The ACN-PCN system of rating airport pavements is designated by the International Civil Aviation Organization (ICAO) as the only approved method for reporting strength.
“The bearing strength of a pavement intended for aircraft of apron (ramp) mass greater than 5.700 kg shall be made available using the aircraft classification number - pavement classification number ACN-PCN method………” (ICAO Annex 14, clause 2.6.2)
The ICAO system for civil airport pavements involves comparison of an airport’s Pavement Classification Number (PCN) with an Aircraft Classification Number (ACN). According to this world-wide ICAO 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. The PCN is formally published in an Aeronautical Information Publication (AIP).
The ACN is a single unique number expressing the relative effect of an aircraft on a pavement for a specified subgrade strength specifying a particular pavement thickness. It consists of a number on a continuous scale, ranging from 0 on the lower end and with no upper limit, that is computed between two pavement types (rigid or flexible), and the subgrade support strength category. ACN values for civil aircraft have been published in ICAO’s Aerodrome Design Manual and in FAA Circular 150/5335-5.
As per October 2007, ICAO revised the alpha factor for four wheel undercarriages. Based on recent findings of full scale pavement tests, ICAO agreed to the following revisions concerning the alpha factor values:
a. change the alpha factor value for all four-wheels per main landing gear from the current 0.825 to 0.80;
b. retain the alpha factor for six-wheels per main landing gear at 0.72; and
c. change the alpha factors for other main landing gears so that the ranking of the damaging effect remains consistent.
The ACN/PCN method is strict towards the ACN procedure, but allows the airport to choose for any method suit-able for the determination of the pavement load bearing capacity or PCN. However, ICAO’s Aerodrome Design Manual, Doc. 9157-AN/901, part 3 Pavements and Aerodromes, Annex 14 to the Convention on International Civil Aviation presents some guidance for the PCN determination.
Harmonising pavement load load-carrying capacity is required to optimise the use of aircraft and to safeguard the pavement’s integrity. However, there are also some flaws within the system.
The Dutch CROW has performed a State-of-the-Art study on PCN Runway Strength Rating and Load Control Systems. The objective of the survey was to clarify on ICAO’s ACN/PCN reporting system for civil airports, and to investigate PCN assessment methods currently utilised by member states. The study did lead to a number of recommendations which can result in a standard method for PCN assessment based on technical evaluation for usage within the Netherlands and/or for NATO practice. The technical report 04-09 can be downloaded from the CROW website.
As a continuation of the literature survey, CROW prepared a ‘Guideline for PCN assignment’ for usage on civil airports in the Netherlands 2013. The Guideline can be used to evaluate rigid and flexible pavement for PCN. CROW-report D13-02 provides guidance on the PCN assignment for reporting the bearing strength of airport pavements using the ACN-PCN system. The Appendices i the report present a field survey for flexible and rigid pavements comprising of deflection measurements, coring and material testing.
It must be mentioned that CROW does not endorse the use of certain software. However, CROW has selected Pavers ® as the preferred program to use for the PCN assignment of flexible and rigid pavements. The Guideline can be downloaded from the CROW-website.
The North Atlantic Treaty Organisation (NATO) decided in 2001 to define their military requirements for airfield pavement strength in terms of the Aircraft Classification Number (ACN), following the ICAO international system of load bearing strength reporting. Currently, NATO is in the transition from the LCN/LCG system to the ACN-PCN system. NATO wishes to determine in a reasonably short time a single methodology for establishing PCN values. The methodology is subject to discussion within the NATO Airfield Services Working Group (ASWG). NATO nations oppose the underlying design ICAO methodology for determining ACN, as suitable for extension for the determination of PCN. The ASWG concluded in their 2003 meeting that a ‘Layered Mechanistic Approach’ is preferred over Emperical Methods.