## 3 Review of the ACN-PCN and ACR-PCR Systems  
### Existing Strength Rating System  
The current ACN-PCN system was developed in the late 1970s and implemented in 1981. Under the current system, aircraft loads are expressed by an ACN, which allows no discretion. The ACN is defined as twice the wheel load (in tones), which, on a single wheel, inflated to 1.25 MPa, causes vertical pavement deflection (calculated at the top of the subgrade) equal to that caused by the actual multi-wheel aircraft gear at its actual gear load and its actual tire pressure [25]. The more wheels in the wheel load group, which is usually a single main gear leg or strut on an aircraft, the more load the wheel group can carry before causing a particular amount of deflection at the subgrade, in a similar manner to multiple axles gear of large trucks being associated with a higher axle group load limit. Importantly, aircraft wheels interact and cause different amounts of relative damage differently for flexible and rigid pavement structures. This reflects the different ways in which the two pavement types resist and accommodate aircraft loads, with flexible pavements spreading the load and dissipating the resulting stress with increasing depth, while rigid pavements bend to resist the load internally, with the highest stress usually occurring at the joints [30]. Because of the different relative effects of different aircraft on these two pavement types, each aircraft has a different ACN for a rigid pavement than it does for a flexible pavement [7]. The interaction between the multiple wheels on an aircraft landing gear also changes with pavement depth. This means that two aircraft with different landing gear configurations but the same ACN for a particular subgrade category will cause relatively different damage to pavements with different thicknesses, where the wheels interact less (thinner pavement) or more (thicker pavement). Furthermore, pavement thickness is significantly affected by subgrade bearing capacity, commonly expressed as the California Bearing Ratio (CBR). Consequently, the application of ACN-PCN therefore changes with subgrade bearing capacity, as a proxy for the pavement thickness, to reflect the degree of wheel interaction at the critical top of subgrade depth below the pavement surface. In the 1970s, when ACN-PCN was developed, the practical calculation of ACN was usually performed using charts, with the subgrade bearing capacity and aircraft weight varying, but for a fixed tire pressure, as shown in the example in Figure 1. Rather than a continually varying ACN across all possible CBR values, subgrades were categorized and a representative CBR value was adopted for each category [25]:  
* Category A. High strength. Represented as CBR 15;
* Category B. Medium strength. Represented by CBR 10;
* Category C. Low strength. Represented by CBR 6;
* Category D. Ultra-low strength. Represented by CBR 3. These subgrade categories remain in the current ACN-PCN system today. The representative values generally reflect the range of CBR values that are generally adopted for pavement thickness determination, with values below CBR 3 generally being improved prior to pavement construction and higher strength subgrades generally categorized with a capped value of CBR 15 (Australia) or CBR 20 of fine-grained soils and CBR 33 for gravelly subgrades (USA) [30]. Similar to the subgrade category simplification, the ACN-PCN system also categorized the tire pressure limits. The limits were (and remain) inherently empirical, in that there is no rational or design-based approach for assessing the allowable tire pressure for any given pavement surface, nor for quantifying the risk of exceeding that limit. In 2008, increases in the tire pressure limits were proposed by aircraft manufacturers [3]. In 2013, the proposed increase in the categorical tire pressure limits of the ACN-PCN system was approved [31]. Table 1 provides the original and revised tire pressure limits.