The ACN-PCN method
Single universal system for reporting strength

States are required to evaluate and publish the strength of airport pavements using ICAOs ACN-PCN system. The method concentrates on classifying the relative damage of aircraft. ICAO foresees that each pavement authority will define a Pavement Classification Number (PCN) by whatever means is considered suitable to indicate the support level of a particular pavement such that all aircraft with a published ACN equal to or less than the reported PCN can use that pavement safely, without load bearing failure or undue damage to the structure.

The ACN-PCN system provides a standardised international airplane/pavement rating system replacing the various S, T, TT, LCN, AUW, ISWL, etc., rating systems throughout the world. In 1981 ICAO promulgated the ACN/PCN method as the single universal system for determining the weight limitation of aircraft operating on airport pavements by a procedure of comparing an airport’s Pavement Classification Number (PCN) with an Aircraft Classification Number (ACN). To avoid accelerated deterioration and excessive maintenance costs and for the safeguarding of pavement integrity and assurance of optimum service life ICAO utilises the ACN /PCN load classification method for reporting pavement strength. According to this world-wide standard, aircraft can safely operate on a pavement if their ACN is less than or equal to the pavement load bearing capacity or PCN. An aircraft having an ACN equal to or less than the PCN can operate without weight restrictions on a pavement.

It must be noted that the ACN/PCN method is not a design or evaluation method, but purely a classification system. Unfortunately the fact that the method of calculating ACN utilises two common design and analysis methods (the CBR equation and Westergaard theories) has led a surprisingly large number of people to assume that it is a design and evaluation method. It is not uncommon for reference to be made to PCN’s calculated by the ACN/PCN method. In fact the ICAO documentation makes it very clear that it is not a design/evaluation method and that the PCN is simply the ACN of the most damaging aircraft that can use the pavement on a regular basis (regular being defined by the operator).

The ACN/PCN method only deals with aircraft weighting in excess of 5.700 kg ( 12,566 lb.) as the airports with pavement for smaller size aircraft need only report the maximum allowable mass and the maximum allowable tire pressure if applicable. The ACN/PCN system ensures that both aircraft and pavement can be utilised to their maxi-mum extend without detrimental effects. According to the Design Manual the method is meant only for publication of pavement strength data in the Aeronautical Information Publication (AIPs). It is not intended for design or evaluation of pavements, nor does it contemplate the use of a specific method by the airport authority either for the design or evaluation of pavements. Although the Design Manual states that any method may be used to determine the load rating of the pavements, it is obvious that the use of layered elastic method in conjunction with calibrated failure criteria is preferred.

The ACN-PCN system is easy to use. Each aircraft is assigned a number that expresses the structural effect on a pavement for a specified pavement type and a standard subgrade category. Each airport operating authority reports site pavement strengths using the same numbering system. The pavement is capable of accommodating unrestricted operations provided the aircraft load number is less than or equal to the pavement strength number. Maximum tire pressure limitations may also be applied to some pavements which may further restrict certain aircraft operations. The ACN is based on the static application on aircraft loads to the pavement surface making them somewhat conservative in nature. Member States to ICAO are required to evaluate and publish the strength of airport systems using the ACN/PCN system. The national CAA publishes weight bearing limits in terms of ACN/PCN in an Aeronautical Information Publication for civil and international use. The intent is to provide planning information for individual flights or multi-flight missions which avoid either overloading of pavement facilities or refused landing permission. The ACN and PCN are defined as follows:

  1. ACN is a number that expresses the relative structural effect of an aircraft on different pavement types for specified standard subgrade strengths in terms of a standard single-wheel load.
  2. PCN is a number that expresses the relative load-carrying capacity of a pavement in terms of a standard single-wheel load.
  3. The system is structured so that a pavement with a particular PCN value can support, without weight restrictions, an aircraft that has an ACN value equal to or less than the pavement's PCN value.
  4. ACN values will normally be provided by the aircraft manufacturers. The ACN has been developed for two types of pavements, flexible and rigid, and for four levels of subgrade strength.
  5. The PCN value is for reporting pavement strength only. The PCN value expresses the results of pavement evaluation in relative terms and cannot be used for pavement design or as a substitute for evaluation. It must be noted that the ACN/PCN method is not a design or evaluation method, but purely a classification system.

Unfortunately the fact that the method of calculating ACN utilises two common design and analysis methods (the CBR equation and Westergaard theories) has led a surprisingly large number of people to assume that it is a design and evaluation method. It is not uncommon for reference to be made to PCN’s calculated by the ACN/PCN method. In fact the ICAO documentation makes it very clear that it is not a design/evaluation method and that the PCN is simply the ACN of the most damaging aircraft that can use the pavement on a regular basis (regular being defined by the operator).

 

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