## 1 Introduction  
Since their first introduction in the early 1900s, aircraft have become larger and heavier. Particularly since WWII, aircraft wheel loads and tire pressures have increased significantly [1, 2] and this requires ever-stronger pavements to be provided by airports. In 1958, the USA Federal Aviation Administration (FAA) of the United States of America (USA) implemented a policy to restrict the development of new aircraft that stressed pavements more than a DCS-50 aircraft operating at 159 t. In 2009, the FAA pavement protection policy was withdrawn, effectively allowing more demanding aircraft to be developed without restriction [3]. Consequently, the provision of airport pavements capable of accommodating any more demanding future aircraft became the responsibility of airport owners. The trend of increasing aircraft tire pressures and individual wheel loads has continued since that time and is not expected to abate in the future [4]. To assist airport owners and operators in controlling and managing the impact of different aircraft on their pavements, the International Civil Aviation Organization (ICAO) introduced an internationally recognized system for airport pavement strength rating in 1981 [5]. The current system is known as Aircraft Classification Number-Pavement Classification Number (ACN-PCN). As explained in detail below, each aircraft, at a specific weight and tire pressure, has a mathematically exact ACN value, depending on the type of pavement and level of subgrade support. The ACN is compared to the PCN value assigned to a specific runway pavement, which represents the bearing strength of the pavement. The PCN is more subjective and can be assigned using a technical or rational basis or via performance-based experience or usage. When the ACN is equal to or below the PCN, the aircraft can operate in an unrestricted manner. However, when the ACN exceeds the PCN, permission must be sought and granted from the airport operator before that aircraft can operate, referred to as a pavement concession or an overload operation. When ACN-PCN was developed in the 1970s, pavement design processes were based on simple mathematics, such as a standard pavement composition, [PERSON]'s single layer theory of stress distribution within a pavement, the equivalent aircraft concept, where other aircraft are converted to an equivalent number of the critical aircraft, and the CBR method of pavement thickness determination [6]. Consequently, ACN-PCN was also based on the same simple mathematics. Since that time, pavement design methods have developed, with modern aircraft pavement design methods routinely based on layered elastic analysis and more sophisticated methods for combing the effect of different aircraft, known as the cumulative damage factor, which relies on Miner's law of damage superposition [7]. Some pavement design methods, including rigid pavement design using the software known as FAAFRIELD [8], also use finite element methods [9]. Despite this, ACN-PCN has remained largely as it was when it was originally developed, with only minor changes over time [7]. Some countries provide only limited guidance on the calculation of a PCN for any given runway. For example, Australian regulations only require a runway to be strong enough for the aircraft that operate on it [10]. This largely reflects the focus on the safety of many modern airport regulators, the fact that pavement strength is rarely the cause of a safety-related issue, and the general transition from state ownership of airports to a combination of private airport ownership of major airports and local government management of regional airports [11]. In contrast, some countries mandate specific protocols for PCN calculation, which is more appropriate in locations where the same regulator also has a prescribed pavement design and specification system in place and provides a significant portion of the funding for new pavement development and existing pavement rehabilitation. The PCN calculation protocol published by the FAA is a good example [12].