Why use ACN-PCN?
ACN-PCN provides insight into determining if existing pavement can support current or projected aircraft operations. This determination may be calculated using a single operation or calculated for multiple operations of an aircraft or aircraft fleet mix. It also serves as an assessment of whether an aircraft may potentially damage the existing pavement and/or reduce its remaining service life. ACN-PCN applies only for pavements with bearing strengths of 12,500 lbs. or greater.
The ACN-PCN Method
ACN is defined as: A number that expresses the relative effect of an aircraft at a given configuration on a pavement structure for a specified standard of subgrade strength.
Typically the official ACN is provided by the Aircraft Manufacturer or can be determined using COMFAA 3.0.
PCN is defined as: A number that expresses the load-carrying capacity of a pavement for unrestricted operations.
The analysis is simple: ensure the ACN is of equal or lesser value than the PCN.
ACN ≤ PCN
What’s behind the numbers?
ACN is determined from a derived wheel load as well as weight of the aircraft and center of gravity for both rigid (concrete) and flexible (bituminous) pavements. Tire pressure is assumed and the frequency of coverages (departures) is calculated at 10,000. Also, four separate subgrade strength categories are applied to the determination, ranging from high to ultra low.
PCN is determined from one of two methods.
- Aircraft Method. This method simply applies the largest ACN value for current aircraft allowed to operate on the specific pavement as the PCN of that pavement. This method is not recommended for long-term use due to the potential to significantly overestimate or significantly underestimate the pavement capacity.
- Technical Method. This method takes into account a number of site-specific variables such as aircraft loading conditions, frequency of operations and pavement support conditions.
COMFAA 3.0 is a FAA developed software that calculates the ACN for both flexible and rigid pavement as well as pavement thicknesses. Please note that COMFAA is not intended to be used as pavement design software. A COMFAA support spreadsheet is also available and provides calculation for equivalent thicknesses for flexible and rigid pavements.
After the PCN is determined, it should be reported to the FAA to be included in the Airport’s Master Record 5010.
Flexible pavements - ACN should not exceed 10% of the reported PCN.
Rigid pavements - ACN should not exceed 5% of the reported PCN.
FAA AC 150/5335-5C, along with COMFAA 3.0 and supporting spreadsheet, and provide detailed instruction for determining and evaluating ACN-PCN. Both can be found at the FAA website: www.FAA.gov.
FAA Advisory Circular 150/5335-5C “Standardized Method of Reporting Airport Pavement Strength – PCN.”