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2Introduction From 1995 to 2004, 71 percent of the worldâs jet aircraft accidents occurred during landing and takeoff and accounted for 41 percent of all onboard and third party fatalities (Boeing, 2005). Landing overruns, landing undershoots, takeoff over- runs, and crashes after takeoff are the major types of accidents that occur during these phases of flight. Records show that while most accidents occur within the boundaries of the run- way strip, most fatalities occur near but off the airport area (Caves, 1996). Currently, Federal Aviation Administration (FAA) stand- ards require runways to include a runway safety area (RSA)â a graded and clean area surrounding the runway that âshould be capable, under normal (dry) conditions, of supporting air- planes without causing structural damage to airplanes or injury to their occupantsâ (FAA, 1989). Its purpose is to improve the safety of airplanes that undershoot, overrun, or veer off the runway. The size of the RSA depends on the type and size of aircraft using the runway. RSA standard dimensions have increased over time. The predecessor to todayâs standard extended only 200 feet from the ends of the runway. Today, a standard RSA can be as large as 500 feet wide and extend 1,000 feet beyond each runway end. The standard dimensions have increased to address higher safety expectations of aviation users and accommodate current aircraft performance. However, applying the new standards to existing airports can be problematic. Many runways do not meet current standards because they were constructed to an earlier standard. The prob- lem is compounded by the fact that the airports are increasingly constrained by nearby land development and other natural fea- tures, or they face costly and controversial land acquisition, or a need for unfeasible wetlands filling projects. The runway safety area standards are prescriptive and its rigid nature results in âaveragedâ degrees of protection being provided across broad ranges of risk levels, such that certain airports have much higher tolerance to risk than others. Ide- ally the risk associated with specific airport and operation factors should be modeled to assess the level of safety being provided by specific conditions of existing or planned RSA. Some intuitively important factors that would affect risk, such as various environmental and operational charac- teristics of the airport, are not considered yet. In current risk assessment methods, factors that determine safety cannot be analyzed independently; however, a rational, systematic identification of safety influencing factors and their interrelationships has never been conducted. This situ- ation impedes the assessment of effects of safety improve- ment opportunities and, consequently, risk management. Moreover, most airfield design rules are mainly determined by a set of airfield reference codes, which only take into ac- count the design aircraft approach speed and the aircraft dimensions (wingspan or tail height). The resulting protection is segregated in widely differing groups that do not necessar- ily reflect many of the actual risk exposure factors. Project Objectives The original objective of this project was to collect histor- ical information related to overrun and undershoot accidents and incidents to develop a comprehensive and organized database with editing and querying capabilities, containing critical parameters, including aircraft, airport, runway, oper- ation, and causal factor and consequence information that could assist the evaluation of runway safety areas. The research team extended the project objective to include the development of risk models for overrun and un- dershoot events. The primary function of the risk models is to support risk management actions for those events by in- creasing the size of the RSA, removing obstacles, construction of arrestor beds or perhaps, where that is not possible, by the introduction of procedural measures or limitations for oper- ations under high-risk conditions. C H A P T E R 1 Background
Three sets of models were developed in this studyâlanding overruns, landing overshoots, and takeoff overruns. Each set is comprised of three parts: probability of occurrence, loca- tion, and consequences. The models can improve the under- standing of overrun and undershoot risks and help airport operators manage these risks. Based on the information described above, the goals for this research project were extended to include: 1. Development of a comprehensive database for aircraft overrun and undershoot accidents and incidents; 2. Determination of major factors affecting the risks of such accidents and incidents; 3. Description of how these factors affect operations and associated risks, to improve understanding on how these events may occur; 4. Development of risk models for probability, location, and consequences for each type of accident: landing overruns (LDOR); landing undershoots (LDUS); and takeoff over- runs (TOOR); 5. Development of a practical approach to use these models for assessing risks on existing RSA under estimated oper- ation conditions; 6. Development of a list of relevant factors that should be reported for aircraft overrun and undershoot accidents so that availability of quality data can be improved for future studies; and 7. Development of prototype software to evaluate risks under specific operation conditions that may serve as the basis for creating analysis software that can be used to assess risks of aircraft overruns and undershoots. Applied to any specific airport, the analysis approach for RSA risk assessment developed in this study will allow users to determine if the risk is relatively high or low and whether there is a need for risk management action. The safety bene- fits provided by possible mitigation measures (e.g., increased size of RSA) can be evaluated using the same approach. In addition, three innovative techniques were incorporated to improve the development of risk models. One major im- provement in the modeling of accident occurrence is the use of normal operations (i.e., nonaccident and nonincident) flight data. With normal operations data (NOD), the number of operations that experience the factor benignly, singly, and in combination can be calculated, so risk ratios can be gener- ated and the importance of risk factors quantified. The second improvement is the use of normalization tech- niques to convert information to a standard nominal airport. Using such normalization procedure allows comparing acci- dent and NOD data for different operation conditions, thus creating a larger pool of relevant information. Finally, the models developed were integrated in a rational probabilistic approach for risk assessment of RSA. Based on historical information for flight operations and weather con- ditions, and considering the configuration of the RSA and presence of obstacles located close to the runway, the proba- bility distribution for accidents involving severe consequences may be estimated. 3
