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23 CHAPTER 4 Consequence Approach Risk is the likelihood of the worst credible consequence for with the probability of aircraft overruns and undershoots for a hazard. Many overruns, veer-offs, and undershoots have an assessment of risk. The approach is rational because it is resulted in aircraft hull loss and multiple fatalities, and there- based on physical and mathematical principles. fore, the worst credible level of consequences may be assumed to be catastrophic, according to the severity classification defined by the FAA and presented in Appendix F. Modeling Approach for Risk In some situations, a pilot may lose control of the aircraft, The basic idea was to assess the effect of different obstacles resulting in the destruction of the equipment with possible at various locations in the vicinity or inside the RSA. The fatalities, even when the aircraft accident takes place inside approach integrates the probability distributions defined by the RSA or the runway; however, in the majority of accidents, the location models with the location, size, and characteristics the RSA will offer some protection to mitigate consequences. of existing obstacles in the RSA and its vicinity. Consequences will depend on the type of structures and the The implementation of the approach required some simpli- level of energy during the aircraft collision. Possible obstacles may include buildings, ditches, highways, fences, pronounced fying assumptions so that it could be integrated with the fre- drops in terrain, unprepared rough terrain, trees, and even quency and location models. The following are the assumptions navigational aids (NAVAID) structures, like approach lighting used: system (ALS) towers and Localizer antennas, particularly if mounted on sturdy structures. 1. Aircraft overrunning, undershooting, or veering off the The energy of the aircraft during the collision is related to runway will strike the obstacle in paths parallel to the run- its speed when it strikes the obstacle, i.e., the greater speeds way direction. This assumption is necessary to define the are expected to result in more severe consequences. Also, the area of influence of the obstacle. consequences will depend on the type of obstacle. An aircraft 2. Four categories of obstacles are defined as functions of the striking a brick building at 40 mph may be destroyed whereas maximum speed that an aircraft may collide with an ob- if the obstacle is a perimeter fence less severe consequences stacle, with small chances of causing hull loss and injuries to are expected to occur. its occupants: The variables assumed to have an impact on consequences a. Category 1: Maximum speed is nil (e.g., cliff at the RSA resulting from overruns, veer-offs, and undershoots are: border, concrete wall). b. Category 2: Maximum speed is 5 knots (e.g., brick Obstacle type, size, and location; buildings). Aircraft size (wingspan) and speed; and c. Category 3: Maximum speed is 20 knots (e.g., ditches, Number of obstacles and location distribution (shadowing). fences). d. Category 4: Maximum speed is 40 knots (e.g., frangible The basic approach is that presented in ACRP Report 3, as structures, ALS). summarized in the ensuing sections. Additional details on how 3. Severe damage and injuries are expected only if the aircraft it was incorporated in the analysis are provided. The approach collides within the central third of the wingspan and with a described in ACRP Report 3 was intended to model accident speed higher than the maximum for that obstacle category. and incident consequences so that they could be combined The concept is explained in the ensuing section.

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24 4. The lateral distribution is random and does not depend on There are three distinct regions in this plot. The first re- the presence of obstacles. This is a conservative assumption gion (green) represents overruns that the aircraft departed because there are events when the pilot will avoid the ob- the runway but the exit speed was relatively low and the air- stacles if he has some directional control of the aircraft. craft came to a stop before reaching the existing obstacle. The accident/incident database contains a number of cases The consequences for such incidents associated with that when the pilot avoided ILS and ALS structures in the RSA. specific obstacle are expected to be none if the x-location is smaller than D0. The main purpose of modeling consequences of aircraft ac- The rest of the curve represents events that the aircraft ex- cidents is to obtain an assessment of risk based on the likelihood ited the runway at speeds high enough for the wreckage path for the worst credible consequence. It was not deemed neces- to extend beyond the obstacle location. However, a portion sary to develop a consequence model for each type of accident, of these accidents will have relatively higher energy and should as was done to model frequency and location. The approach can result in more severe consequences, while for some cases the be used to address any of the five types of incidents included aircraft will be relatively slow when hitting the obstacle so that in the analysis. catastrophic consequences are less likely to happen. Using the The basic idea is to use the location models to estimate the location model, if x-location is between D0 and D0+, it may incident occurrences for which the aircraft will have high en- be assumed that no major consequences are expected if the ergy when striking an obstacle, thus resulting in serious con- obstacle is present. sequences. It should be noted that neither of the models used The value of is estimated based on aircraft deceleration in the approach provides an estimate of the aircraft speed; over different types of terrain (paved, unpaved, or EMAS) however, using the location model and the average aircraft and crashworthiness speed criteria for aircraft. It should be deceleration during a runway excursion, it is possible to infer noted that depends on the type of terrain, type and size of the probability that the speed is above a certain level when aircraft, and type of obstacle. Frangible objects in the RSA are reaching the obstacle. Figure 30 is used to illustrate the case less prone to causing severe consequences. It also should be for overruns and help understand the principle. This approach noted that lighter aircraft may stop faster and the landing gear was introduced in ACRP Report 3. configuration also may have an effect on the aircraft deceler- The x-axis represents the longitudinal location of the wreck- ation in soft terrain, but these factors are not accounted for in age relative to the runway departure end. The y-axis is the prob- this approach. ability that the wreckage location exceeds a given distance "x." Using this approach, it is possible to assign three scenarios: In this example, an obstacle is located at a distance D0 from the probability that the aircraft will not hit the obstacle (green the departure end, and the example scenario being analyzed region--resulting in none or minor consequences); the prob- is an aircraft landing overrun incident. Figure 30 shows an ex- ability that the aircraft will hit the obstacle with low speed and ponential decay model developed for the specific accident energy (yellow region--with substantial damage to aircraft but scenario, in this case, landing overruns. minor injuries); and the probability that the aircraft will hit the obstacle with high energy (orange region--with substantial damage and injuries). For those events with low energy when impacting the ob- stacle, it is possible to assume that, if no obstacle was present, Probability d the aircraft would stop within a distance from the location Exceeds x of the obstacle. The problem is then to evaluate the rate of Prob of Incident with d < Do P{d Do} these accidents having low speeds at the obstacle location, and this is possible based on the same location model. This Prob of Incident with d > Do P{d > Do} low energy probability can be estimated by excluding the cases when the Plow = P{d > Do} - P{d > Do+ } speed is high and the final wreckage location is significantly P{d > Do+ } Prob of Incident with d > Do + beyond the obstacle location. high energy Phigh = P{d > Do+ } To complement the approach it is necessary to combine the longitudinal and transverse location distribution with the Do Do+ Distance x from runway end presence, type, and dimensions of existing obstacles. The basic Do is distance to Obstacle, d is distance the aircraft came to stop approach is represented in Figure 31 for a single and simple Area (yellow) between Do and Do+ represents % occurrences at obstacle. low speed (energy) when hitting obstacle (low consequences) Laterally, if part of the obstacle is within the yellow zone, as Figure 30. Approach to model consequences of over- shown in Figure 32a, medium consequences are expected; how- run accidents. ever, if any part of the obstacle is within the orange zone, as