| dc.description.abstract | The International Civil Aviation Organization (ICAO) has adopted the Aircraft Classification Number / Pavement Classification Number (ACN/PCN) method as the standardized method for reporting airfield pavement bearing strength since 1980’s. Recently, the Federal Aviation Administration (FAA) has been circulating a draft Advisory Circular 150/5335-5B to provide specific guidance and revisions on how to arrive at a more reliable PCN. The primary objectives of this study are to investigate its fundamental principles, the reasoning of the newly-proposed revisions, and the effects on the PCN determination.
The original development of ACN/PCN methodology and several rigid airfield pavement design procedures were first reviewed. Parameter studies on the effects of ACN/PCN determinations due to different aircraft types, gear loads, subgrade strengths, traffic mix, etc. were subsequently conducted. The newly revised approach by introducing a cumulative damage factor method for computing PCN based on equivalent traffic was also discussed. The results indicated that the PCN method is significantly influenced by the conversion of equivalent annual departures of the selected critical airplane, interior or edge stress determination, and a representative is-situ subgrade modulus for both the existing approach and the newly revised approach.
ACN is originally defined as a number expressing the relative effect of an airplane at a given weight on a pavement structure for a specified standard subgrade. The concept of a single wheel load was employed to define the interaction of various gear loads and pavement without specifying pavement thickness as an ACN parameter. Since pavement thickness generally has much higher structural effect than subgrade strength, subgrade strength can be omitted along with pavement thickness when determining the relative effect. It was therefore proposed that a specific subgrade category (say “B”) be used for ACN determination.
PCN is originally defined as a number expressing the load-carrying capacity of a pavement for “unrestricted operations” and a concrete working stress of 399 psi (2.75 MPa) was also assumed in this ACN/PCN approach. What were “unrestricted operations” really meant? According to the specified concrete fatigue relationship, unlimited number of load repetitions is achievable when the stress ratio (defined by the ratio of critical stress and flexural strength) is relatively small. In addition to no weight restriction (up to maximum takeoff weight), unrestricted operations are preferably considered to be able to sustain “unlimited number of load repetitions” as well.
As such, a standard aircraft with a dual wheel gear load of 180,000 lbs was introduced. For consistency consideration, interior stress prediction models developed by Lee, et al. was used for critical stress estimations. An equivalent annual departure conversion factor was developed in this study using the concept of cumulative damage factors. A specific subgrade category (say “B”) was adopted. A modified PCN determination approach either with or without traffic mix consideration was subsequently developed.
Based on the results of nondestructive testing, the concepts of random sampling, central limit theorem, and confidence intervals for hypothesis testing were adopted to determine various design inputs. It was also proposed that a single representative design input for the entire runway pavement be determined by the lower limit of 95% confidence level to derive a more consistent and repeatable PCN value. Several case studies were conducted to illustrate the potential problems of the existing PCN procedures and the benefits of the proposed revisions.
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