The Aircraft Classification Number
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.
ACNs are calculated using prescribed technical method of ICAO
Under the ACN-PCN system, each aircraft has assigned an ACN 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 130 or more for heavy aircraft. ACN values are published for both flexible and rigid pavements and at four (4) subgrade categories that span the range of subgrade and bearing support values normally encountered. The ranges of subgrade strength covered by these standard subgrade categories (designated as A, B, C and D) are shown below.
ACNs must be calculated using a prescribed technical method, which is clearly defined. An aircraft’s ACN is calculated from its weight, its wheel layout, its tire pressure, and to the ICAO strength category. ACN’s are calculated according to the computer listings given in the ICAO Airport Pavement Design Manual.
For flexible pavements, the S-77-1 method is utilized. Likewise, rigid pavement ACN is computed by a subset of the PCA pavement computer program, except the variable term of working stress is replaced by a constant standard value. To obtain a classification number for a multi-wheel undercarriage an ESWL has to be calculated and to do this, a pavement thickness must be defined. The ACN-PCN method defines the thickness, known within the method as ’Reference thickness‘ by designing a realistic pavement on a given subgrade. The reference thickness is obtained by using specific design methods for flexible and rigid pavements.
The ICAO "Aerodrome Design Manual - Part 3 - Appendix 2" contains computer programs (source code) for the calculation of ICAO ACN's for aircraft operation on both rigid and flexible pavements. The ICAO ACN Fortran source code has been rewritten and recompiled by Transport Canada into two (2) executable stand-alone programs. The original input/output formats of the ICAO ACN programs were followed as closely as possible. The internal program calculations and equations of the ICAO programs were also followed and incorporated in the new .exe files. The result is the computation of aircraft ACN values that are fully compatible with the ICAO ACN/PCN strength reporting system for airfield pavements. The programs are available at specialist websites.
ICAO has approved on October 16, 2007 the changing of the numerical coefficients entitled "Alpha factor" involved for flexible pavement in calculating the ACN of an aircraft. In the ACN of flexible pavement, the alpha factor or pavement reduction factor, is used to take into account the effect of gear geometry in computing pavement design curves for flexible pavements. The original deflection-based design method overstated the damage caused by multi-wheel gears and this was later corrected by introducing the alpha factor. Studies undertaken in some States had indicated that the classical pavement design curves for dual-tandem landing gears were extrapolated for main landing gears with six wheels. Additionally, there was a lack of data points on the original alpha factor curve and any extrapolation results in a higher value of the alpha factor, thus predicting higher stresses in the pavement, whereas the additional set of wheels were intended to distribute the load better. In order to resolve this technical anomaly, some States decided to carry out full scale pavement tests. In the meantime, an interim alpha factor of 0.72 for six wheels per landing gear would be used by aircraft manufacturers, until such time the results of the test were available. Recent findings of full scale pavement tests conducted by one State had been reviewed by the Aerodromes Panel. The Commission agreed to the following revisions concerning the alpha factor:
- Change alpha factor value for all four wheels per main landing gear from the current 0.825 to 0.80;
- Retain the alpha factor value for six-wheels per landing gear at 0.72; and
- Change the alpha factors for other main landing gears so that the ranking of the damaging effects remains consistent.
The revised Alpha Factors generally result in lower ACNs. Hence, if the PCN is based on four wheel aircraft, the PCN’s reported after 2007 are slightly lower than those reported before October 16, 2007. The confusion of differences between ‘Old’ and ‘New’ ACN and PCN numbers was the prime reason to adopt the new ACR-PCR system. Its effective date is July 2020 and must be full integrated by November 2024. During transition, both systems will remain available. An ACR and PCR are approximately 10 times that of an ACN and PCN.