The ACR-PCR system

The use of the standardized method of reporting pavement strength applies only to pavements at public use airports with bearing strengths of 12,500 pounds (5,700 kg) or greater. The method of reporting pavement strength for pavements of less than 12,500 pounds (5,700 kg) is to only report the gross weight and gear configuration of the aircraft that can be accommodated.

Harmonizing pavement load load-carrying capacity is required to optimize the use of aircraft and to safeguard the pavement’s integrity. The ACR-PCR system is structured so a pavement with a particular PCR value can support an aircraft that has an ACR value equal to or less than the pavement’s PCR value. This is possible because ACR represents a scale for relative pavement damage and PCR values are computed using the States technical method for structural pavement design.

The ACR-PCR (Aircraft Classification Rating – Pavement Classification Rating) system for rating airport pavements will replace the ACN-PCN method.

  • Proposed amendment to Annex 14 on ACR-PCR has gone through State review with no objections.
  • After formal adoption by the ICAO Air Navigation Committee, the ANC agreed on a four-year transition period from ACN-PCN to ACR-PCR:
    • Effective date July 2020
    • Full applicability November 2024
    • During transition, both systems will remain available
  • An updated ADM Part 3 with the new procedures has been published .
  • Training on ACR-PCR is planned

Definition of ACR

ACR is a number that expresses the relative effect of an aircraft at a given configuration on a pavement structure for a specified standard subgrade strength. The aircraft manufacturer provides the official computation of an ACR value. For rigid and flexible pavements, the aircraft landing gear support requirements are determined by the layer elastic method (LEA) for each subgrade support category. An ACR is approximately 10 times that of an ACN.

Definition of PCR

PCR is a number that expresses the load-carrying capacity of a pavement for unrestricted operations. The PCR is assessed and assigned by the airport using the ICAO States computational method. An PCR can be based on LEA for flexible pavement or advanced Westergaard equations for rigid pavement.

The ACR/PCR method is strict towards the ACR procedure, but allows the airport to choose for any method suit-able for the determination of the pavement load bearing capacity or PCR. However, ICAO’s Aerodrome Design Manual, Doc. 9157-AN/901, part 3 Pavements and Aerodromes, Annex 14 to the Convention on International Civil Aviation presents some guidance for the PCR determination.

Limitations of the ACR-PCR System

The ACR-PCR system is only intended as a method that airport operators can use to evaluate acceptable operations of aircraft. It is not intended as a pavement design or pavement evaluation procedure, nor does it restrict the methodology used to design or evaluate a pavement structure. There is no mathematical correlation between the previous ICAO pavement strength reporting ACN-PCN and the new ICAO ACR-PCR system.

Assigning PCR to a pavement using PAVERS

The method of PCR evaluation is left up to the airport. ICAO recommends to relate PCR to the pavement life and to tie it to the annual volume of traffic, implying a pavement to have a variable PCR as a function of the desired structural pavement life. A sound relation with pavement life is obtained by adopting mechanistic design/evaluation systems with criteria that appear to be yielding reasonable results. Many of these procedures are based on linear, elastic theory coupled with empirical relationships for relating computed stress/strain to allowable aircraft load. This approach is well understood and well documented. The elastic layer mechanistic/empirical methods are also very adaptable to new criteria. For example, it is not very difficult to add/remove/modify the criteria (fatigue relationships or transfer functions). This makes it attractive since results from continuing research and development could be incorporated as necessary.

Implementation of calibrated design criteria into modern software tool such as PAVERS allow the designer to access the full advantages of the layered elastic method, including treatment of wander, and quickly produce PCR-numbers that are consistent with the original design concept. The Pavers concept is equipped to calculate PCR-numbers. A summary list of the steps required for PCR-assignment as based on the Technical evaluation method using the PAVERS program is explained in full detail.

NATO also shifts to the ACR-PCR system

The North Atlantic Treaty Organisation (NATO) decided to define their military requirements for airfield pavement strength in terms of the Aircraft Classification Rating (ACR), following the ICAO international system of load bearing strength reporting. Currently, NATO is in the transition from the ACN-PCN system to the ACR-PCR system. NATO wishes to determine in a reasonably short time a single methodology for establishing PCR values. The methodology is subject to discussion within the NATO Airfield Services Working Group (ASWG). NATO nations oppose the underlying design ICAO methodology for determining ACR, as suitable for extension for the determination of PCR.