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15 Based on the survey results, approximately 12% of agen- Structural Evaluation cies reported using roughness surveys on runways and 4% of agencies on taxiways and other facilities (see Table 2). The overall structural strength of airport pavements is evalu- ated using the Aircraft Classification NumberPavement Clas- sification Number (ACN-PCN) method outlined in the draft Pavement Friction FAA Advisory Circular on Standardized Method of Reporting Airport Pavement Strength--PCN (2009). The PCN captures Pavement friction is the force that resists the motion between the relative strength of the pavement structure (considering a a vehicle tire and a pavement surface. Pavement friction is a standard subgrade) and the ACN provides guidance to airport significant safety concern for aircraft with greater weight and operators regarding the relative effect of an aircraft on the landing speeds, such as turbojet aircraft, particularly when pavement structure. The PCN evaluation is not routinely the pavement is wet. The Guide for Pavement Friction (Hall used for the planning of pavement M&R treatments and was at al. 2009) provides general technical information on pave- not included in the survey. ment friction. According to the survey (see Table 2), 18% of airports, FAA Advisory Circular on Measurement, Construction, typically large airports, carry out periodic network-level sur- and Maintenance of Skid-resistant Airport Pavement Sur- veys using a Falling Weight Deflectometer (FWD) on run- faces (2004) provides guidelines for designing skid-resistant ways and 12% of surveyed airports reported FWD surveys of airport pavement surfaces and for on-going monitoring and taxiways and other facilities. The average frequency of the evaluation of pavement friction. The Circular also describes FWD surveys on runways was 3.7 years. recommended procedures to measure pavement friction and provides specific friction levels required for safe aircraft Procedures for FWD testing are outlined in FAA Advisory operations. These friction levels can be used to plan and carry Circular on Use of Nondestructive Testing Devices in the out appropriate M&R actions. Evaluation of Airport Pavements (2004). For project-level analysis, structural evaluation is discussed in chapter seven. For airfield pavements, friction is typically evaluated on runways only. Twenty-two percent of responding agencies reported that they evaluate pavement friction on runways. In CONDITION ANALYSIS addition, 8% of airport agencies reported measuring friction on taxiways (see Table 2). Pavement condition analysis utilizes pavement condition data in pursuit of the following outcomes: Presence of Foreign Object Debris Assessment of the overall condition of the pavement net- The presence of foreign object debris is evaluated using the work. For example, Figure 7 shows the results of a PCI Foreign Object Damage/Debris (FOD) Index. The FOD survey for a small Michigan airport (Michigan Airports Index is determined from the PCI calculated by considering Division 2007). The objective assessment of the condi- only the distresses/severity levels capable of producing FOD tion of the asset is also useful in meeting the accounting (Pavement Engineering Assessment Standards 2004). The recommendations of the Governmental Accounting FOD index is generally not used at major airports. Standards Board (1999). TW927 (36) A01 (34) RW1533 (34) THSOUTH (100) THW (21) TWA (100) TWA (100) TWB (100) RW1836 (100) A02 (70) THEAST1 (29) RW1533 (34) THEAST2 (29) Pavement Condition Index 100 - 86 40 - 26 85 - 71 25 - 11 70 - 56 10 - 0 55 - 41 FIGURE 7 Example of a graphical display of Pavement Condition Index.