## 4 Potential Improvements to ACR-PCR
### Light Aircraft Provision  
The strength rating of pavements for light aircraft is generally not an issue for rigid pavements, because even the minimum practically constructable concrete thickness is likely to be adequate for all light aircraft. However, it is an issue for flexible pavements, which are far more common for airports that cater to light aircraft, either for flight training or in remote and regional areas [7]. The ACR-PCN method of pavement strength rating applies to pavements intended to support aircraft exceeding 5700 kg in total weight. That is, it does not apply to aircraft up to 5.7 tones [5]. Pavements for the lighter aircraft are instead rated using a simple weight limit, usually 5700 kg, or are considered to be unrated. The reason for this is that most aircraft with a total weight of up to 5700 kg are supported by single main gear wheels. Recalling that the different subgrade categories are primarily to reflect the different interaction between the multiple wheels on larger aircraft, the pavement thickness, and therefore the subgrade category, is not important for single wheeled aircraft, because single wheels can not interact with other wheels. Consequently, for single-wheel aircraft, a total aircraft mass is just as effective as a more complex ACN (or ACR) in representing the relative amount of pavement damage caused by the aircraft. The first challenge with this system, which is not changing under ACR-PCR, is the potential for confusion between a 5700 kg rating, a pavement that is unrated, and a pavement that leaves the element of the aerodrome information publication blank. A review of smaller airport entries from the Australian aerodrome information publication [50] shows examples of all three of these entries. There are subtle differences between a blank space where the rating should be, a declaration of unrated, and a 5700 kg rating. However, in practice, these all have the same effective outcome, which is a default 5700 kg limitation. The publication of the strength rating for these light aircraft pavements should be made clear, so there is consistency and clarity. The second challenge for these light pavements is that the FAA's FAAFRIELD [8] software, which is the most accessible and most commonly used airport pavement thickness method in the world, does not cater to pavements intended only for these light aircraft. The simplest and thinnest flexible pavement that can be considered in thickness design mode in FAAFRIELD is 102 mm of P-401 (asphal concrete) over 152 mm of P-209 (crushed rock base). For a standard 20-year design life and a low 1200 annual departures, the weight of a single-wheeled aircraft that can be supported by this minimum pavement is 7.4 tones up to 26 tones, depending on the subgrade bearing strength, as detailed in Table 6. Consequently, there is a gap between the lowest-strength pavements that can be designed using FAAFRIELD and pavements that are considered to be too low in strength for the ACR-PCR method to apply. Furthermore, the gap is larger for pavements with higher levels of subgrade bearing capacity, because the minimum pavement thickness can support a much larger aircraft when the subgrade support is high. As an example of this challenge, for a typical subgrade with modulus 80 MPa (CBR 8), the minimum 256 mm thick flexible FAAFRIELD pavement can support single-wheel aircraft up to 9.9 tones. There is no effective way to design and rate a pavement, using FAAFRIELD, for an aircraft between 5700 kg and 9900 kg. Similarly, for a 200 MPa subgrade (CBR 20), this gap increases to between 5700 kg and 26,000 kg. There are many aircraft that fall into this range, including Cessna Citation V, Shorts 360, BeechJet-400, and military jets such as the F-16 and F/A-18.