CROW Guideline on PCN Assignment released
Guideline harmonises PCN Assignment (CROW-report D13-02)

ICAO Member States have agreed to evaluate and publish the bearing strength of airport pavements using the ACN-PCN system.Although there is a great amount of material published on how to compute an ACN, ICAO has not specified regulatory guidance as how an airport authority is to arrive at a PCN, but has left it up to the airport authority as to how to perform this task. This is a result of member states reluctance to agree on an international standardized method of pavement evaluation, but rather to rely on their own internally developed procedures. As a consequence, PCNs can vary, depending on the nationally used method of design and evaluation of airport pavements.

However, it is important to have an unambiguous, generally accepted methodology for computing pavement damage, to allow airport operators and pavement engineers to adequately design pavements to accommodate new aircraft, and to allow airlines to anticipate airport pavement weight restrictions in planning their operations and in deciding which aircraft to purchase. An established and industry recognized engineering method appropriate to the pavement construction type should be used to determine the structural capability of a pavement to support proposed aircraft loads and traffic levels.

To harmonize and arrive at reproducible PCNs, the CROW Coordinating Committee on Airport Pavements developed in 2005 a ‘Guideline on PCN assignment’ that prescribes the structural evaluation of jointed rigid and flexible pavements. The Guideline is to be used to assign a PCN by means of technical evaluation for civil airport pavements on a national level. This guideline presents the procedure for the calculation of a technical PCN in full detail. The basis for the PCN-determination is the use of Layered Elastic Analysis and calibrated failure criteria derived from material testing and/or full-scale pavement tests. An uniform set of pavement transfer functions (performance models) and material characterization (mechanical properties) are described and procedures to access these properties are given. It should be noted that the transfer functions presented in this guideline are typical for Dutch construction materials and subgrades. For reporting the results of the technical PCN assessment, a datasheet with all relevant parameters used concerning the pavement, materials, PCN-life and fleet usage is presented.

The Guideline was primarily intended for use in the Netherlands, however, can very well be used by other Countries and/or NATO nations, provided that they use or test the characteristics of their own road construction materials. The Guideline is to be used to assign a PCN by means of technical evaluation for civil airport pavements on a national level. The CROW-guideline for assigning a technical PCN-value focuses on standardization of the pavement models, the calculation steps, the assessment and selection of material characteristics (transfer functions), the structural pavement life, the design criterion in relation to the true pavement damage, reliability concept as well as traffic and wander. This guideline can be used in the Netherlands by airport pavement engineers or regulatory be prescribed by national Civil Aviation Authorities to arrive at realistic pavement designs and comparable PCNs. The PCN value is published in an Aeronautical Information Publication (AIP). This guideline can also be used by NATO to establish a STANAG that would well be accepted among the nations.

The Guideline can be used to evaluate rigid and flexible pavement for PCN based on revised design for a new pavement, for newly constructed pavements (as-built situation) and for in-service pavements already subjected to traffic.

 

The PCN-method presented in the Guideline is known as the Technical evaluation method, and requires knowledge of the pavement and its traffic, as well as a basic understanding of engineering methods that are utilized in pavement evaluation in order to be successfully implemented. This Guideline harmonises the pavement models, the calculation steps, the assessment and/or selection of material characteristics (strength, transfer functions), the structural pavement life, the design criterion in relation to the true pavement damage as well as traffic and wander. The method complies to the PCN-method as in use by the Dutch Ministry of Defense (2007). Use is made of implemented mechanistic design/evaluation systems with criteria that appear to be yielding reasonable results. These procedures are based on linear, elastic theory coupled with empirical relationships for relating computed stress/strain to allowable aircraft load. This approach is well understood and well documented. The elastic layer mechanistic/empirical methods are also very adaptable to new criteria. For example, it is not very difficult to add/remove/modify the criteria (fatigue relationships or transfer functions). This makes it attractive as a NATO standard too, since results from continuing material research and development could be incorporated as necessary. With the current emphasis and requirements for better design/evaluation methods at NATO, a standard that would be well accepted among the Nations could also be established.

The Guideline has been revised in 2013 to cover amendments to ICAO’s Annex 14 in respect to the ACN-PCN method.
ICAO has approved on October 16, 2007 the changing of the numerical coefficients entitled "Alpha factor" involved, for flexible pavement type, in calculating the ACN of the aircraft. The ICAO decision led to recalculate the "flexible" ACN of landing gears of aircraft which have a total of at least four wheels. Aircraft manufacturers were requested to revise the ACN values for their respective aircraft types on various subgrade categories, currently available as guidance material in Doc 9157, Part 3, Table A5-1. ACNs of today’s civil aircraft may be found in a number of different sources.
Secondly, the tire pressure category and designations has been revised to comply to the actual tire pressures of modern heavy gear aircraft as has been proposed in 2012.

CROW-report D13-02 explains the PCN assignment in full detail, and provide worked examples of PCN calculations with APSDS (flexible only) and PAVERS (both flexible and rigid pavement). The standardized method is to be used for reporting strength of Dutch civil airport pavement. Chapter 2 reports on the officially published procedures for the PCN assessment of rigid and flexible pavements. The PCN assignment is presented in Chapter 3. Chapter 4 contains worked examples and PCN assessments for rigid and flexible pavement using PaversŪ-software. Based on the information contained in this guideline, the user can determine the PCN or load-carrying capacity of an airfield pavement. The Guideline presents a number of test methods to assess in a standardized manner, the material and fatigue transfer functions to be used for PCN-evaluation in the Netherlands. The mechanical properties of typical Dutch road building materials are given in appendix 1 to 3. These properties may also be used for the design of a pavement. Test methods are presented to assess the dominant properties of several pavement materials and subgrade. Appendix 1 focuses on bituminous materials whereas appendix 2 presents the properties of cement concrete according to Dutch Standards. The properties of base and sub base materials are presented in appendix 3. Appendix 4 presents the results of a worked example using APSDSŪ-software. Appendix 5 has the status of Directive and presents the PCN data format to be mandatory reported in submitting the results of a PCN-assessment to the Netherlands CAA. Appendix 6 presents the Addendum and describes the measurement program comprising of field measurements and laboratory testing as well as the determination of the material performance properties.

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