Pavement classification number
The pavement classification number is an International Civil Aviation Organization standard used in combination with the aircraft classification number to indicate the strength of a runway, taxiway or airport apron. This helps to ensure that the runways etc. are not subjected to excessive wear and tear, thus prolonging their usable life and promoting safe aircraft operations.
How it works
The PCN is 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 or a flexible 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-stabilised clay. A subgrade of D would be very weak, like uncompacted soil.
Flexible pavements
The flexible pavements have four subgrade categories:| Category | Strength | Value | Range of California bearing ratio Values |
| High | A | CBR 15 | Above 13% |
| Medium | B | CBR 10 | Between 8% and 13% |
| Low Strength | C | CBR 6 | Between 4% and 8% |
| Ultra Low | D | CBR 3 | Below 4% |
Rigid pavements
The rigid pavements have four subgrade categories:| Category | Strength | Value | Range of k values |
| High | A | 150 MN/m² | Above 120 MN/m² |
| Medium | B | 80 MN/m² | Between 60 and 120 MN/m² |
| Low Strength | C | 40 MN/m² | Between 25 and 60 MN/m² |
| Ultra Low | D | 20 MN/m² | Below 25 MN/m² |
Tire pressure
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 | 1.5 MPa |
| Medium | Y | 1.25 MPa |
| Low | Z | 0.5 MPa |
The fifth and final part describes how the first value was worked out, a T indicating a technical evaluation, and a U indicating usage – a physical testing regime.