## 3 Review of the ACN-PCN and ACR-PCR Systems
### Existing Strength Rating System
This change allowed an increase in aircraft tire pressure from 1.50 MPa to 1.75 MPa to operate on airports with a Category X tire pressure limit. This affected over 40% of runways around the world at that  
Figure 1: Example of an ACN chart for the B737-800. time [32] and was supported by only minimal physical testing for relative rut resistance, at an outdoor test track in southern France during winter [33]. Consequently, the justification for these changes has been questioned [34] but they are now well-established and accepted as part of the ACN-PCN system. Some ICAO member states, such as Australia, prefer to publish an actual aircraft tire pressure limit rather than a category [35] and this reflects the common use of chip seals [36] for runway surfacing in regional parts of Australia [37]. The full PCN description for any given runway is a multi-element expression, such as the example in Equation (1). The main element is the number against which the ACN is compared. It is intended to protect the pavement structure from overload, primarily based on permanent subgrade deformation, also known as pavement rutting. The tire pressure limit is secondary, is compared to the operating tire pressure of the aircraft, and is intended to protect the surface from high near-surface stress. For an aircraft to operate in an unrestricted manner, the ACN must not exceed the PCN and the aircraft tire pressure must not exceed the tire pressure category or limit. When operating in an unrestricted manner is not permitted, either because of a higher ACN number or a higher tire pressure, a pavement concession must be sought by the aircraft operator and granted (or denied) by the airport owner or manager. PCN 58/F/A/X/T (1)  
where:  
58 is the numerical element against which the ACN is compared. F is to indicate a Flexible pavement, rather than R for Rigid. B is the category of subgrade category detailed above. X is the tire pressure limit category (Table 1). T is to indicate a technical assessment rather than a U for a usage-based assessment. Unlike the ACN of an aircraft, the PCN of a runway is more open to interpretation. The optional usage-based assignment of the PCN effectively allows an airport to assign any PCN desired to any given runway. If an over-rating results in accelerated pavement damage, the airport is responsible for rectifying that. In contrast, in the USA, the FAA requires the use of a standardized and prescriptive method of PCN determination, based on software known as COMFAA [13]. This ensures that all airports in the USA have a consistent approach to their aircraft pavement strength rating and minimizes the risk of accelerated pavement deterioration. The ACN-PCN system generally applies to pavements supporting critical aircraft with a mass of 5700 kg and above. For pavements that are designed for aircraft below that weight, a 5700 kg or lower total aircraft mass restriction is generally applied. That reflects the fact that most aircraft with a weight of 5700 kg or below have only a single wheel in each of the two main aircraft gears. For single-wheeled aircraft, the depth of pavement makes no difference to the relative effect of the aircraft because there are no other wheels for that single wheel to interact with. Consequently, for aircraft with a single main gear wheel, there is no practical benefit of determining an ACN, compared to a simple aircraft weight limit. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Original Tire Pressure Limits** & **Revised Tire Pressure Limits** \\ \hline W & Unlimited & Unlimited \\ \hline X & 1.50 MPa & 1.75 MPa \\ \hline Y & 1.10 MPa & 1.25 MPa \\ \hline Z & 0.50 MPa & 0.50 MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Tire pressure category limits.