NATO shifts to the ACN-PCN system in 2001
The North Atlantic Treaty Organisation (NATO) decided in 2001 to define their military requirements for airfield pavement strength in terms of the Aircraft Classification Number (ACN), following the ICAO international system of load bearing strength reporting. Currently, NATO is in the transition from the LCN/LCG system to the ACN-PCN system. NATO wishes to determine in a reasonably short time a single methodology for establishing PCN 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 ACN, as suitable for extension for the determination of PCN. The ASWG concluded in their 2003 meeting that a ‘Layered Mechanistic Approach’ is preferred over Emperical Methods.
LCN versus PCN
Because ICAO lays down the method of determining ACN for both flexible and rigid pavements, this step has been achieved easily. NATO has published individual ACN values for the range of aircraft likely to use its airfields in Standard NATO Agreement 7131 ACN-PCN and Allied Engineering Publication 46a. NATO’s has replaced the older Load Classification (LCN) system in NATO Criteria and Standards for Airfields with ACN.
Nations reporting PCN to NATO
The February 2003 ASWG meeting decided to let nations use their own national design theories to determine PCN. An inquiry of the U.S. Corps amongst several nations learned that the pavement design method used by individual nations lead to different pavement thickness (and PCNs). Consequently, different nations get different results. This, and the fact that the PCN reporting system does not reflect the actual pavement life are considered as shortfalls of the ACN-PCN system. Therefore additional information such as the number of aircraft passes used to make the calculation must be provided (runway only). Nations shall provide the following information:
- Airfield name, runway(s) and PCN value(s);
- Type of aircraft for which the PCN values are based;
- Number of aircraft passes used to make the calculation.
The additional information allows NATO to compare NATO airfield strength values as required and shown in the previous table.
MPA = Maritime Patrol Aircraft; AAR = Air to Air Refuelling.
ACN for several NATO aircraft groups
NATO Ad-hoc Pavement meetings
A small group of national pavement experts of several NATO nations meet each other on a regular basis to discuss pavement topics. Items such as bearing capacity (STANAG 7131 Ed1; AEP-46A ACN/PCN), runway friction and braking conditions (STANAG 3634 Ed4) and visual inspections and pavement management (AEP-56) are discussed for broad usage within NATO nations.
NATO Standardization Agreement (STANAG) for PCN-method
With the current world situation, there is an increasing need for NATO standards with respect to airfield pavement design to support joint military operations. It is critical to mission planners that methods steps are taken to insure that global methods for reporting the structural capacity of an airfield are available. Several Nations have implemented 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). This makes it attractive since results from continuing research and development could be incorporated as necessary. With the current emphasis and requirements for better design/evaluation methods, NATO drafted a Standardization Agreement or STANAG to be used among the Nations.