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.
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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.
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