Introduction
In the context of the continuously evolving landscape of aviation, evaluating and maintaining up-to-date and efficient runway pavement systems at international airports is of paramount importance. This article delves deep into the transition from the ACN-PCN to the ACR-PCR system for airport pavement strength rating, a new system that optimizes evaluation of pavement strengths.
ACN-PCN SYSTEM OVERVIEW: The Precursor to the Latest Reporting System
Historical Context
Since 1981, the Aircraft Classification Number-Pavement Classification Number (ACN-PCN) system has been the standard for the evaluation of aircraft pavement. The new ACR-PCR method, a revised version of the ACN-PCN, is growing in acceptance and global implementation under the endorsement of the International Civil Aviation Organization (ICAO).
Mathematical Foundations
In the ACN-PCN system, the Aircraft Classification Number (ACN) serves as a mathematically precise value for the evaluation of aircraft pavement. With the new ACR-PCR system, the ACR is calculated taking into account the aircraft weight, tire pressure, and the subgrade bearing capacity of the airfield pavement. The ACN serves as an indicator of the aircraft’s impact on the pavement subgrade and is subsequently compared to the Pavement Classification Number (PCN) assigned to the pavement.
PCN Calculations
Unlike ACN, the PCN is not strictly mathematical; it’s influenced by various factors and may involve discretion on the part of the airport owner. The PCN is generally determined using Federal Aviation Administration FAA guidelines, which include:
Equivalent Annual Departures: Using the concept of equivalent annual departures, traffic mixtures are consolidated into a single representative aircraft.
Cumulative Damage Factor (CDF): This factor is crucial in determining PCN. Cumulative Damage Factor (CDF): This factor carries significant weight in the PCR calculation within the new ACR-PCR system. This measurement considers the combined effect of all aircraft types and their frequencies, computed for the chosen representative aircraft.
Material Equivalent Factors: When the pavement composition is non-standard in an aerodrome, material equivalent factors are used to convert it to a standard thickness for PCN determination under the new rating – pavement classification rating system.
It’s worth noting that, since the conversion to the new system in 2020, challenges have arisen due to inconsistencies between the mathematical models used for pavement design and those used for PCN calculations within the latest reporting system. These challenges, associated with the previous system, form the foundation for the transition towards the new ACR-PCR method in the industry.
Functional Mechanism
The rule is simple: if the ACN is equal to or less than the PCN, the aircraft can operate on that pavement without restrictions. If the ACN is higher, the aircraft may be excluded, or may be allowed to operate subject to weight and/or frequency limitations.
Shortcomings and Inconsistencies
However, the ACN-PCN system has limitations, particularly when it comes to the inconsistency between the models used for ACN calculation and those used for pavement design. This has led to situations where a pavement designed for a specific aircraft is assigned a PCN that restricts that aircraft’s operation—a clear paradox.
WHY THE CHANGE TO ACR-PCR IS NECESSARY
Addressing the Limitations
The ACR-PCR system is designed to rectify the limitations of the ACN-PCN method. It introduces:
- Pavement Usage Optimization: Allows for more efficient use of airport pavements.
- Consistency: Ensures that the models used for pavement design and strength rating are aligned.
- Improved Lifespan Predictability: Offers better forecasts of when pavement will need to be replaced or repaired.
The table below shows the main inconsistencies between the ACN/PCN System and the Airport Pavement Design Methods
| Subject | ACN/PCN System | Airport Pavement Design Methods |
| Consideration of Landing Gears | Equivalencies to single wheels (ESWL, alpha factors) |
All wheels are considered for the pavement response computation |
| System Rationale | Empirical method that does not consider the actual mechanical response of the pavement | Mechanistic-empirical The FAA developed FAARFIELD using failure models based on full-scale tests. Layered elastic and three-dimensional finite element-based structural analysis for flexible and rigid airfield pavements respectively. |
| Methodology | Based on the CBR design procedure 1983 | Gradually moving to rational methods based on the Linear Elastic Analysis (LEA*) |
| Pavement Material Characteristics | Equivalencies to a standard material | Pavement material characteristics considered explicitly for the pavement response computation (E- modulus & Poisson’s ratio) |
Technological Advancements
One of the significant advancements is the use of layered elastic methods to calculate “critical strain,” which serves as a more accurate indicator of pavement damage compared to the older deflection models.
Regulatory Changes
Approved by ICAO in 2019, the ACR-PCR system will become mandatory in September, 2024. This means that all ICAO member states must transition by this date.
UPCOMING FAA DEADLINE: SEPTEMBER 30, 2024
The FAA has emphasized the importance of this transition by setting a deadline. Non-compliance could lead to penalties and operational difficulties for 14 CFR Part 139 certificated airports. The deadline is not merely a bureaucratic requirement but is rooted in ensuring the safety and operational efficiency of airfields. Guidelines for the ACR_PCR method and deadline information are outlined in this FAA document, available here.
DETAILED OVERVIEW OF ACR-PCR
PCR Concept
Similar to the PCN, the PCR represents the pavement structural bearing strength (on the ACR scale) for unrestricted operations.
Fundamental Changes
- Strain over Deflection: ACR-PCR uses strain as the damage indicator, making it more accurate.
- Individual Wheel Consideration: The system takes into account each wheel, avoiding approximations.
- Material Specificity: Real-world materials are considered in calculations, enhancing accuracy.
System Methodology and Definitions
- ACR: This is a relative indicator of the aircraft’s effect on a given pavement.
- PCR: Indicates the load-carrying capacity of a pavement without specifying a particular aircraft.
DETERMINATION OF NUMERICAL PCR VALUE
Two Approaches
- Using Aircraft Method: A simpler approach that uses historical aircraft data.
- Technical Evaluation Method: A more rigorous method requiring in-depth engineering evaluation.
Steps for PCR Determination: Technical Evaluation Method
The method involves:
- Data Collection: Gathering all relevant pavement data.
- Aircraft Mix Definition: Identifying the type and number of aircraft expected to use the pavement.
- ACR Computation: Calculating the ACR for each aircraft type.
- CDF Calculation: Computing the maximum Cumulative Damage Factor.
- Critical Aircraft Selection: Adjusting the annual departures to match the maximum CDF.
- Weight Adjustment: Adjusting the critical aircraft weight to obtain a CDF of 1.0.
- Final PCR Computation: The PCR value is derived from the ACR of the critical aircraft.
- Reporting: The final PCR value is reported, along with the critical aircraft. The PCR should be published in the Aeronautical Information Publication (AIP) according to the format defined in ICAO Annex 14 (§2.6.6)
Consequences of PCR Inaccuracies
Under-estimated PCR (Overestimated CDF):
Aircraft weight/annual departure restriction, loss of airport revenues, under use of pavement
Over-estimated PCR (Underestimated CDF):
More traffic acceptance (weight/volume) than what the pavement can endure over its design life.
Premature pavement damage, increase of maintenance, repairs, and rehabilitation costs.
Is your airport equipped for the ACR-PCR transition?
The shift to the ACR-PCR system presents both challenges and opportunities for airports. While it offers a chance to optimize pavement use, inspection, and maintenance, the transition can be difficult for airports lacking historical data on existing pavement composition and subgrade strength. A thorough grasp of the ACR-PCR requirements is essential for a smooth transition
The Importance of Field Testing
An accurate PCR determination often requires rigorous field testing. Methods such as Heavy Weight Deflectometer (HWD) testing provide invaluable data for assessing the subgrade’s properties. At QES, we offer state-of-the-art HWD testing services using our Dynatest 8082 HWD machine.
Summary and Conclusion
Transitioning from PCN to PCR system is a significant step forward in aligning pavement design methodologies with strength ratings. With the looming FAA deadline, understanding this transition’s intricacies is vital. The ACR-PCR system stands to benefit operators of airports worldwide, airlines, and the broader aviation community by providing a more rational, consistent, and technically sound approach to pavement classification.
For further consultation or inquiries, feel free to contact QES, your trusted partner in airport pavement engineering solutions.