programs reveal cockpit crew errors. Airlines themselves have been concerned about enforcement penalties that might be imposed by the FAA if operational violations are identified. And the aviation industry in general has been concerned about adverse consequences if news organizations or plaintiffs' lawyers were able to use the Freedom of Information Act to access FOQA data supplied to the FAA in confidence for the purpose of improving safety. Several major U.S. airlines are now cautiously initiating FOQA programs, but these are based on individual agreements between unions and management. They also depend on assurances of the FAA administrator that information revealed for the purposes of improving flight safety will not be used in a punitive way. Proposals for FAA regulations granting similar protections are also being drafted. Industry safety experts and organizations like the Flight Safety Foundation have long advocated the need for comprehensive, nonpunitive FOQA reporting systems. The potential benefits are obvious, and removing the threat of punitive action would significantly increase support for FOQA programs. Military Aircraft Although many military aircraft are fitted with various types of recorders, including (in some cases) crash survivable FDRs, the U.S. military has often been reluctant to require crash-survivable FDRs, even on military versions of commercial aircraft where recording capability is readily available. The investigation of the YF-22 accident, which is discussed in Chapter 2, indicated that investigators would have had a much more difficult time identifying the details of the APC event without the sophisticated data recording system that was installed on this developmental aircraft. The death of the Secretary of Commerce in 1996 in the crash of a military transport has led to additional emphasis on the installation of crash-survivable FDRs on some military aircraft; however, that will not address the problem of adequately investigating APC-related aspects of crashes involving operational combat aircraft.

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--> Accident Investigations In the past, PIO phenomena have not generally been recognized as a potential cause of accidents involving commercial aircraft in service. Although the data collected by FDRs on commercial aircraft have been adequate to identify flight path oscillations, for the most part these data have not been adequate to determine definitively if APC caused the oscillations or if the accidents involved APC phenomena. Most new FBW commercial aircraft have experienced one or more APC events during development, some of them severe. The sophisticated flight test instrumentation fitted to development aircraft enabled those APC events to be identified and the problems eliminated before the aircraft was put into operation. Once in service, however, the aircraft FDRs and QARs can not detect PIO problems, except in the most fortuitous circumstances. Therefore, investigations of commercial accidents seldom mention PIO as a contributory factor. Nevertheless there may have been a few APC-related incidents in operational service. Airbus, which has more than 700 FBW aircraft in airline operation, has more FBW experience than any other manufacturer. In all the flight hours accumulated by this fleet to date, 10 possible PIO incidents have been identified. Although Airbus acknowledges only three as genuine PIOs, the problems associated with these 10 incidents have been identified and fixed. One of them is described in case study 4 (Chapter 2). Because APC events may appear in operational service, improvements in the capabilities of FDRs and QARs will make it easier for investigators to determine the extent to which APC phenomena are present in specific incidents and accidents, provided recorder sampling rates are adequate. Therefore, the ability to detect APC problems could be enhanced by educating reviewers of FOQA programs, as well as accident investigators, regarding the existence of APC hazards and how to identify them.