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8 Figure 4. Wingtip clearance for taxilanes. The issue was given to a multidisciplined group called the aerodrome design and operations only, and Volume II deals Airport Obstructions Standards Committee (AOSC) to resolve. with the design of heliports. After much deliberation, this group grandfathered runways and Statistics have shown that approach and landing is the most taxiways meeting existing separation standards into the airport critical phase of a flight because the aircraft must follow a pre- design standards and determined further analysis was neces- cise and stable approach path despite the challenging circum- sary. In the interim, separation standards for construction of stances that characterize this flight phase--aircraft engine new runways or taxiways were increased to 500 ft for Cat II/III power is at its minimum, weather conditions on the ground operations involving Group V aircraft and for Cat I approaches may pose difficulties for landing, and habitation and land devel- involving Group VI aircraft. For Group VI aircraft making a opments surrounding airports can be significantly impacted by Cat II/III approach, a separation of 550 ft would be required deviations from the approach path. between new runways and taxiways. The AOSC work is docu- Moreover, aircraft may sometimes touchdown before the mented in AOSC Decision Document #04 (AOSC, 2005). runway arrival end or reject the takeoff and depart the runway if something goes wrong. These are the main reasons runways have a safety area. The FAA refers to this area as an RSA, whereas ICAO Rationale the ICAO defines an area with an equivalent function, con- To harmonize the development of aviation (including air- sisting of the runway strip plus the runway end safety area ports) globally, the ICAO was established toward the end of (RESA). The objective of this area is to reduce the risk of dam- World War II with the signing of the Convention on Interna- age to aircraft running off a runway and protect aircraft flying tional Civil Aviation (also known as the Chicago Convention) over a runway during takeoff or landing operations. The safety on December 7, 1944. Since then, ICAO has developed and area applies not only to the airspace on or around an airport but also to the ground itself. updated international specifications on all aspects of aviation. In Annex 14 to the Chicago Convention, first published in 1949, the ICAO promulgated specifications on airport design Runway Strip Width and operations (ICAO, 1949). ARCP's basis for the specifications of the graded portion of ICAO has kept pace with technological developments in the the strip is the acceptable risk of occurrence of aircraft veer- aircraft industry and kept Annex 14 current to provide to its offs. From the information available, there seems to be no Contracting States (or Member Nations) the minimum safety defined basis for the development of the specification of the full specifications for designing new airports and upgrading exist- strip width for the protection of over-flying aircraft using the ing ones to handle succeeding generations of newer, larger, and CRM. ARCP identified the following factors in the definition heavier aircraft. of the strip width: Many of the current standards and recommended practices (SARPs) contained in Chapter 3 of Annex 14 were defined by Aircraft approach speed the Aerodrome Reference Code Panel (ARCP) in 1981 (ICAO, Wingspan 1981). As part of the process to define a new reference code for Aircraft mass airports, ARCP also undertook a fundamental review of SARPs Type of approach (visual or instrument) based on a more rational approach. In 1990, Annex 14 was separated into two volumes (ICAO, Statistical data on aircraft veer-off events presented at the 1990). Volume I now contains international specifications on 8th Air Navigation Conference in 1974 were the basis for the

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9 analysis. The frequency of veer-offs exceeding a given dis- Separation Distance between a Runway tance from the runway centerline were determined according and a Taxiway to the type of operation and class of aircraft. To maintain a safety area around the runway, a runway A parallel taxiway is located such that no part of the largest strip width of 984 ft (300 m) is specified for instrument aircraft expected to operate on the parallel taxiway would runways, and a runway strip of 492 ft (150 m) is speci- penetrate into the adjacent runway strip. This is intended to fied for smaller, non-instrument runways. The strip is accommodate any potential veer-off of a landing aircraft when symmetrically located on either side of the runway center- the taxiway is being used and also to provide a sterile area, free line. The central portion is required to be graded to certain of obstacles that may endanger an aircraft executing a missed specified slopes so that it is less likely that an aircraft will approach or balked landing maneuver. The separation distance suffer substantial damage during runway veer-offs. These is expressed as follows: requirements are used to establish a safety area around the SW WS runway in which only those objects that must be located SRWY-TWY = + 2 2 there are permitted, subject to the applicable frangibility criteria being met. where SRWY-TWY is the distance between the centerlines of a runway Airfield Separations and a parallel taxiway, Every aircraft landing or taking off on a runway must pro- SW is the runway strip width, and ceed along a system of taxiways. During these movements, WS is the aircraft wingspan. aircraft should be protected by wide, obstacle-free areas; thus, Figure 5 depicts the main factors in the ICAO rationale. even the circulation areas (taxiways and aprons) must be Although a link taxiway for entry into and exit from the located at suitable distances apart and at a specified distance runway is located within the runway strip, whenever the from the runway. runway is in use, an aircraft on a link taxiway is required to The early specifications in ICAO Annex 14 were based on stop and hold at a distance of 295 ft (90 m) from the run- the layouts of airfields that existed at the time--mostly military way centerline (ICAO Code E) (ICAO, 2004). For aircraft airfields--that were deemed to be examples of best practice. It designated ICAO Code F, where the wingspan is greater is not uncommon to see older airports with runways from 98 than 213 ft (65 m) but not more than 262 ft (80 m), the to 197 ft (30 to 60 m) wide. Similarly, the circulation taxiways holding position location is at a distance of 350 ft (107 m) were separated from the runways so that aircraft on the taxi- from the runway centerline (ICAO, 2004). This distance way did not cause major risk to aircraft landing or taking off may need to be increased for certain operational conditions, from the runway. and this minimum holding distance should be reviewed if it Since then, numerous ICAO studies, undertaken with assis- interferes with radio navigational aids provided for the tance from ICAO technical panels and study groups, have fine- runway. tuned the specifications as actual aircraft performance results The separation distances between a runway and fixed objects and airport experience have become available. Improvements other than visual aids required for air navigation purposes are in aircraft manufacturing technology, better training, and the the following (ICAO, 2006a): availability of modern visual aids have also contributed to this fine-tuning. Within 254 ft (77.5 m) of the runway center line of a pre- The broad principles governing airfield separations are cision approach runway Cat I, II, or III where the code explained in greater detail in the following sections. number is 3 or 4 and the code letter is F; Figure 5. Runway/parallel taxiway separation distance.

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10 Within 197 ft (60 m) of the runway centerline of a precision as permitted and added to the semi-wingspan). The safety approach runway Cat I, II, or III where the code number is buffer illustrated in Figure 6 is determined using the follow- 3 or 4; and ing equation: Within 148 ft (45 m) of the runway centerline of a precision approach runway Cat I where the code number is 1 or 2. Z = TCS - WS - C where In addition, no mobile object shall be permitted on this part of the runway strip during the use of the runway for Z is the safety buffer, landing or takeoff. TCS is the taxiway centerline separation, WS is the aircraft wingspan, and C is the clearance between the outer main gear wheel and Separation Distance between a Taxiway the taxiway edge (maximum allowable lateral deviation). and Another Taxiway When deriving this specific dimension for Code F aircraft, it Taxiways are vital facilities of an airport on and around was considered that the steering mechanism of the Code F air- which certain safety areas must be provided at all times to craft would not be worse than that of the Code E aircraft and, ensure that a taxiing aircraft does not collide with another air- thus, the same deviation value of 15 ft (4.5 m) was retained. craft or an object. A primary assumption here is that an air- However, because it was felt that the wider, swept-back wings craft taxiing on a taxiway may deviate from its centerline; thus, using a "permissible" deviation, the taxiway strip width and of modern aircraft might not permit a pilot in the cockpit to taxiway width are determined. The taxiway strip, like the run- see where the wingtip would be, the safety buffer was increased way strip, should be clear of objects that may endanger taxiing in proportion to the wingspan increase vis--vis that for a aircraft. Furthermore, all other taxiways and objects that need Code E aircraft. to be on the operational areas of an airport are built to meet these criteria. Separation Distance between Taxiway and Object For two aircraft traveling in opposite directions on two parallel taxiways, it initially was deemed appropriate to pro- The separation distance between the taxiway and a station- vide for deviations of both aircraft from their respective cen- ary object is specified in order to ensure that a taxiing aircraft's terlines toward each other. It was considered necessary to wingtip does not collide with any stationary object. Because ensure that in such deviations there was still an adequate the object is not moving, only one deviation of the aircraft safety margin between the wingtips. While the permissible itself is taken into account along with the safety buffer, which value of the deviation of an aircraft from the taxiway center- includes all other factors that may cause further deviation of line is one factor, abnormal conditions like steering malfunc- the taxiing aircraft. The relationship illustrated in Figure 7 is tions, very slippery pavement conditions, low visibility, and expressed as follows: poor markings could not be ruled out. Thus, the concept of a WS safety buffer was introduced to provide an additional safety STWY-OBJ = +C + Z margin to the separation distance. 2 The safety buffer, Z, is equal to the difference between the where half width of the taxiway strip and the semi-wingspan of the largest aircraft in a given category (whose outer wheel is located STWY-OBJ is the separation, at the edge of the paved taxiway, in other words, full deviation WS is the wingspan, Figure 6. Parallel taxiway separation geometry.

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11 Figure 7. Taxiway/apron taxiway-to-object geometry. Figure 8. Aircraft stand taxilane-to-object geometry. C is the clearance between the outer main gear wheel and the WS STXL-OBJ = +C + Z taxiway edge (maximum allowable lateral deviation), and 2 Z = safety margin distance (for example, 10.5 m and 13 m for aircraft Codes E and F, respectively). where Separation Distance between a Taxilane STXL-OBJ is the separation distance between the taxilane and an Object centerline and an object. For the clearance distance required on an aircraft taxilane, In the case of Code F aircraft, again the 1.6-ft (0.5-m) the safety margins are reduced due to the slow taxiing speed of increase in the Z value was engineering judgment to account aircraft and the availability of a visual docking guidance system for the larger wingspan aircraft. or a marshaller to accurately guide the aircraft. The condition For taxiways and taxilanes, the same document establishes is illustrated in Figure 8. Therefore, for Code E aircraft opera- that the minimum separation distance is equal to the wingspan tions, the C value was reduced to 8 ft (2.5 m) and the safety plus max lateral deviation plus increment. ICAO Annex 14-- buffer (Z) was reduced to 25 ft (7.5 m). The formula is the same Aerodromes (2006b) contains the standards and recommended as the formula for the distance between a taxiway and an object. practices on airport separations.