Aircraft Classification Number (ACN)
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 (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 (=.09 ton/ft2). 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).
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.
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%).
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 to 120 MN/m3).
C. Low Strength – k = 40 MN/m3 (150 lb/in3) (For values between 25 to 60 MN/m3).
D. Ultra Low Strength – k = 20 MN/m3 (75 lb/in3) (All k values below 25 MN/m3).
Pavement Classification Number (PCN)
The Pavement Classification Number (PCN) is an International Civil Aviation Organization standard used in combination with the Aircraft Classification Number (ACN) to indicate the strength of a runway, taxiway or airport ramp (or apron). This helps to ensure that the airport ramp is not subjected to excessive wear and tear, thus prolonging its life.
Although important for the runway the major use of this number is for the apron. On landing the aircraft is light on fuel and usually less than 5% of the weight of the aircraft touches the runway in one go. On takeoff the aircraft is heavy but as the aircraft accelerates the weight gradually moves from the wheels to the wings. It is while the aircraft is being loaded and taxiing prior to departure, that the apron experiences significant loads from aircraft weight.
Typically this is only used for asphalt or concrete runways and would not be used for grass or gravel.
The PCN is actually expressed as a five part code, separated by forward-slashes, describing the piece of pavement concerned.
The first part is a numeric value expressing the actual assessed strength of the pavement. The second part is a letter: either an R or an F, depending on whether the pavement itself is of a rigid (concrete) or a flexible (asphalt) design.
The third part is another letter from A to D expressing the strength of what is underneath the pavement, known as the subgrade. So a subgrade of A would be very strong, most likely a reinforced concrete subbase. A subgrade of D would be very weak, most likely uncompacted soil.
The fourth part is either a letter, or a number with units expressing the maximum tire pressure that the pavement can support. In terms of letters, W is the highest, indicating that the pavement can support tires of any pressure, the others are as follows:
Pavement Class ———— Maximum Tire Pressure
W—————————– No Limit
X —————————– 1.5 MPa (217 psi)
Y —————————– 1.0 MPa (145 psi)
Z —————————– 0.5 MPa (72 psi)
The fifth and final part just describes how the first value was worked out, a T indicates technical evaluation, and a U indicates usage — a physical testing regime.
So a PCN of 80/R/B/W/T means that the underlying (probably concrete) has a bearing strength of 80, is rigid, it’s on a medium subgrade, has no limit on tire pressure, and this has been calculated through technical evaluation.