Aircraft classification number

The aircraft classification number (ACN) is a number expressing the relative effect of an aircraft on the runway pavement for a specified standard subgrade category, using a method defined by the International Civil Aviation Organization (ICAO).

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.

Using the ACN 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 strength, without specifying a particular pavement thickness.

The ACN is twice the derived single-wheel load expressed in thousands of kilograms, with single-wheel tire pressure standardized at 1.25 megapascals (= 181 psi). Additionally, the derived single-wheel load is a function of the sub-grade strength.

The ACN of an airplane is a function of not only its weight but also the design parameters of its landing gear such as the distances between the wheels of a multiple-wheel landing gear assembly.

The pavement’s strength is denoted by its pavement classification number (PCN).

The load exerted on a pavement by the landing gear of an airplane is denoted as its ACN, or airplane classification number. The ACN is not permitted to exceed the PCN of the runway to be used, in order to prolong pavement life and prevent possible pavement damage.

The ACN is defined for only four subgrade categories (high, medium, low, and ultra low).

The ACN-PCN method

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 5700 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 worldwide 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).

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 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 PCN with an 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.

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 maximum extent 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

ACN reporting

ACN values for selected aircraft have been calculated by the International Civil Aviation Organization (ICAO) using two computer programs, one for rigid pavements and the other for flexible pavements.

Manufacturers are required to calculate ACNs for new aircraft as they come into service and publish the results in flight manuals. The tables give ACN values for two weights, one at the maximum total weight authorized and the other at the operating weight when empty. If an aircraft is operating at an intermediate weight, the ACN value can be calculated by a linear variation between the limits. Extrapolation is not permissible. ACN is calculated with respect to the center of gravity position, which yields the critical loading on the critical gear. Normally, the aftmost center of gravity, or CG position, appropriate to the maximum gross apron (ramp) mass, or ramp weight, is used to calculate the ACN. In exceptional cases, the forwardmost CG position may make the nose-landing gear loading more critical.

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 http://www.tc.gc.ca/CivilAviation/.../software.htm

The FAA also has a large amount of guidance material available on their website. Advisory Circular AC 150/5335-5B provides further guidance on ACN and PCN calculations and the relationship between the two numbers. The FAA also provides a more user friendly version of the ICAO computer programs, although that code has been translated from the original Fortran language to Visual Basic. The FAA software is COMFAA and is available from their software download page at: http://www.airporttech.tc.faa.gov/naptf/download/index1.asp#soft

Subgrade support strength category

The ranges of subgrade strength covered by these standard subgrade categories (designated as A, B, C and D) are shown below.

Flexible pavements

The flexible pavements have four subgrade categories:

• A. High Strength – CBR 15 (All CBR above 13%).
• B. Medium Strength – CBR 10 (For CBR between 8% to 13%).
• C. Low Strength – CBR 6 (For. CBR between 4% to 8%).
• D. Ultra Low Strength – CBR 3 (For CBR below 4%).

Rigid pavements

The rigid pavements have four subgrade categories:

• A. High Strength – Subgrade k = 150 MN/m³ (550 lb/in³) (All k values above 120 MN/m³).
• B. Medium Strength – k = 80 MN/m³ (300 lb/in³) (For values between 60 and 120 MN/m³).
• C. Low Strength – k = 40 MN/m³ (150 lb/in³) (For values between 25 and 60 MN/m³).
• D. Ultra Low Strength – k = 20 MN/m³ (75 lb/in³) (All k values below 25 MN/m³).

Updates

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.

Aircraft ACN list