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32 Development of a Tool to Analyze Limitations Airfield Separations The methodology developed in this study has some limita- An analysis process for each type of airfield separation is tions. Risk of collision between two aircraft, or an aircraft and presented in Appendix A. The procedures are simple to use, an object, estimated with this methodology is applicable only and the instructions include practical examples to help to straight parallel segments of taxiways and taxilanes. the user. Although the lateral deviation data in taxiing operations The methodology serves as a screening tool to support used to develop the risk plots were measured only for the MOS requests involving airfield separations when the stan- B-747 aircraft, it is assumed that smaller aircraft have lateral de- dards cannot be met. The process helps to quantify the risk viation distributions that have smaller ranges. Thus, the model levels for non-standard conditions and, based on criteria rec- can be considered conservative when applied to smaller air- ommended in this report, in agreement with the FAA risk craft. However, the taxiway deviation models used in this study matrix, it is possible to evaluate the feasibility of approval of were developed from lateral deviation data collected on taxi- an MOS request (FAA, 2010). ways with centerline lights. Therefore, the conspicuity of the Different procedures are provided depending on the type taxiway/taxilane centerline is an added risk mitigation measure of airfield separation. It is possible to analyze separations in- that should be used when justifying an MOS request. volving runways, taxiways, taxilanes, and objects. A specific The FAA/ICAO CRM during missed approach was devel- approach was used for each type of separation. To facilitate oped based on data for two- and three-engine jet airplanes. The the application of the methodology, risk plots are presented veer-off models developed under this study are based on data for each ADG, and when the analysis involves specific aircraft, from veer-off accident/incident reports taken from several risk plots based on wingtip clearance are provided. countries and for aircraft with MTOW larger than 5,600 lb. The collision risk during the approach phase of landing is modeled for missed approach during instrument approaches Recommended Risk Criteria for Taxiway under Cat I and II. This is assumed to be the highest risk con- and Taxilane Separations dition, and the phase when the pilot is under visual condi- tions is not modeled in the risk curves presented. An extensive survey of historical taxiway and taxilane inci- CRM risk is estimated for an aircraft located on the center- dents helped assess the major factors involved in these events. line of a parallel taxiway. The taxiing aircraft is of the same Due to the slow speeds of aircraft on taxiways and taxilanes ADG as the approaching aircraft, and the maximum tail as compared to aircraft speeds on runway operations, even height for the ADG is taken to characterize the obstacle lo- under adverse weather conditions or slippery pavements, the cated in the taxiway. The same plots may be used to assess pilot was able to stop the aircraft as soon as it departed the risks associated with other types of obstacles at a certain dis- paved surface of the taxiway. Further, historical taxiway col- tance from the runway centerline; however, such obstacles lision events were not related to taxiway deviations. In almost must be lower than the maximum tail height of the ADG used every accident/incident, the collision occurred because there to develop the charts. was another aircraft or movable object inside the OFA of the taxiing aircraft. Recommendations for Future Work One major conclusion is that the existing standards pro- vide an excellent level of safety and that the risk is lower than Effort to Collect Taxiway/Taxilane one accident in 1 billion operations. Even when another air- Deviation Data craft or object was in the path, resulting in a collision, there As described in previous sections of this report, many of has never been a serious injury associated with the accidents, the separation standards were developed during World and the damages have been limited mostly to the wingtip of War II and were based on engineering judgment. These the aircraft. standards have helped maintain very high levels of safety, as Based on the available evidence, the worst credible conse- evidenced by the fact that there is no history of collisions quence for a taxiway or taxilane according to the FAA risk between two aircraft taxiing in parallel routes. matrix is major damage to aircraft and/or minor injury to With the increase in traffic volume and the need to increase passengers/workers, major unplanned disruption to airport airport capacity, many airports are restricted in their ability operations, or serious incident (FAA, 2010). For major con- to increase existing airfield separations to introduce opera- sequences, the maximum acceptable level of likelihood is tion of larger aircraft. Although the FAA permits MOS based "remote." In this case, a remote event is expected to occur on formulas developed for this purpose, the formulas were once every year or 2.5 million departures, whichever occurs developed based on engineering judgment, rather than using sooner. a probability approach.
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33 Recent FAA studies on aircraft deviation for large aircraft Cat I and II. The CRM model was based on the limited data have demonstrated the feasibility of collecting data to develop available at the time it was developed. Aircraft technology and risk models. However, these studies have focused on large air- navigational aids have improved significantly since then. As craft on taxiway segments with centerline lights. There is a need it is expected that airport capacity will need to increase two- to collect additional data for various categories of aircraft, for or threefold in the near future, it is necessary to develop a both taxiway and taxilane segments, under various environ- more rational approach to more accurately assess the level of mental conditions, with and without conspicuous centerline safety. Many airports still rely on visual and non-precision markings, and with and without centerline lights. approaches, and for these categories, that analysis can be Such studies should not be undertaken to modify the cur- made only by using Part 77 imaginary surfaces obstruction rent standards, but they can support MOS processes when the evaluation to obtain a very basic assessment of risk. evaluation of shorter-than-standard distances is necessary. A risk-based model for the assessment of visual segment or non-precision approaches would benefit many airports in the United States and abroad, particularly for the evaluation of Effort to Collect Aircraft Deviation Data airfield areas. Therefore, studies that address risk assessment during Landing and Takeoff Operations for aircraft operations associated with movable or fixed ob- The development of risk plots for the airborne phase of jects within or in the vicinity of airports would greatly bene- landing used the FAA/ICAO CRM for instrument approach fit the aviation industry.