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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 apron (or ramp). 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 the PCN numerical value, indicating the load-carrying capacity of the pavement. This is always reported as a whole number, rounded from the determined capacity. The value is calculated based on a number of factors, such as aircraft geometry and a pavement's traffic patterns, and is not necessarily the direct bearing 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 (most typically concrete) or a flexible (most typically asphalt) design.
The third part is another letter from A to D expressing the strength of what is underneath the pavement section, known as the subgrade. So a subgrade of A would be very strong, like concrete-stabilized clay. A subgrade of D would be very weak, like uncompacted soil.
Flexible pavements The flexible pavements have four subgrade categories as given below:
|HIGH||A||CBR 15 (All CBR above 13%)|
|Medium||B||CBR 10 (For CBR between 8% to 13%)|
|Low Strength||C||CBR 6 (For. CBR between 4% to 8%)|
|Ultra Low||D||CBR 3 (For CBR below 4%)|
Rigid pavements The rigid pavements have four subgrade categories as given below:
|HIGH||A||k = 150 MN/m3 (550 lb/in3) (All k values above 120 MN/m3)|
|Medium||B||k = 80 MN/m3 (300 lb/in3) (For values between 60 to 120 MN/m3)|
|Low Strength||C||k = 40 MN/m3 (150 lb/in3) (For values between 25 to 60 MN/m3)|
|Ultra Low||D||k = 20 MN/m3 (75 lb/in3) (All k values below 25 MN/m3)|
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. Concrete surfaces can support tire pressures greater than those of existing commercial aircraft and are therefore nearly always rated W. Other letter classifications are as follows:
|Category||Pavement class||Maximum tire pressure|
|Unlimited||W||No Pressure Limit|
|High||X||1.75 MPa (254 psi)|
|Medium||Y||1.25 MPa (181 psi)|
|Low||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 pavement's value indicating load-carrying capacity is 80 (unitless), is rigid (and thus is likely concrete), is on a medium-strength subgrade, has no limit on tire pressure, and has been calculated through technical evaluation.