|The Pavement Classification Number|
|Some guidance to PCN assignment||PCN reporting format|
ICAO requires that the strength of pavements for aircraft with mass greater than 12,500 lb ( 5,700 kg) be made available using ACN-PCN method by reporting all of the following information:
Pls. Note that the most damaging aircraft doesn't have to be the aircraft within the fleet mix having the highest ACN. The critical aircraft for PCN evaluation is the aircraft resulting in the highest damage af all aircraft within the fleet mix. An analytical technical evaluation is preferred over empirical methods (i.e. CBR-method), revealing the true material strength using calibrated failure criteria for all pavement materials and subgrade.
In the most fundamental terms, the determination of a rating in terms of PCN is a process of deciding on the maximum allowable gross weight of a selected critical airplane for a pavement knowing its ACN at that weight, reporting it as PCN. This process can be as simple as knowing the operational gross weight of each aircraft that is currently using the pavement and looking up its ACN (referred to as the Using aircraft method). This method can be applied with limited knowledge of the existing aircraft and pavement characteristics. The second method is more complex and referred to as Technical evaluation. In order to be successfully implemented, the technical evaluation requires an intimate knowledge of the pavement and its traffic, as well as basic understanding of engineering methods that are utilized in pavement design.
The ICAO PCN pavement strength reporting system involves publishing a five (5) part strength code in the form of 51 FDWT for flexible pavements or 62 R/B/W/T for rigid concrete pavements. If desired, PCNs may be published to an accuracy of 1/10th of a whole number. Briefly, the first number is the reported PCN value on a scale of 1 to about 130, with 1 representing a weak pavement and 130 a very strong pavement. The second part of the code is either an "F" for flexible pavement systems or "R" for rigid pavement systems. The third part is a letter code A, B, C, or D indicating the subgrade/bearing strength, with A representing a high supporting strength and D a very low strength. The fourth part indicates the tire pressure limitation in MPa if applicable (X, Y, Z otherwise W). The fifth and final part of the PCN code indicates the evaluation method used to determine the pavement strength - "T" if derived from an engineering study or "U" if based on satisfactory aircraft usage.
Example of reporting PCN
New tire pressure
categories and designations
beyond published limits and ICAOs overload guidance
Pavements if overloaded beyond their original design strength can deteriorate very quickly leading to both FOD and roughness safety problems for aircraft. According to clause 19.1 ’Overload operations‘ of ICAO Annex 14, occasional movements (5 percent) by aircraft on flexible pavements with ACN values no more than 10 percent above the reported PCN should not adversely affect the pavement. For rigid pavement types, the ACN should not exceed the reported PCN by 5 percent. Overloads beyond these limits should be based on the results of a detailed engineering study that compares the individual aircraft load to the structural capability of the pavement. The implications of allowing overload operations should be fully understood by the Airport Authority in terms of the accelerated structural deterioration and the reduction in pavement service life which may occur. When overloads are allowed, the pavement should be inspected regularly by the airport authority to ensure that unacceptable structural damage is not taking place.