This publication provides guidance for using the standardized International Civil Aviation Organization (ICAO) method to report airport pavement strength and to deal with the overload operations issues. This standardized method is known as ACN/PCN method.
Using this method, it is possible to express the effect of individual aircraft on different pavements by a single unique number which varies according to pavement type and subgrade strengths, without specifying a particular pavement thickness. This number is the Aircraft Classification Number (ACN). Conversely, the load carrying capacity of a pavement can be expressed by a single unique number, without specifying a particular aircraft . This number is the Pavement Classification Number (PCN).
The system is structured so that pavement with particular PCN value can support without weight restriction, an aircraft which has an ACN value equal to or less than the pavement’s PCN value. This is possible because ACN and PCN values are computed using the same technical basis.
Airport pavements will deteriorate rapidly if they are subjected to aircraft loadings that exceed the structural design strength of the pavement. To prevent pavements from being overloaded and failing structurally, their strengths must be evaluated and reported and aircraft loads should not be allowed to exceed the reported strengths.
It is commonly thought that the most critical loading on an airfield pavement occurs when an aircraft touches down on a runway surface - but this is not the case. As a general rule, less than 50% of the weight of an aircraft impacts on the runway at touchdown - most of the aircraft weight actually remains airborne. Also, aircraft are considerably lighter on touchdown having lost a great deal of their weight due to the burning of fuel in flight. During the takeoff run, an aircraft gradually "transfers weight" from the landing gear to the wings as it accelerates along the runway towards rotation speed.
The most critical loading is static and occurs when the aircraft is parked on the apron prior to departure. At this time, the aircraft is at its maximum weight being full with both payload and fuel. The aircraft loading remains critical as it travels over the taxiway route to the runway end and gradually lessens as it accelerates through the takeoff run.
Pavement surface stability is also critical under heavy aircraft loads, especially when an aircraft turns on an asphalt surface - as when moving from a taxiway onto a runway threshold or when performing a 180 degree turn at a runway end to prepare for takeoff. High tire pressures can easily tear or shove an unstable asphalt surface.
An engineering system is used for the control of aircraft loadings on airside surfaces. This system developed by the International Civil Aviation Organization (ICAO) which is known as the Aircraft/Pavement Classification Number (ACN/PCN) system which was introduced for worldwide use in the mid-1980´s.
Each aircraft is assigned a number that expresses the structural loading effect of the aircraft on a pavement for a specified pavement type and standard subgrade category. Each airport operating authority shall report site pavement strengths using the same numbering system. The pavement is capable of accommodating unrestricted operations by an aircraft provided the aircraft load number is less than or equal to the pavement strength number.
In order that pavement strengths can be reported and understood internationally, ICAO developed a very similar ACN/PCN system. Under the international ACN/PCN system, each aircraft has been assigned an ACN (Aircraft Classification Number) that indicates design thickness requirements for the aircraft on a more expanded scale that ranges from an ACN of 5 for light aircraft to an ACN of 120 or more for heavy aircraft.
Some typical ACN values for aircraft are shown in Appendix (A) of this publication.
To comply with ICAO requirements, Jordan Civil Aviation Authority (JCAA) report pavement strengths in the format which includes standard international PCN codes. A PCN code has five parts - the first part is the actual PCN number, the second part is the pavement type ("F" for "flexible" or "R" for "Rigid"), and the third part indicates the subgrade strength category ("A" for high and "D" for low strength). The fourth part lists any tire pressure limitation which may apply - a "W" indicates that there is no tire pressure limit. The fifth part indicates the evaluation method used to determine the pavement strength - "T" if derived from an engineering study or "U" if based on satisfactory aircraft usage.
The PCN number indicates the suitability of a pavement area for unrestricted operations by any aircraft that has an ACN and tire pressure not exceeding the limits which stated pavement type and subgrade strength category
In compliance with ICAO requirements JCAA specifies the pavement overload operations limitation in attachment (A) of the JCAA Publication AN 14-I as Follows:-
5.1 Overloading of pavements can result either from loads too large, or from a substantially increased application rate, or both. Loads larger than the defined (design or evaluation) load shorten the design life, whilst smaller loads extend it. With the exception of massive overloading, pavements in their structural behavior are not subject to a particular limiting load above which they suddenly or catastrophically fail. Behavior is such that a pavement can sustain a definable load for an expected number of repetitions during its design life. As a result, occasional minor over-loading is acceptable, when expedient, with only limited loss in pavement life expectancy and relatively small acceleration of pavement deterioration. For those operations in which magnitude of overload and/or the frequency of use do not justify a detailed analysis, the following criteria are suggested:
a) for flexible pavements, occasional movements by aircraft with ACN notexceeding 10 per cent above the reported PCN should not adversely affect the pavement ;
b) for rigid or composite pavements, in which a rigid pavement layer provides a primary element of the structure, occasional movements by aircraft with CAN not exceeding 5 per cent above the reported PCN should not adversely affect the pavement
d) the annual number of overload movements should not exceed approximately 5 per cent of the total annual aircraft movements.
5.2 Such overload movements should not normally be permitted on pavements exhibiting signs of distress or failure. Furthermore, overloading should be avoided during any periods of thaw following frost penetration, or when the strength of the pavement or its subgrade could be weakened by water. Where overload operations are conducted, the appropriate authority should review the relevant pavement condition regularly, and should also review the criteria for overload operations periodically since excessive repetition of overloads can cause severe shortening of pavement life or require major rehabilitation of pavement.
5.3 ACNs for several aircraft types For convenience, several aircraft types currently in use have been evaluated on rigid and flexible pavements founded on the four subgrade strength categories in Chapter 2, 2.6.6 b) and the results tabulated in the ICAO Aerodrome Design Manual, Part 3.
Aircraft Classification Numbers (ACNs)
Source : Transport , Aerodrome Safety