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:

  • Pavement Classification Number
  • Pavement type
  • Subgrade strength category
  • Maximum allowable tire pressure category or maximum allowable tire pressure value
  • Evaluation method
The bearing strength of airfield pavements must be measured, analyzed, evaluated and reported so that the operating weight of aircraft allowed to use the pavements can be controlled. The PCN number indicates the suitability of a pavement area for unrestricted operations by any aircraft that has an ACN and tire pressure not exceeding the limits reported in PCN format of stated pavement type and subgrade strength category. The method of PCN pavement evaluation is left up to the airport, under the approval of the regulating CAA. Some guidance to the selection of an appropriate PCN is provided in Chapter 3, ‘Evaluation of pavements’ of the Aerodrome design manual  Although ICAO does not give specified regulatory guidance on how to determine a PCN, it states that the PCN must represent a relation between allowable load i.e. the ACN of the critical i.e. most damaging aircraft and the structural pavement life:
  1. The PCN is to be based on the ACN values of the most damaging aircraft(s) that use the pavement on a regular basis (regular being defined by the operator).
  2. The PCN determination must consider the number of coverage’s during the pavement’s life.

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.

PCN
Numeric Value
Pavement Type Subgrade Strength
Category
Allowable Tire Pressure (revised limits) Method of PCN
Determination
31 R – Rigid
F – Flexible
A – High
B – Medium
C – Low
D – Ultra low
W – Unlimited
X – High, limited to 1,75 MPa
Y – Medium, limited to 1,25 MPa
Z – Low, limited to 0,5 MPa
T – Technical
U – Using aircraft

Example of reporting PCN
A flexible pavement with a subgrade CBR of 7, a numeric value of 31 determined by a technical method, and having an allowable tire pressure of 1,6 MPa is reported as 31/F/C/X/T.

New tire pressure categories and designations
The tire pressure limits were recently revised to accommodate the tire pressures of modern aircraft. The results of two independent series of full-scale tests showed that increasing tire pressure from 15 bar to 17.5 bar did not adversely affect pavement rutting to any significant degree at HMA temperatures representative for airport operations in hot and temperate climates. The rutting as measures in the tests had no effect on the structural capacity of the lower layers of the pavement structures. That is, the life of the pavements would not have been decreased as a consequence of increasing tire pressure. The result of an ACI survey of pavement condition at worldwide airports in all climatic regions support the conclusions of the full-scale tests. The Pavement Study Group of the ICAO’s Aerodromes Panel reached a consensus in 2010 that tire pressure categories could be formally and permanently changed to be more consistent with the tire pressures of current and future aircraft without affecting the performance of airfield studies.

Aircraft overloads beyond published limits and ICAOs overload guidance
In the life of a pavement, it is possible that either the current or future traffic will load the pavement in such a manner that the assigned PCN rating is exceeded. Loads larger than the original design or as-built evaluated load may shorten the pavement service life, while smaller loads will usually have a minimal effect on pavement deterioration. Aircraft with load ratings greater than the reported pavement load ratings may still be allowed to use the pavement subject to the approval of the airport operating authority.

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.

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