Advantages and shortfalls of the system
Harmonising pavement load load-carrying capacity is required to optimise the use of aircraft and to safeguard the pavement’s integrity. However, there are also some flaws within the system.
Overcoming drawbacks of the ACN/PCN method:
The ACN/PCN method is easy to use and has several advantages:
- It is a numerical system based on a method of classifying the aircraft rather than the pavement. This makes it universally applicable but leaves the operator complete freedom of choice over the method of evaluation.
- The method of calculating the ACN allows full account to be taken of the undercarriage spacing and tire pressure. The method used to determine the effect of multiple wheel undercarriages are the most accurate yet included in a numerical classification system and there is unlikely to be any substantial future improvement. The ACN/PCN method provides an accurate measure of the damaging effect of an aircraft on a much wider range of pavement thickness’ than the LCN/LCG system.
- The ACN/PCN method is the only ICAO recognised classification system. The majority of commercial airports of the world are in conformance with ICAO’s intent to publish pavement strength in terms of PCN.
- The ACN/PCN method provides overload guidelines for rigid and flexible pavements.
A number of methods can be used by an airport authority to determine the rating of a pavement in terms of PCN.
The first method is known as the Using aircraft method, and can be applied with limited knowledge of the existing traffic and runway characteristics. The terminology Using aircraft simply means that the PCN is based on the aircraft currently and satisfactorily using the pavement, and there are no engineering methods or technical analysis employed to arrive at this sort of PCN.
The second method, known as the Technical evaluation method, requires a much more intimate knowledge of the pavement and its traffic, as well as a basic understanding of engineering methods that are utilized in pavement evaluation in order to be successfully implemented. All of the factors that contribute towards pavement analysis, such as existing and forecasted traffic, aircraft characteristics, pavement design parameters, and engineering experience are applied in arriving realistic and comparable PCNs.
The method of PCN evaluation is left up to the airport. ICAO recommends to relate PCN to the pavement life and to tie it to the annual volume of traffic, implying a pavement to have a variable PCN 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 tools allow the designer to access the full advantages of the layered elastic method, including treatment of wander, and quickly produce PCN-numbers that are consistent with the original design concept. The Pavers concept is equipped to calculate PCN-numbers. A summary list of the steps required for PCN-assignment as based on the Technical evaluation method is as follows:
- Determine the pavement's structural design life;
- Assess the pavement structure in terms of constructed thickness’, elastic moduli and Poisson ratio’s;
- Determine the pavement’s layer fatigue properties, including subgrade CBR;
- Determine the traffic volume in terms of type of aircraft and number of past and future operations of each aircraft that the pavement had and will experienced over its PCN pavement life course;
- Look up or calculate the ACNs of the aircraft at its operating empty (OEW) and maximum weight and at maximum takeoff weight (MTOW);
- Determine the degree of lateral wander for the pavement under consideration;
- Determine the critical pavement layer i.e. the constructed layer with the lowest bearing capacity or highest damage factor;
- Determine the critical aircraft of the forecasted fleet mix in terms of structural damage by simply taking the aircraft with the highest ACN or by determining the damage factors by means of calculation. A damage factor is the reciproke value of the allowable number of allowable aircraft passes. The ACNs at OEW and MTOW of that so-called critical aircraft are to be used in the PCN evaluation.
- Calculate the accumulated pavement damage in terms of Palmgren-Miner due to the forecasted fleet (incl. wander and for the PCN-pavement life course);
- Compute the Allowable Gross Weight (AGW) of the critical aircraft by varying the load of the undercarriage of the critical aircraft resulting in the same Miner damage as computed in the previous step;
- Once the allowable load (or weight) is established, the determination of the PCN value is a process of converting that load to a standard relative damage value (i.e. ACN). Look up the ACN using the published ACN data, and calculate the ACN of the critical aircraft at its allowable, maximum weight;
- Assign the ACN of the critical aircraft at the allowable, maximum weight to be the PCN of the pavement.
The Pavers program has all capabilities to assist you in arriving at realistic and reproducible PCN-numbers that are consistent with with the true pavement life, material strengths and fleet mix usage.