## 4 Potential Improvements to ACR-PCR
### Number of Subgrade Categories  
Further to the above discussion regarding the subgrade categories, limits, and representative values, the actual need for subgrade categories is also questioned. As stated above, the representation of the various subgrade categories each by single modulus or CBR value creates the potential for errors in the degree of represented wheel interaction at  
Figure 5: Pavement thickness for A350-900 on different subgrade supports. depth. Given that one of the aims of the transition from ACN-PCN to ACR-PCR was to avoid anomalous differences between design and strength rating, such differences should have been avoided whenever practically possible. In the case of subgrade bearing capacity, the category-based approach was necessary when ACN-PCN was developed in the late 1970s. That is because, at that time, the mathematics required to calculate the interaction between aircraft wheels on a landing gear was performed manually and was time-consuming. Consequently, most ACN values were calculated by a formula or from a simple graph, such as the example in Figure 1. Similarly, routine pavement design was performed using charts or nomographs published by local authorities, such as the FAA [6]. In fact, it was not until 2002 that the FAA made the software known as COMFAA [13] available for ACN calculation, along with LEDFAA in 2014 for pavement thickness determination [45]. Because of the different degrees of wheel interaction for pavements of different thicknesses, multiple formulae of charts were required for different degrees of subgrade support. To simplify the publication of formulae or charts for routine ACN calculation, the four subgrade categories were established to span the range of reasonable subgrade bearing capacity levels expected in common pavement design methods. In contrast, contemporary pavement thickness determination is predominantly performed in software, with APSDS [46], Alize-Airport [47], and FAARFIELD [8] being common examples. This software generally uses subgrade modulus values for subgrade support characterization, generally rounded to the nearest 10 MPa, equivalent to a whole CBR value. Once this subgrade elastic modulus used for design purposes is known, it could easily be entered into software for ACR calculation. The software known as ICAO-ACR [48] has already been made available by the FAA for the calculation of ACR values. Once the aircraft and pavement are selected, the software generates an ACR value for each of the four subgrade categories almost instantly. However, it would be just as reasonable to calculate ACR values for any other subgrade modulus, from the equivalent of CBR 3 to CBR 15, with lower and higher CBR-equivalent values combined with the minimum and maximum values, respectively. Alternatively, the user could enter their subgrade modulus and ICAO-ACR could generate an ACR value for a rigid and flexible pavement at that subgrade modulus value only. Either way, the calculation time would remain almost instant because the time-consuming manual calculations required in the 1970s are now performed so quickly by the software. By increasing the precision of the subgrade support characteristic, from a broad category to the actual modulus of CBR value, anomalies resulting from differences in the aircraft wheel interaction, due to the different depths of the subgrade under the different pavement thicknesses required, would be further reduced. Given that reducing such anomalies was one aim of the transition from ACN-PCN to ACR-PCR, it would be advantageous if it is practically implementable. Contemporary computer power and the cessation of the need to manually determine wheel interactions at depth with laborious manual mathematics enables ACN (and ACR) values to be determined almost instantly. Consequently, there is no barrier to adopting actual subgrade support modulus or CBR value, which should be known from pavement thickness determination anyway.