Decoding the PCN for Airport Pavement Strength
By Cory Kesler, PE – The standard for reporting airport pavement strength has evolved over the years as new types of aircraft, gear configurations, and international travel have increased the need for a standardized reporting method. Airport managers frequently receive calls from charter pilots asking about pavement strength at their airports. Through the Aircraft Classification Number-Pavement Classification Number (ACN-PCN) method, airport managers have a way to determine if their pavements can support aircraft landings at their airport. However, the ACN-PCN method is a new concept to many airport managers. Thankfully, there are several useful tips to help you understand pavement strength using the ACN-PCN method.
The Need for Standardization
In the past, the method for reporting pavement strength in the United States was based on gear configuration and aircraft weight. This was commonly reported as maximum single wheel, dual wheel, dual tandem, and double dual tandem, reported in thousands of pounds. If an aircraft had a certain gear configuration, or an international pilot who was accustomed to a different standard or units, it was challenging to determine whether your pavement could support the aircraft.
Once I received a request from an airport manager about a C-130 aircraft that wanted to land at their airport for military training exercises. The C-130 had a single tandem gear configuration, which in the past was not one of the common gear configurations for reporting pavement strength. The airport had information on the pavement strength for single wheel and dual wheel loadings, however the C-130 weight was between these two values. Providing a quick, correct, answer to the airport manager was not possible, and we needed to research and perform additional calculations.
To mitigate situations like this, the International Civil Aviation Organization (ICAO), developed a single international pavement strength reporting method that used a single number and subgrade category, regardless of gear configuration. This method was the Aircraft Classification Number-Pavement Classification Number (ACN-PCN) method. In simplest terms, if your aircraft’s ACN is lower than the PCN of the pavement, the pavement can support your aircraft.
In the mid 2000s, the FAA started to require that pavement strength for runways be reported using the ACN-PCN method.
What is the Aircraft Classification Number (ACN)?
The aircraft classification number (ACN) is a number that reflects the effect of an aircraft on a pavement structure for a standard subgrade strength. The subgrade strength ranking is either A, B, C, or D; with A being the highest and D the lowest. The ACN of an aircraft at a given weight increases as the subgrade strength decreases.
What Does It Mean?
ACN numbers can range from 1 to 103. The ACN is double the derived singe wheelload (SWL) expressed in thousands of kilograms. For example, if the derived SWL of an aircraft is 20,000 lbs. the ACN would be 18.1 (20,000 lbs x 2/2.2 lbs/kg)/1000=18.1.
The ACN is calculated by the aircraft manufacturer for each subgrade category and is reported in the aircraft’s flight manual. If a pilot does not know their aircraft’s ACN, they can find it there. Below is an example of information a pilot should have to determine the ACN of their aircraft.
What is the Pavement Classification Number (PCN)?
The Pavement Classification Number is a number that shows the load-carrying capacity of the pavement for unrestricted operations. The PCN is calculated using:
- Subgrade strength.
- Pavement type.
- Pavement thickness.
- Aircraft fleet mix.
- Aircraft operations.
A Five-Part Code
The PCN is reported as a five-part code and includes the following information:
1. Numerical PCN Value
2. Pavement Type (R or F), Rigid or Flexible
3. Subgrade Category (A,B,C, or D)
4. Allowable Tire Pressure (W,X,Y, or Z)
5. Calculation Method (A or T), Aircraft or Technical
Software provided by the FAA and ICAO, calculate PCNs based on these factors. Once all these factors are considered, a PCN code is reported for the various pavement sections on the airfield. For example, if a PCN code for a pavement section is 32/F/B/X/T, this means:
- PCN value is 32
- Flexible Pavement (F)
- B Category Subgrade
- Allowable Tire Pressure of X=218 psi
- Technical Method (We recommend that the Technical Method be used to calculate the PCN method for your pavements. Your consultant can assist you with these calculations.)
Two Useful Tips for Airport Managers
Many airports have various pavement sections on the airfield that will result in different PCN codes. Your PCN codes may even vary on a single runway. The figure below shows an example of varying PCN codes on an airfield. Using this information alone, it may still be challenging to report your pavement strength to an inquiring pilot. Below are some useful tips to assist you in using the ACN-PCN method.
Note: It is very important to refer to the entire PCN code (32/F/B/X/T) and not just the PCN value (32). The ACN for the aircraft changes based on the subgrade strength. For example, the ACN of a 737-800 varies from 43 to 55 depending on the subgrade category.
- Simplify the PCN map of your airfield. Group pavements along a typical landing, taxiing, and parking route into a single PCN code. This will provide a simple reference to determine if the pavements on an airfield will support the aircraft where it is likely to operate. For example, you may have recently reconstructed your runway to current design standards, but your taxiways are scheduled to be reconstructed in future years. In this case you would want to use the calculated PCN code of the taxiways when considering how heavy an aircraft can be to utilize the airport. If an aircraft lands, they will most likely taxi and park at an airport’s FBO or apron.
- Keep a list or table (see example below) of ACN value for aircraft based on your subgrade category that may want to land at your airport. It’s a good idea to include aircraft in your list that have unusual gear configurations such as the C-130.
|Aircraft||Gross Weight (lbs)||PCN|
|King Air B100||11,500||2|
Real World PCN and ACN Scenarios
Let’s say your consultant has graciously provided a PCN code map of your airfield based on a subgrade category ‘B,’ and a table of ACNs for mix of aircraft that corresponds to your subgrade category at the airport. A charter pilot of a G-V calls to ask about pavement strength. You refer to your table and see that a G-V has an ACN of 28. Your PCN map indicates that your runway, taxiway, and apron have PCN codes of 30. In this case, since you have all flexible pavements, you would tell the pilot that your pavement strength is a 30/F/B/X/T. Since your PCN is greater than the ACN, your pavements can support the aircraft.
Conversely, the next day, you get a call from the pilot of a 737 who wants to fly in. Knowing your PCN is 30, you refer to the table and see that the 737 has an ACN of 45 for your subgrade category. If after telling the pilot your PCN code, they still indicate that they are going to want to land, this should raise a red flag and you may want to clarify or have them verify their intentions. (See table above for both examples)
It is always good practice to report what the PCN code is for the pavement to the pilot and not specifically tell them your airport can support their aircraft. The pilot is responsible for determining whether they should use the airport depending on a variety of factors including reported pavement strength.
The ACN-PCN method for pavement strength can become complex depending on the variations in your airfield pavements, subgrades, and aircraft fleet mix. If you would like to discuss practical options and tools to assist you regarding pavement strength, please reach out anytime.
Cory Kesler, PE is a senior engineer with 20 years of experience in design and construction of airport pavements and facilities at a variety of general aviation and commercial service airports in the West. He is based in Morrison-Maierle’s Helena office and can be reached at firstname.lastname@example.org.
Technical reviews provided by Heather Mosser, PE, and Scott Bell, PE.