Advisory

Circular U.S. Department 

of Transportation 

Federal Aviation 

Administration

Subject: STANDARDIZED METHOD OF Date: DATE, 09/28/06 AC No: 150/5335-5A 

REPORTING AIRPORT PAVEMENT Initiated by: AAS-100 Change: 

STRENGTH - PCN 

1. THE PURPOSE OF THIS ADVISORY CIRCULAR. 

This advisory circular (AC) provides guidance for using the standardized International Civil 

Aviation Organization (ICAO) method to report airport pavement strength. The standardized 

method is known as the Aircraft Classification Number – Pavement Classification Number 

(ACN-PCN) method. 

2. WHAT THIS AC CANCELS. 

This AC cancels AC 150/5335-5, Standardized Method of Reporting Airport Pavement Strength – 

PCN, dated June 15, 1983. 

3. WHO THIS AC AFFECTS. 

The International Civil Aviation Organization (ICAO) requires member countries to report 

pavement strength information for a variety of purposes. The ACN-PCN method has been 

developed and adopted as an international standard and has facilitated the exchange of pavement 

strength rating information. This AC provides specific guidance on how to report airport 

pavement strength using the standardized method. 

4. RELATED READING MATERIAL. The publications listed in Appendix 4 provide further 

information on the development and use of the ACN-PCN method. 

David L. Bennett 

Director, Office of Airport Safety and Standards 
AC 150/5335-5A 09/28/06 

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TABLE OF CONTENTS 

CHAPTER 1. INTRODUCTION ...................................................................................................1 

1.0 BACKGROUND ......................................................................................................................1 

1.1 DEVELOPMENT OF A STANDARDIZED METHOD.........................................................1 

a. Definition of ACN ............................................................................................................1 

b. Definition of PCN.............................................................................................................1 

c. System Methodology ........................................................................................................1 

1.2 APPLICATION ........................................................................................................................1 

1.3 LIMITATIONS OF THE ACN-PCN SYSTEM.......................................................................1 

CHAPTER 2. DETERMINATION OF AIRCRAFT CLASSIFICATION NUMBER..................3 

2.0 DETERMINATION OF THE ACN. ........................................................................................3 

2.1 SUBGRADE CATEGORY. .....................................................................................................3 

Table 2-1. Standard Subgrade Support Conditions for Rigid Pavement ACN Calculation ...3 

Table 2-2. Standard Subgrade Support Conditions for Flexible Pavement ACN Calculation3 

2.2 OPERATIONAL FREQUENCY..............................................................................................3 

2.3 RIGID PAVEMENT ACN. ......................................................................................................3 

2.4 FLEXIBLE PAVEMENT ACN. ..............................................................................................4 

2.5 ACN CALCULATION.............................................................................................................4 

2.6 VARIABLES INVOLVED IN DETERMINATION OF ACN VALUES...............................4 

CHAPTER 3. DETERMINATION OF ACN VALUES USING COMFAA.................................5 

3.0 ANNOUNCEMENT OF COMFAA SOFTWARE APPLICATION.......................................5 

3.1 ORIGIN OF THE COMFAA PROGRAM...............................................................................5 

3.2 COMFAA PROGRAM.............................................................................................................5 

a. ACN Computation Mode..................................................................................................5 

b. Pavement Thickness Mode:..............................................................................................5 

3.3 INTERNAL AIRPLANE LIBRARY. ......................................................................................5 

3.4 EXTERNAL AIRPLANE LIBRARY. .....................................................................................6 

3.5 USING THE COMFAA PROGRAM.......................................................................................6 

Figure 3-1. Operation of the COMFAA Program in ACN Mode...........................................7 

Figure 3-2. Operation of the COMFAA Program in Pavement Thickness Mode ..................8 

CHAPTER 4. DETERMINATION OF PCN NUMERICAL VALUE..........................................9 

4.0 PCN CONCEPT........................................................................................................................9 

4.1 DETERMINATION OF NUMERICAL PCN VALUE. ..........................................................9 

4.2 USING AIRPLANE METHOD TO DETERMINE PCN. .......................................................9 

a. Assumptions of the Using Airplane Method. ...................................................................9 

b. Inaccuracies of the Using Airplane Method. ....................................................................9 

4.3 TECHNICAL EVALUATION METHOD TO DETERMINE PCN........................................9 

a. Determination of the PCN Value....................................................................................10 

b. Concept of Equivalent Traffic. .......................................................................................10 

c. Counting Airplane Operations. .......................................................................................10 

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4.4 LIMITATIONS OF THE PCN. ............................................................................................ 11 

4.5 REPORTING THE PCN........................................................................................................ 11 

a. Numerical PCN Value.................................................................................................... 11 

b. Pavement Type............................................................................................................... 11 

Table 4-1. Pavement Codes for Reporting PCN.................................................................. 11 

i) Flexible Pavement............................................................................................. 11 

ii) Rigid Pavement. ................................................................................................ 11 

iii) Composite Pavement......................................................................................... 12 

c. Subgrade Strength Category .......................................................................................... 12 

d. Allowable Tire Pressure................................................................................................. 12 

TABLE 4-2. Tire Pressure Codes for Reporting PCN......................................................... 12 

i) Tire Pressures on Rigid Pavements................................................................... 12 

ii) Tire Pressures on Flexible Pavements .............................................................. 12 

e. Method Used to Determine PCN. ............................................................................ 13 

f. Example PCN Reporting.......................................................................................... 13 

g. Report PCN Values to the FAA............................................................................... 13 

APPENDIX 1. EQUIVALENT TRAFFIC.................................................................................... 1 

1.0 EQUIVALENT TRAFFIC....................................................................................................... 1 

1.1 EQUIVALENT TRAFFIC TERMINOLOGY. ....................................................................... 1 

a. Arrival (Landing) and Departure (Takeoff). .................................................................... 1 

b. Pass................................................................................................................................... 2 

Figure A1-1. Traffic Load Distribution Patterns.................................................................... 2 

c. Coverage. ......................................................................................................................... 3 

d. Operation.......................................................................................................................... 4 

e. Traffic Cycle and Traffic Cycle Ratio. ............................................................................ 4 

Table A1-1. TC/C Ratio for Flexible Pavements – Additional Fuel Not Obtained............... 5 

Table A1-2. TC/C Ratio for Flexible Pavements – Additional Fuel Obtained...................... 5 

Table A1-3. TC/C Ratio for Rigid Pavements – Additional Fuel Not Obtained ................... 6 

Table A1-4. TC/C Ratio for Rigid Pavements – Additional Fuel Obtained .......................... 6 

1.2 EQUIVALENT TRAFFIC BASED ON GEAR TYPE........................................................... 6 

Table A1-5. Conversion Factors to Convert from One Landing Gear Type to Another....... 7 

Table A1-6. Equivalency Conversion to a Dual Tandem (2D) Gear Type............................ 8 

Table A1-7. Equivalency Conversion to a Dual Gear (D) Type............................................ 8 

1.3 EQUIVALENT TRAFFIC BASED ON LOAD MAGNITUDE. ........................................... 9 

Table A1-8. Equivalent Traffic Cycles Based on Load Magnitude..................................... 10 

APPENDIX 2. PCN DETERMINATION EXAMPLES............................................................... 1 

1.0. THE USING AIRPLANE METHOD..................................................................................... 1 

1.1 USING AIRPLANE EXAMPLE FOR FLEXIBLE PAVEMENTS ....................................... 2 

Table A2-1. Using Airplane and Traffic for a Flexible Pavement......................................... 2 

1.2 USING AIRPLANE EXAMPLE FOR RIGID PAVEMENTS............................................... 3 

Table A2-2. Using Airplane and Traffic for a Rigid Pavement............................................. 3 

2.0 THE TECHNICAL EVALUATION METHOD. .................................................................... 4 

2.1 TECHNICAL EVALUATION FOR FLEXIBLE PAVEMENTS .......................................... 4 

2.2 TECHNICAL EVALUATION EXAMPLES FOR FLEXIBLE PAVEMENTS. ................... 7 

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a. Flexible Pavement Example 1. .........................................................................................7 

Figure A2-1. Flexible Pavement Example Cross-Section ......................................................7 

Table A2-3. Technical Evaluation Critical Airplane Determination......................................8 

Table A2-4. Equivalent Annual Departures of the Critical Airplane .....................................9 

b. Flexible Pavement Example 2. .......................................................................................10 

c. Flexible Pavement Example 3. .......................................................................................10 

d. Flexible Pavement Example 4. .......................................................................................11 

2.3 TECHNICAL EVALUATION FOR RIGID PAVEMENTS.................................................11 

2.4 TECHNICAL EVALUATION EXAMPLES FOR RIGID PAVEMENTS...........................13 

a. Rigid Pavement Example 1.............................................................................................13 

Figure A2-2. Rigid Pavement Example Cross-Section.........................................................14 

Table A2-5. Rigid Pavement Technical Evaluation Traffic .................................................14 

Table A2-6. Technical Evaluation Critical Airplane Determination...................................15 

Table A2-7. Equivalent Annual Departures of the Critical Airplane ...................................15 

b. Rigid Pavement Example 2.............................................................................................16 

c. Rigid Pavement Example 3.............................................................................................17 

APPENDIX 3. PAVEMENT OVERLOAD EVALUATION BY THE ACN-PCN SYSTEM .....1 

1.0 ICAO PAVEMENT OVERLOAD EVALUATION GUIDANCE. .........................................1 

1.1 OVERLOAD GUIDANCE.......................................................................................................1 

1.2 ADJUSTMENTS FOR FLEXIBLE PAVEMENT OVERLOADS. ........................................2 

a. Flexible Pavement Overload Illustration 1. ......................................................................3 

b. Flexible Pavement Overload Illustration 2. ......................................................................3 

Figure A3-1. B747-400 Flexible Pavement ACN Versus Gross Weight ...............................3 

Figure A3-2. B747-400 Flexible Pavement Life Versus ACN...............................................4 

Table A3-1. Data for Constructing Flexible Pavement Life Curves for B747-400................5 

Figure A3-3. B747-400 Flexible Pavement Life ....................................................................6 

c. Flexible Pavement Overload Illustration 3. ......................................................................7 

Table A3-2. Flexible Pavement Overload Airplane Added....................................................7 

Table A3-3. Flexible Pavement New Airplane Equivalent Traffic ........................................8 

1.3 ADJUSTMENTS FOR RIGID PAVEMENT OVERLOADS .................................................8 

a. Rigid Overload Illustration 1. ...........................................................................................8 

Figure A3-4. B747-400 Rigid Pavement ACN Versus Gross Weight....................................9 

Figure A3-5. B747-400 Rigid Pavement Life Versus ACN.................................................10 

Table A3-4. Data for Constructing Rigid Pavement Life Curves for B747-400..................10 

Figure A3-6. B747-400 Rigid Pavement Life.......................................................................12 

b. Rigid Pavement Overload Illustration 2. ........................................................................13 

Table A3-5. Rigid Pavement Overload Example with New Airplane..................................13 

Table A3-6 Equivalent Annual Departures of the Critical Airplane .....................................14 

APPENDIX 4. RELATED READING MATERIAL......................................................................1 

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CHAPTER 1. INTRODUCTION 

1.0 BACKGROUND. The United States is a member of the International Civil Aviation 

Organization (ICAO) and is bound by treaty agreements to comply with the requirements of 

ICAO to the maximum extent practical (see Federal Aviation Administration (FAA) Order 

2100.13, FAA Rulemaking Policies, Chapter 11). Annex 14 to the Convention of International 

Civil Aviation-Aerodromes requires that each member country publish information on the 

strengths of all public airport pavements in its own Aeronautical Information Publication (AIP). 

FAA reports pavement strength information to the National Airspace System Resources (NASR) 

database and published pavement strength information in the Airport Master Record (Form 5010) 

and the Airport/Facility Directory (AFD). 

1.1 DEVELOPMENT OF A STANDARDIZED METHOD. In 1977, ICAO established a 

Study Group to develop a single international method of reporting pavement strengths. The study 

group developed and ICAO adopted the Aircraft Classification Number - Pavement Classification 

Number (ACN-PCN) method. Using this method, it is possible to express the effect of an 

individual airplane on different pavements by a single unique number that varies according to 

airplane weight and configuration (e.g. tire pressure, gear geometry, etc.), pavement type, and 

subgrade strength. This number is the Aircraft Classification Number (ACN). Conversely, the 

load-carrying capacity of a pavement can be expressed by a single unique number, without 

specifying a particular airplane or detailed information about the pavement structure. This number 

is the Pavement Classification Number (PCN). 

a. Definition of ACN. ACN is defined as a number that expresses the relative effect 

of an airplane at a given weight on a pavement structure for a specified standard subgrade 

strength. 

b. Definition of PCN. PCN is a number that expresses the load-carrying capacity of 

a pavement for unrestricted operations. 

c. System Methodology. The ACN-PCN system is structured so a pavement with a 

particular PCN value can support, without weight restrictions, an airplane that has an ACN value 

equal to or less than the pavement’s PCN value. This is possible because ACN and PCN values 

are computed using the same technical basis. 

1.2 APPLICATION. The use of the standardized method of reporting pavement strength 

applies only to pavements with bearing strengths of 12,500 pounds (5 700 kg) or greater. The 

method of reporting pavement strength for pavements of less than 12,500 pounds (5 700 kg) 

bearing strength remains unchanged. 

1.3 LIMITATIONS OF THE ACN-PCN SYSTEM. The ACN-PCN system is only intended 

as a method of reporting relative pavement strength so airport operators can evaluate acceptable 

operations of airplanes. It is not intended as a pavement design or pavement evaluation 

procedure, nor does it restrict the methodology used to design or evaluate a pavement structure. 

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AC 150/5335-5A 09/28/06 

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CHAPTER 2. DETERMINATION OF AIRCRAFT CLASSIFICATION NUMBER 

2.0 DETERMINATION OF THE ACN. The airplane manufacturer provides the official 

computation of an ACN value. Computation of the ACN requires detailed information on the 

operational characteristics of the airplane such as maximum aft center of gravity, maximum ramp 

weight, wheel spacing, tire pressure, and other factors. 

2.1 SUBGRADE CATEGORY. The ACN-PCN method adopts four standard levels of 

subgrade strength for rigid pavements and four levels of subgrade strength for flexible pavements. 

These standard support conditions are used to represent a range of subgrade conditions as shown 

in Tables 2-1 and 2-2. 

Table 2-1. Standard Subgrade Support Conditions for Rigid Pavement ACN Calculation 

Subgrade 

Strength Category 

Subgrade Support 

k-Value 

pci (MN/m3) 

Represents 

pci (MN/m3

) 

Code 

Designation 

High 552.6 (150) k > 442 ( >120) A 

Medium 294.7 (80) 221<k<442 (60<k<120) B 

Low 147.4 (40) 92<k<221 (25<k<60) C 

Ultra Low 73.7 (20) k<92 (<25) D 

Table 2-2. Standard Subgrade Support Conditions for Flexible Pavement ACN Calculation 

Subgrade 

Strength Category 

Subgrade Support 

CBR-Value Represents 

Code 

Designation 

High 15 CBR > 13 A 

Medium 10 8<CBR<13 B 

Low 6 4<CBR<8 C 

Ultra Low 3 CBR<4 D 

2.2 OPERATIONAL FREQUENCY. Operational frequency is defined in terms of coverages 

that represent a full-load application on a point in the pavement. Coverages must not be confused 

with other common terminology used to reference movement of airplanes. As an airplane moves 

along a pavement section it seldom travels in a perfectly straight path or along the exact same 

path as before. This movement is known as airplane wander and is assumed to be modeled by a 

statistically normal distribution. As the airplane moves along a taxiway or runway, it may take 

several trips or passes along the pavement for a specific point on the pavement to receive a fullload application. It is easy to observe the number of passes an airplane may make on a given 

pavement, but the number of coverages must be mathematically derived based upon the 

established pass-to-coverage ratio for each airplane. 

2.3 RIGID PAVEMENT ACN. For rigid pavements, the airplane landing gear flotation 

requirements are determined by the Westergaard solution for a loaded elastic plate on a Winkler 

foundation (interior load case), assuming a concrete working stress of 399 psi (2.75 MPa). 

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2.4 FLEXIBLE PAVEMENT ACN. For flexible pavements, airplane landing gear flotation 

requirements are determined by the California Bearing Ratio (CBR) method for each subgrade 

support category. The CBR method employees a Boussinesq solution for stresses and 

displacements in a homogeneous, isotropic elastic half-space. To standardize the ACN 

calculation and to remove operational frequency from the relative rating scale, the ACN-PCN 

method specifies that ACN values be determined at a frequency of 10,000 coverages. 

2.5 ACN CALCULATION. Using the parameters defined for each type of pavement section, a 

mathematically derived single wheel load is calculated to define the landing gear/pavement 

interaction. The derived single wheel load implies equal stress to the pavement structure and 

eliminates the need to specify pavement thickness for comparative purposes. This is achieved by 

equating the thickness derived for a given airplane landing gear to the thickness derived for a 

single wheel load at a standard tire pressure of 18l psi (1.25 MPa). The ACN is defined as two 

times the derived single wheel load (expressed in thousands of kilograms). 

2.6 VARIABLES INVOLVED IN DETERMINATION OF ACN VALUES. Because 

airplanes can be operated at various weight and center of gravity combinations, ICAO adopted 

standard operating conditions for determining ACN values. The ACN is to be determined at the 

weight and center of gravity combination that creates the maximum ACN value. Tire pressures 

are assumed to be those recommended by the manufacturer for the noted conditions. Airplane 

manufacturers publish maximum weight and center of gravity information in their Airplane 

Characteristics for Airport Planning (ACAP) manuals. 

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CHAPTER 3. DETERMINATION OF ACN VALUES USING COMFAA 

3.0 ANNOUNCEMENT OF COMFAA SOFTWARE APPLICATION. To facilitate the use 

of the ACN-PCN system, FAA developed a software application that calculates ACN values 

using the procedures and conditions specified by ICAO. The software is called COMFAA and it 

may be downloaded along with its source code and supporting documentation from the FAA 

website. The program is useful for determining an ACN value under various conditions however, 

the user should remember that official ACN values are provided by the airplane manufacturer. 

3.1 ORIGIN OF THE COMFAA PROGRAM. Appendix 2 of the ICAO Aerodrome Design 

Manual, Part 3, Pavements, provides procedures for determining the Aircraft Classification 

Number (ACN). The appendix provides program code for two FORTRAN software applications 

capable of calculating the ACN for various airplanes on rigid and flexible pavement systems. 

The computer program listings in Appendix 2 of the ICAO manual were optically scanned and 

the FORTRAN code translated into Visual Basic 6.0 for incorporation into COMFAA. 

3.2 COMFAA PROGRAM. The COMFAA software is a general purpose program that 

operates in two computational modes: ACN Computation Mode and Pavement Thickness Mode. 

a. ACN Computation Mode: 

• Calculates the ACN number for airplanes on flexible pavements. 

• Calculates the ACN number for airplanes on rigid pavements. 

• Calculates flexible pavement thickness based on the ICAO procedure (CBR 

method) for default values of CBR (15, 10, 6, and 3). 

• Calculates rigid pavement slab thickness based on the ICAO procedures 

(Portland Cement Association method, interior load case) for default values of 

k (552.6, 294.7, 147.4, and 73.7 lb/in3 [150, 80, 40, and 20 MN/m3]). 

 

 Note: Thickness calculation in the ACN mode is for specific conditions 

identified by ICAO for determination of ACN. For flexible pavements a 

standard tire pressure of 181 psi (1.25 MPa) and 10,000 coverages is specified. 

For rigid pavements an allowable stress level of 399 psi is identified by ICAO. 

These parameters seldom represent actual design criteria used for pavement 

design. The thickness calculated in ACN mode has little meaning to pavement 

design requirements and should not be used for determining allowable 

pavement loading. 

 

 b. Pavement Thickness Mode: 

• Calculates total flexible pavement thickness based on the FAA CBR method 

specified in AC 150/5320-6D, Airport Pavement Design and Evaluation, for 

CBR values and coverage levels specified by the user. 

• Calculates rigid pavement slab thickness based on the FAA Westergaard 

method (edge load analysis) specified in AC 150/5320-6D for k values and 

coverage levels specified by the user. 

3.3 INTERNAL AIRPLANE LIBRARY. COMFAA contains an internal library of airplanes 

covering most large commercial and US military airplanes currently in operation. The internal 

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AC 150/5335-5A 09/28/06 

library is based on airplane information provided directly by airplane manufacturers or obtained 

from ACAP Manuals. The default characteristics of airplanes in the internal library represent the 

ICAO standard conditions for calculation of ACN. These characteristics include center of gravity 

at the maximum aft position for each airplane in the ACN mode, whereas the pavement thickness 

mode center of gravity is fixed to distribute 95 percent of the max gross load to the main landing 

gear for all airplanes. 

3.4 EXTERNAL AIRPLANE LIBRARY. COMFAA allows for an external airplane library 

where characteristics of the airplane can be changed and additional airplanes added as desired. 

Functions permit users to modify the characteristics of an airplane and save the modified airplane 

in the external library. There are no safeguards in the COMFAA program to assure that airplane 

parameters in the external library are feasible or appropriate. The user is responsible for assuring 

all data is correct. 

When saving an airplane from the internal library to the external library, the COMFAA program 

will calculate the tire contact area based upon the gross load, maximum aft center of gravity, and 

tire pressure. This value is recorded in the external library and is used for calculating the pass-tocoverage (P/C) ratio in the pavement thickness mode. Since the tire contact area is constant, the 

P/C ratio is also constant in the pavement thickness mode. This fixed P/C ratio should be used for 

converting passes to coverages for pavement thickness determination and equivalent airplane 

operations. 

3.5 USING THE COMFAA PROGRAM. Using the COMFAA program to calculate ACN 

values is visually interactive and intuitive. The user selects the desired airplane, confirms the 

physical properties of the airplane, and clicks on the ACN Flexible or ACN Rigid button to 

determine the ACN for the four standard subgrade conditions. The program includes a help file 

assist the user. Figures 3-1 and 3-2 detail the operation of the COMFAA program. 

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1. Select 

Aircraft 

Group 

2. Select 

Aircraft 

3. Confirm 

Aircraft 

Parameters 

4. Click to Calculate ACN Values Calculated ACN Values 

Figure 3-1. Operation of the COMFAA Program in ACN Mode 

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AC 150/5335-5A 09/28/06 

1. Select 

Aircraft 

Group 

2. Select 

Aircraft 

5. Click to Calculate Pavement Thickness 

3. Confirm 

Aircraft 

Parameters 

4. Click to Enter CBR or k-value 

Calculated Pavement Thickness 

Figure 3-2. Operation of the COMFAA Program in Pavement Thickness Mode 

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CHAPTER 4. DETERMINATION OF PCN NUMERICAL VALUE 

4.0 PCN CONCEPT. In fundamental terms, the determination of a pavement rating in terms of 

PCN is a process of determining the ACN for the selected critical or most demanding airplane 

and reporting the ACN value as the PCN for the pavement structure. Under these conditions, any 

airplane with an ACN equal to or less than the reported PCN value can safely operate on the 

pavement subject to any limitations on tire pressure. 

4.1 DETERMINATION OF NUMERICAL PCN VALUE. Determination of the numerical 

PCN value for a particular pavement can be based upon one of two procedures. The procedures 

are known as the “using” airplane method and the “technical” evaluation method. ICAO 

procedures permit member states to determine how PCN values will be determined based upon 

internally developed pavement evaluation procedures. Either procedure may be used to 

determine a PCN, but the methodology must be reported as part of the posted rating. 

4.2 USING AIRPLANE METHOD TO DETERMINE PCN. The using airplane method is a 

simple procedure where ACN values for all airplanes currently permitted to use the pavement 

facility are determined and the largest ACN value is reported as the PCN. This method is easy to 

apply and does not require detailed knowledge of the pavement structure. 

 a. Assumptions of the Using Airplane Method. An underlying assumption with 

the using airplane method is that the pavement structure has the structural capacity to 

accommodate all airplanes in the traffic mixture and that each airplane is capable of operating on 

the pavement structure without restriction. 

 b. Inaccuracies of the Using Airplane Method. The accuracy of this method is 

greatly improved when airplane traffic information is available. Significant over-estimation of 

the pavement capacity can result if an excessively damaging airplane, which uses the pavement 

on a very infrequent basis, is used to determine the PCN. Likewise, significant under-estimation 

of the pavement capacity can lead to uneconomic use of the pavement by preventing acceptable 

traffic from operating. Although there are no minimum limits on frequency of operation before 

an airplane is considered part of the normal traffic, the reporting agency must use a rational 

approach to avoid overstating or understating the pavement capacity. Use of the using airplane 

method is discouraged on a long-term basis due to the concerns listed above. 

4.3 TECHNICAL EVALUATION METHOD TO DETERMINE PCN. The strength of a 

pavement section is difficult to summarize in a precise manner and will vary depending upon the 

unique combination of airplane loading conditions, frequency of operation, and pavement support 

conditions. The technical evaluation method attempts to address these and other site-specific 

variables to determine reasonable pavement strength. In general terms, for a given pavement 

structure and given airplane, the allowable number of operations (traffic) will decrease as the 

intensity of pavement loading increases (increase in airplane weight). It is entirely possible that 

two pavement structures with different cross-sections will report similar strength. However, the 

permissible airplane operations will be considerably different. This discrepancy must be 

acknowledged by the airport operator and may require operational limitations administered 

outside of the ACN-PCN system. All of the factors involved in determining a pavement rating 

are important, and it is for this reason that pavement ratings should not be viewed in absolute 

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AC 150/5335-5A 09/28/06 

terms, but rather as estimations of a representative value. A successful pavement evaluation is 

one that assigns a pavement strength rating that considers the effects of all variables on the 

pavement. 

The accuracy of a technical evaluation is better than the using airplane procedure but requires a 

considerable increase in time and resources. Pavement evaluation may require a combination of 

on-site inspections, load-bearing tests and engineering judgment. It is common to think of 

pavement strength rating in terms of ultimate strength or immediate failure criteria. However, 

pavements are rarely removed from service due to instantaneous structural failure. A decrease in 

the serviceability of a pavement is commonly attributed to increases in surface roughness or 

localized distress, such as rutting or cracking. Determination of the adequacy of a pavement 

structure must not only consider the magnitude of pavement loads but the impact of the 

accumulated effect of traffic volume over the intended life of the pavement. 

 a. Determination of the PCN Value. The PCN numerical value is determined from 

an allowable load rating. While it is important not to confuse the PCN value with a pavement 

design parameter, the PCN is developed in a similar fashion. An allowable load rating is 

determined by applying the same principles as those used for pavement design. The process for 

determining the allowable load rating takes factors such as frequency of operations and 

permissible stress levels into account. Allowable load ratings are often stated in terms of 

airplane gear type and maximum gross airplane weight, as these variables are used in the 

pavement design procedure. Missing from the stated load rating, but just as important, is 

frequency of operation. In determining an allowable load rating, the evaluation must address 

whether the allowable load rating represents the pavement strength over a reasonable frequency 

of operation. Once the allowable load rating is established, the determination of the PCN value is 

a simple process of determining the ACN of the airplane representing the allowable load and 

reporting the value as the PCN. 

 b. Concept of Equivalent Traffic. The ACN-PCN method is based upon design 

procedures that establish one airplane as the critical or most demanding on the pavement 

structure. Calculations necessary to determine the PCN can only be performed for one airplane 

at a time. The ACN-PCN method does not directly address how to represent a traffic mixture as a 

single airplane. To address this limitation, FAA uses the equivalent airplane concept to 

consolidate entire traffic mixtures into one representative airplane. The procedures for 

establishing an equivalent airplane from a traffic mixture are provided in Appendix A1. 

 c. Counting Airplane Operations. When evaluating or designing a pavement 

section, it is important to account for the number of times the pavement will be stressed. As 

discussed in paragraph 2.2, an airplane may have to pass over a given section of pavement 

numerous times before the portion of pavement considered for evaluation receives one full stress 

application. While statistical procedures exist to determine the passes required for one full stress 

application, the evaluation of a pavement section for PCN determination must also consider how 

airplanes use the pavement in question. A conservative approach is used in pavement design 

procedures where it is assumed that each airplane using the airport must land and take off once 

per cycle. Since the arrival or landing weight of the airplane is usually less than the departure 

weight, the design procedure only counts one pass at the departure weight for analysis. These 

departures are usually discussed in term of departures per year or annual departures. A detailed 

discussion of traffic analysis is provided in Appendix 1. 

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4.4 LIMITATIONS OF THE PCN. The PCN value is for reporting relative pavement strength 

only and should not be used for pavement design or as a substitute for evaluation. Pavement 

design and evaluation are complex engineering problems that require detailed analyses. They 

cannot be reduced to a single number. The PCN rating system uses a continuous scale to 

compare pavement strength where higher values represent pavements with larger load capacity. 

4.5 REPORTING THE PCN. The PCN system uses a coded format to maximize the amount of 

information contained in a minimum number of characters and to facilitate computerization. The 

PCN for a pavement is reported as a five-part number where the following codes are ordered and 

separated by forward slashes. 

 Numerical PCN value 

 Pavement type, 

 Subgrade category, 

 Allowable tire pressure, and 

 Method used to determine the PCN. 

An example of a PCN code is 80/R/B/W/T, which is further explained in paragraph 4.5.f. 

a. Numerical PCN Value. The PCN numerical value is a relative indication of the 

load-carrying capacity of a pavement in terms of a standard single wheel load at a tire pressure of 

181 psi (1.25 MPa). The PCN value should be reported in whole numbers, rounding off any 

fractional parts to the nearest whole number. For pavements of diverse strengths, the controlling 

PCN numerical value for the weakest segment of the pavement should normally be reported as 

the strength of the pavement. Engineering judgment may be required here in that if the weakest 

segment is not in the most heavily used part of the runway, then another representative segment 

may be more appropriate to determine PCN. 

 b. Pavement Type. For the purpose of reporting PCN values, pavement types are 

considered to function as either flexible or rigid structures. Table 4-1 lists the pavement codes for 

the purposes of reporting PCN. 

Table 4-1. Pavement Codes for Reporting PCN 

Pavement Type Pavement Code 

Flexible F 

Rigid R 

 

 i) Flexible Pavement. Flexible pavements support loads through bearing 

rather than flexural action. They comprise several layers of selected materials designed to 

gradually distribute loads from the surface to the layers beneath. The design ensures that load 

transmitted to each successive layer does not exceed the layer’s load-bearing capacity. 

 ii) Rigid Pavement. Rigid pavements employ a single structural layer, 

which is very stiff or rigid in nature, to support the pavement loads. The rigidity of the structural 

 11 
AC 150/5335-5A 09/28/06 

layer and resulting beam action enable rigid pavement to distribute loads over a large area of the 

subgrade. The load-carrying capacity of a rigid structure is highly dependent upon the strength of 

the structural layer, which relies on uniform support from the layers beneath. 

 iii) Composite Pavement. Various combinations of pavement types and 

stabilized layers can result in complex pavements that could be classified as between rigid or 

flexible. A pavement section may comprise multiple structural elements representative of both 

rigid and flexible pavements. Composite pavements are most often the result of pavement surface 

overlays applied at various stages in the life of the pavement structure. If a pavement is of 

composite construction, the pavement type should be reported as the type that most accurately 

reflects the structural behavior of the pavement. The method used in computing the PCN is the 

best guide in determining how to report the pavement type. For example, if a runway is 

composed of a rigid pavement with a bituminous overlay, the usual manner of determining the 

load-carrying capacity is to convert the pavement to an equivalent thickness of rigid pavement. In 

this instance, the pavement type should be reported as a rigid structure. A general guideline is 

that when the bituminous overlay reaches 75 to 100 percent of the rigid pavement thickness the 

pavement can be considered as a flexible pavement. It is permissible to include a note stating that 

the pavement is of composite construction but only the rating type, “R” or “F”, is used in the 

assessment of the pavement load capacity. 

 c. Subgrade Strength Category. As discussed in Paragraph 2-1, there are four 

standard subgrade strengths identified for calculating and reporting ACN or PCN values. The 

values for rigid and flexible pavements are reported in Tables 2-1 and 2-2. 

 d. Allowable Tire Pressure. Table 4-2 lists the allowable tire pressure categories 

identified by the ACN-PCN system. The tire pressure codes apply equally to rigid or flexible 

pavement sections; however, the application of the allowable tire pressure differs substantially for 

rigid and flexible pavements. 

TABLE 4-2. Tire Pressure Codes for Reporting PCN 

Category Code Tire Pressure Range 

High W No pressure limit 

Medium X Pressure limited to 218 psi (1.5 MPa) 

Low Y Pressure limited to 145 psi (1.00 MPa) 

Very Low Z Pressure limited to 73 psi (0.50 MPa) 

 i) Tire Pressures on Rigid Pavements. Airplane tire pressure will have 

little effect on pavements with Portland cement concrete surfaces. Rigid pavements are 

inherently strong enough to resist tire pressures higher than currently used by commercial 

airplanes and can usually be rated as code W. 

 ii) Tire Pressures on Flexible Pavements. Tire Pressures may be restricted 

on asphaltic concrete, depending upon the quality of the asphalt mixture and climatic conditions. 

Tire pressure effects on an asphalt layer relate to the stability of the mix in resisting shearing or 

densification. A poorly constructed asphalt pavement can be subject to rutting due to 

consolidation under load. The principal concern in resisting tire pressure effects is with stability 

12 
09/28/06 AC 150/5335-5A 

or shear resistance of lower quality mixtures. A properly prepared and placed mixture that 

conforms to FAA specification Item P-401 can withstand substantial tire pressure in excess of 

218 psi (1.5 Mpa). Item P-401, Plant Mix Bituminous Pavements, is provided in AC 150/5370-

10B, Standards for Specifying Construction of Airports. Improperly prepared and placed 

mixtures can show distress under tire pressures of 100 psi (0.7 MPa) or less. Although these 

effects are independent of the asphalt layer thickness, pavements with well-placed asphalt of 4 to 

5 inches (10.2 to 12.7 cm) in thickness can generally be rated with code X or W, while thinner 

pavement of poorer quality asphalt should not be rated above code Y. 

 e. Method Used to Determine PCN. Two pavement evaluation methods are 

recognized in the PCN system. If the evaluation represents the results of a technical study, the 

evaluation method should be coded T. If the evaluation is based on “using airplane” experience, 

the evaluation method should be coded U. Technical evaluation implies that some form of 

technical study and computation were involved in the determination of the PCN. Using airplane 

evaluation means the PCN was determined by selecting the highest ACN among the airplanes 

currently using the facility and not causing pavement distress. PCN values computed by the 

technical evaluation method should be reported to the NASR database and shown in the FAA 

Form 5010, Airport Master Record. Publication of a using airplane evaluation in the Airport 

Master Record, Form 5010, and the NASR database is permitted only by mutual agreement 

between the airport owner and FAA. 

 f. Example PCN Reporting. An example of a PCN code is 80/R/B/W/T—with 80 

expressing the PCN numerical value, R for rigid pavement, B for medium strength subgrade, W 

for high allowable tire pressure, and T for a PCN value obtained by a technical evaluation. 

 g. Report PCN Values to the FAA. Once a PCN value and the coded entries are 

determined, the PCN code should be reported to the appropriate regional FAA Airports Division, 

either in writing or as part of the annual update to the Airport Master Record, FAA Form 5010-l. 

The regional office will forward the PCN code to FAA headquarters where it will be disseminated 

by the National Flight Data Center through aeronautical publications such as the Airport/Facility 

Directory (AFD) and the Aeronautical Information Publication (AIP). An airplane's ACN can 

then be compared with the published PCN to determine if the airplane can safely operate on the 

airport’s runways, subject to any limitation on tire pressure. 

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14 
09/28/06 AC 150/5335-5A - APPENDIX 1 

APPENDIX 1. EQUIVALENT TRAFFIC 

1.0 EQUIVALENT TRAFFIC. A detailed method, based on the procedures originally 

introduced in AC 150/5320-6C (outdated version) is presented to allow the calculation of the 

combined effect of multiple airplanes in the traffic mix for an airport. This combined traffic is 

brought together into the equivalent traffic of a critical airplane. This is necessary in that the 

procedure used to calculate ACN allows only one airplane at a time. By combining all of the 

airplanes in the traffic mix into an equivalent critical airplane, calculation of a PCN that includes 

the effects of all traffic becomes possible. It is recognized that there are other methods of 

determining equivalent traffic. However, the method described here has been developed and used 

over a period of years by FAA. 

The assessment of equivalent traffic, as described in this section, is needed only in the process of 

determining PCN using the technical method and may be disregarded when the using airplane 

method is employed. 

 

In order to arrive at a technically derived PCN, it is necessary to determine the maximum 

allowable or commonly sustained gross weight of the critical airplane (i.e. the allowable weight 

on a given landing gear configuration). This in turn requires that the pavement design and 

airplane loading characteristics be examined in detail. Consequently, the information presented 

in this appendix appears at first to apply to pavement design rather than a PCN determination. 

However, with this knowledge in hand, an engineer will be able to arrive at a PCN that will have 

a solid technical foundation. 

1.1 EQUIVALENT TRAFFIC TERMINOLOGY. In order to determine a PCN, as based on 

the technical evaluation method, it is necessary to define common terms used in airplane traffic 

and pavement loading. The terms arrival, departure, pass, coverage, load repetition, operation, 

and traffic cycle are often used interchangeably by different organizations when determining the 

effect of airplane traffic operating on a runway. It is important not only to determine which of the 

airplane movements need be counted when considering pavement stress, but how these terms 

apply in relation to the pavement design and evaluation process. In general, and for the purpose 

of this document, they are differentiated as follows: 

a. Arrival (Landing) and Departure (Takeoff). Typically, airplanes arrive at an 

airport with a lower amount of fuel than is used at takeoff. As a consequence, the stress loading 

of the wheels on the runway pavement is less when landing than at takeoff due to the lower 

weight. This is true even at the touchdown impact in that there is still lift on the wings, which 

alleviates the dynamic vertical force. Because of this, the FAA pavement design procedure only 

considers departures and ignores the arrival traffic count. However, if the airplanes do not 

receive additional fuel at the airport, then the landing weight will be substantially the same as the 

takeoff weight (discounting the changes in passenger count and cargo), and the landing operation 

should be counted as a takeoff for pavement stress loading cycles. In this latter scenario, there are 

two equal load stresses on the pavement for each traffic count (departure), rather than just one. 

Regardless of the method of counting load stresses, a traffic cycle is defined as one takeoff and 

one landing of the same airplane, subject to a further refinement of the definition in the following 

text. 

 A1-1 
AC 150/5335-5A - APPENDIX 1 09/28/06 

b. Pass. A pass is a one-time transaction of the airplane over the runway pavement. 

It could be an arrival, a departure, a taxi operation, or all three, depending on the loading 

magnitude and the location of the taxiways. Figure A1-1 shows typical traffic patterns for 

runways having either parallel taxiways or central taxiways. A parallel taxiway requires that 

none or very little of the runway be used as part of the taxi movement. A central taxiway requires 

that a large portion of the runway be used during the taxi movement. 

Landing

Takeoff

Figure A1-1a. Runway with Parallel Taxiway

Takeoff

Landing

Taxi Taxi

Figure A1-1b. Runway with Central Taxiway 

Figure A1-1. Traffic Load Distribution Patterns 

 i) Parallel Taxiway Scenario. In the case of the parallel taxiway, as shown 

in Figure A1-1a, two possible loading situations can occur. Both of these situations assume that 

the passenger count and cargo payload are approximately the same for the entire landing and 

takeoff cycle: 

 1) If the airplane obtains fuel at the airport, then a traffic cycle consists 

of only one pass since the landing stress loading is considered at a reduced level, which is a 

fractional equivalence. For this condition only the takeoff pass is counted, and the ratio of passes 

to traffic cycles (P/TC) is 1. 

 2) If the airplane does not obtain fuel at the airport, then both landing 

and takeoff passes should be counted, and a traffic cycle consists of two passes of equal load 

stress. In this case, the P/TC ratio is 2. 

A1-2 
09/28/06 AC 150/5335-5A - APPENDIX 1 

 ii) Central Taxiway Scenario. For a central taxiway configuration, as shown 

in Figure A1-1b, there are also two possible loading situations that can occur. As was done for 

the parallel taxiway condition, both of these situations assume that the payload is approximately 

the same for the entire landing and takeoff cycle: 

 1) If the airplane obtains fuel at the airport, then both the takeoff and 

taxi to takeoff passes should be counted since they result in a traffic cycle consisting of two 

passes at the maximum load stress. The landing pass can be ignored in this case. It is recognized 

that only part of the runway is used during some of these operations, but it is conservative to 

assume that the entire runway is covered each time a pass occurs. For this situation, the P/TC 

ratio is 2. 

 2) If the airplane does not obtain fuel at the airport, then both the 

landing and takeoff passes should be counted, along with the taxi pass, and a traffic cycle consists 

of three passes at loads of equal magnitude. In this case, the P/TC ratio is 3. 

iii) A simplified, but less conservative, approach would be use a P/TC ratio of 

1 for all situations. Since a landing and a takeoff only apply full load to perhaps the end third of 

the runway (opposite ends for no shift in wind direction), this less conservative approach could be 

used to count one pass for both landing and takeoff. However, the FAA recommends conducting 

airport evaluations on the conservative side, which is to assume any one of the passes covers the 

entire runway. 

c. Coverage. When an airplane moves along a runway, it seldom travels in a 

perfectly straight line or over the exact same wheel path as before. It will wander on the runway 

with a statistically normal distribution. One coverage occurs when a unit area of the runway has 

been traversed by a wheel of the airplane main gear. Due to random wander, this unit area may 

not be covered by the wheel every time the airplane is on the runway. The number of passes 

required to statistically cover the unit area one time on the pavement is expressed by the pass to 

coverage (P/C). 

Although the terms coverage and P/C ratio have commonly been applied to both flexible and rigid 

pavements, the P/C ratio has a slightly different meaning when applied to flexible pavements as 

opposed to rigid pavements. This is due to the manner in which flexible and rigid pavements are 

considered to react to various types of gear configurations. For gear configurations with wheels 

in tandem, such as dual tandem (2D) and triple dual tandem (3D), the ratios are different for 

flexible and rigid pavements, and using the same term for both types of pavements may become 

confusing. It is incumbent upon the user to select the proper value for flexible and rigid 

pavements. 

Aircraft passes can be determined (counted) by observation but coverages are used by the 

COMFAA program. The P/C ratio is necessary to convert passes to coverages for use in the 

program. The P/C ratio for any airplane can be determined from the COMFAA program. This 

ratio is different for each airplane because of the different number of wheels, main gear 

configurations, tire contact areas, and load on the gear. Although the ratio will change slightly for 

each airplane when the tire contact area varies due to changes in applied load, for the purposes of 

this document, the P/C ratio will be reported as the ratio obtained from the COMFAA program in 

pavement thickness mode. In the pavement thickness mode the P/C is based upon the 

 A1-3 
AC 150/5335-5A - APPENDIX 1 09/28/06 

manufacturer’s recommended tire deflection at 95 percent of the gross load on the main landing 

gear(s). 

The P/C ratios for gears with wheels in tandem are different for flexible and rigid pavement. This 

difference occurs because of the method in which the flexible and rigid pavements are assumed to 

handle stress. It is assumed that the flexible pavement loading pattern has a series of stress peaks, 

depending on the number of wheels in tandem, while a rigid pavement acts as a single deflecting 

plate, with only one stress peak per group of wheels. Generally, a single- or dual-gear 

arrangement will provide only one load stress per pass, regardless of the pavement type, since 

there is only one set of wheels traversing a given place on the pavement. However, a dual tandem 

gear stresses a flexible pavement twice since there are two repetitions of the load on flexible 

pavement, and it stresses a rigid pavement once due to the effect of only one stress loading per 

group of wheels. Likewise, a triple dual tandem gear stresses the flexible pavement three times 

but only one time for rigid pavement. Gear configurations with tandem spacing exceeding 72 

inches (182 cm) are treated as individual load peaks for flexible and rigid pavements in the 

COMFAA program. 

d. Operation. The meaning of this term is unclear when used in pavement design or 

evaluation. It could mean a departure at full load or a landing at minimal load. It is often used 

interchangeably with pass or traffic cycle. When this description of an airplane activity is used, 

additional information should be supplied. It is usually preferable to use the more precise terms 

described in this section. 

e. Traffic Cycle and Traffic Cycle Ratio. As has been discussed, a traffic cycle can 

include a landing pass, a takeoff pass, a taxi pass or all three. For pavement design or evaluation, 

the ratio of traffic cycles to coverages (TC/C) in flexible pavement, rather than passes to 

coverages, is required since there could be one or more passes per traffic cycle. When only one 

pass on the operating surface is assumed for each traffic count, then the P/C ratio is sufficient. 

However, when situations are encountered where more than one pass is considered to occur 

during the landing to takeoff cycle, then the TC/C ratio is necessary in order to properly account 

for the effects of all of the traffic. These situations occur most often when there are central 

taxiways or fuel is not obtained at the airport. 

Equation A1-1 translates the P/C ratio to the TC/C ratio for flexible and rigid pavements by 

including the previously described ratio of passes to traffic cycles (P/TC): 

÷= TCPCPCTC (Equation A1-1) 

 Where: 

 TC = Traffic Cycles 

 C = Coverages 

 P = Passes 

Determination of the TC/C ratio can best be illustrated by examples. Table A1-1 shows typical 

ratios for flexible pavement runways for situations in which fuel is not obtained at the airport. 

Typical values of the P/C ratio are shown in this table, but different ratios can be substituted for 

other airplanes. Refer to Figure A1-1 for guidance in determining the number of passes used for 

A1-4 
09/28/06 AC 150/5335-5A - APPENDIX 1 

each traffic count. Note that the number of traffic cycles to complete one coverage is reduced 

considerably for a runway with a central taxiway, as opposed to one with a parallel taxiway. The 

effect of this is that a runway with a central taxiway will experience more load stresses for each 

traffic count than one with a parallel taxiway. 

Table A1-1. TC/C Ratio for Flexible Pavements – Additional Fuel Not Obtained 

Taxiway Type 

Typical Dual 

Gear (D) 

Typical Dual 

Tandem Gear (2D) 

Typical Triple Dual 

Tandem Gear (3D) 

P/C 3.6 1.8 1.4 

P/TC - Parallel 2 2 2 

P/TC - Central 3 3 3 

TC/C - Parallel 1.8 0.9 0.7 

TC/C - Central 1.2 0.6 0.5 

 

Table A1-2 shows the same information for a situation in which additional fuel is obtained at the 

airport. From a comparison of these two tables, it is seen that for a runway having a central 

taxiway where fuel is not obtained at the airport, there are more traffic cycles than for a runway in 

which a parallel taxiway exists and fuel is obtained at the airport. For example, the typical dual 

gear TC/C for a central taxiway indicated in Table A1-1 is 1.2 compared with that of 3.6 for the 

parallel taxiway indicated in Table A1-2, resulting in three times the number of passes for each 

traffic count. Additionally, as the number of wheels increases, the TC/C ratio decreases, 

regardless of the taxiway configuration. The effect of this is that there are more loading cycles in 

terms of coverages per traffic count on flexible pavement with the increased number of wheels. 

Table A1-2. TC/C Ratio for Flexible Pavements – Additional Fuel Obtained 

Taxiway Type 

Typical Dual 

Gear (D) 

Typical Dual 

Tandem Gear (2D) 

Typical Triple Dual 

Tandem Gear (3D) 

P/C 3.6 1.8 1.4 

P/TC - Parallel 1 1 1 

P/TC - Central 2 2 2 

TC/C - Parallel 3.6 1.8 1.4 

TC/C - Central 1.8 0.9 0.7 

Table A1-3 shows typical ratios for rigid pavements for situations in which fuel is not obtained at 

the airport, while Table A1-4 shows the same information for cases in which additional fuel is 

obtained at the airport. The same comparison as above is seen in which a different number of 

traffic cycles occur between the runways with differing taxiway configurations. However, unlike 

the flexible pavement example, the ratio of traffic cycles to load stress is not very sensitive to 

gear configuration. For example, in Tables A1-3 and A1-4, both the dual and dual-tandem gears 

have the same TC/C ratio, while the triple dual tandem gear is only slightly different. The effect 

of this is that for the same taxiway type and fuel-loading situation, the level of load repetitions per 

traffic cycle on rigid pavement is virtually the same, regardless of the gear configuration. 

 A1-5 
AC 150/5335-5A - APPENDIX 1 09/28/06 

Table A1-3. TC/C Ratio for Rigid Pavements – Additional Fuel Not Obtained 

Taxiway Type 

Typical Dual 

Gear (D) 

Typical Dual 

Tandem Gear (2D) 

Typical Triple Dual 

Tandem Gear (3D) 

P/C 3.6 3.6 4.2 

P/TC - Parallel 2 2 2 

P/TC - Central 3 3 3 

TC/C - Parallel 1.8 1.8 2.1 

TC/C - Central 1.2 1.2 1.4 

Table A1-4. TC/C Ratio for Rigid Pavements – Additional Fuel Obtained 

Taxiway Type 

Typical Dual 

Gear (D) 

Typical Dual 

Tandem Gear (2D) 

Typical Triple Dual 

Tandem Gear (3D) 

P/C 3.6 3.6 4.2 

P/TC - Parallel 1 1 1 

P/TC - Central 2 2 2 

TC/C - Parallel 3.6 3.6 4.2 

TC/C - Central 1.8 1.8 2.1 

1.2 EQUIVALENT TRAFFIC BASED ON GEAR TYPE. In order to complete the 

equivalent traffic calculation, all other significant airplanes in the traffic mix must first be 

converted to a critical airplane in terms of gear type and traffic cycles so this other traffic is 

accounted for in the overall pavement design life. Then, the converted gear types must be in turn 

converted to a critical airplane equivalent in terms of load magnitude. The critical airplane is an 

airplane that regularly uses the pavement and that has the greatest thickness requirements, based 

on its individual operational characteristics. 

Table A1-5 provides conversion factors for common gear configurations that are used to convert 

a given gear type to that of the critical airplane. After this conversion, each airplane in the traffic 

mix, and its corresponding traffic cycles, will be represented by the same gear configuration as 

the critical airplane. 

A1-6 
09/28/06 AC 150/5335-5A - APPENDIX 1 

Table A1-5. Conversion Factors to Convert from One Landing Gear Type to Another 

To Convert From 

Gear Type (N) 

To Gear 

Type (M) 

Multiply Traffic 

Cycles By 

S D 0.80 

S 2D 0.51 

S 3D 0.33 

D S 1.25 

D 2D 0.64 

D 3D 0.41 

2D S 1.95 

2D D 1.56 

2D 3D 0.64 

3D S 3.05 

3D D 2.44 

3D 2D 1.56 

2D/2D2 D 1.56 

2D/2D2 2D 1.00 

The general equation for this conversion is: 

 (Equation A1-2) ( ) NM 8.0 −

 Where: 

M = the number of wheels on the critical airplane main gear. 

N = the number of wheels on the converted airplane gear. 

Tables A1-6 and A1-7 provide examples of the use of gear configuration conversion factors Table 

A1-6 shows the gear equivalencies for a dual tandem (2D) gear in a sample traffic mix, while 

Table A1-7 shows the same gear equivalencies for a dual gear (D). The equivalent traffic cycles 

totals are shown for comparison purposes only and are not necessary for critical airplane 

calculations. It can be seen from a comparison of these totals that the selection of the critical 

airplane is very important for the overall evaluation process in that an incorrect selection leads to 

an erroneous number of equivalent traffic cycles. This is evident in Table A1-6 where the overall 

total of annual traffic cycles is 15,200, compared with the total equivalent dual tandem traffic 

cycles of 12,632, whereas in Table A1-7, the total equivalent dual traffic cycles is 19,720. 

 A1-7 
AC 150/5335-5A - APPENDIX 1 09/28/06 

Table A1-6. Equivalency Conversion to a Dual Tandem (2D) Gear Type 

Airplane 

Gear 

Type 

Annual 

Traffic 

Cycles (TC) 

Conversion 

Factor 

Total 

Equivalent 

(2D) TC 

727-200 D 400 0.64 256 

737-300 D 6,000 0.64 3840 

A319-100 D 1,200 0.64 768 

B747-400 2D/2D2 3,000 1.0 3,000 

B767-200ER 2D 2,000 1.0 2,000 

DC8-63 2D 800 1.0 800 

A300-B4 2D 1,500 1.0 1,500 

B777-200 3D 300 1.56 468 

15,200 12,632 

 

Table A1-7. Equivalency Conversion to a Dual Gear (D) Type 

Airplane 

Gear 

Type 

Annual 

Traffic 

Cycles (TC) 

Conversion 

Factor 

Total 

Equivalent 

(D) TC 

727-200 D 400 1.0 400 

737-300 D 6,000 1.0 6,000 

A319-100 D 1,200 1.0 1,200 

B747-400 2D/2D2 3,000 1.56 4,680 

B767-200ER 2D 2,000 1.56 3,120 

DC8-63 2D 800 1.56 1,248 

A300-B4 2D 1,500 1.56 2,340 

B777-200 3D 300 2.44 732 

15,200 19,720 

A1-8 
09/28/06 AC 150/5335-5A - APPENDIX 1 

1.3 EQUIVALENT TRAFFIC BASED ON LOAD MAGNITUDE. After the airplanes have 

been grouped into the same gear configuration, it is necessary to determine the total equivalent 

traffic cycles of each airplane in terms of the critical airplane as based on the relative load 

magnitude. As for the gear type conversion procedure (Paragraph 1.2), this step requires a 

previously selected critical airplane. 

When computing equivalent traffic cycles of the critical airplane based on load magnitude, there 

are several simplifying rules to use: 

 • For the purpose of equivalent traffic cycle calculations, it is generally sufficient to use 

single wheel loads based on 95 percent of gross airplane weight on the main gear. 

 • Since it is difficult to determine current or projected operational weights, the FAA 

recommends using maximum taxiing gross weights of each airplane for this calculation. 

After gear types for the airplanes of the traffic mix are converted to that of the critical airplane 

(Paragraph 1.2), the traffic cycles of each airplane must be converted to equivalent traffic cycles 

of the critical airplane. This conversion addresses the effect of wheel load magnitude and may 

be calculated by applying Equation A1-3: 

W

W R Log R Log

1

2

1 2 ×= or ( ) 12

21

WW = RR (Equation A1-3) 

 Where: 

 R1 = Equivalent traffic cycles of the critical airplane 

 R2 = Traffic cycles of a given airplane expressed 

 in terms of the critical airplane landing gear 

 W1 = Wheel load of the critical airplane 

 W2 = Wheel load of the airplane in question 

Table A1-8 shows how the above calculations are combined to determine the equivalent traffic 

cycles of the critical airplane. For this example, assume that the B747-400 is the critical airplane. 

It can be seen that the original 3,000 annual traffic cycles of the B747-400 have increased to an 

equivalent 7,692 due to the combined effect of the other airplanes in the traffic mix. The R2

column is from Table A1-6. 

 A1-9 
AC 150/5335-5A - APPENDIX 1 09/28/06 

Table A1-8. Equivalent Traffic Cycles Based on Load Magnitude 

Airplane 

 

Operating 

Weight lb 

(W2) 

Single Wheel 

Load, lb 

(R2) 

 (2D) TC

(W2/W1)

1/2

Wheel 

Load 

Ratio 

(R1) 

Equivalent 

B747-400 

TC 

727-200 185,000 43,938 256 0.95 194 

737-300 130,000 30,875 3,840 0.796 716 

A319-100 145,000 34,438 768 0.841 268 

B747-400 820,000 48,688 (W1) 3,000 1.000 3,000 (Critical Airplane) 

B767-200ER 370,000 43,938 2,000 0.950 1,368 

DC8-63 330,000 39,188 800 0.897 403 

A300-B4 370,000 43,938 1,500 0.950 1,041 

B777-200 600,000 47,500 468 0.988 434 

12,632 7,424 

Note that a sensitive factor in this table is the single wheel load and its ratio to the critical airplane 

single wheel load. Any changes in the single wheel load magnitude are reflected in the wheel 

load ratio, which is used as an exponent in the calculation of equivalent traffic cycles. For 

example, the 727-200 equivalent traffic is shown to decrease from 256 to 194. Alternately, the 

B777-200 equivalent traffic decreases from 468 to 434 due to the relative magnitude of the single 

wheel loads. 

A1-10 
09/28/06 AC 150/5335-5A - APPENDIX 2 

APPENDIX 2. PCN DETERMINATION EXAMPLES 

1.0. THE USING AIRPLANE METHOD. The using airplane method of determining PCN is 

presented in the following steps. This procedure can be used when there is limited knowledge of 

the existing traffic and runway characteristics. It is also useful when engineering analysis is 

neither possible nor desired. Airport authorities should be more careful in the application of a 

using airplane PCN in that the rating has not been rigorously determined. 

There are two basic steps required to arrive at a using airplane PCN: 

• Determine the airplane with the highest ACN in the traffic mix currently using the 

runway. This is the critical airplane. 

• Assign the ACN of the critical airplane as the PCN. 

These steps are explained in greater detail: 

1. Assign the pavement surface type as code F or R. 

2. From available records, determine the average strength of the pavement subgrade. If the 

subgrade strength is not known, make a judgment of High, Medium, Low, or Ultra Low. 

3. Determine which airplane has the highest ACN from a list of airplanes that are presently using 

the runway, based on the surface type code assigned in Step 1 and the subgrade code in Step 

2. ACN values may be determined from the COMFAA program or from ACN graphs found 

in the manufacturer’s published ACAP manuals. Use the same subgrade code for each of the 

airplanes when determining the maximum ACN. Base ACNs on the highest operating weight 

of the airplanes at the airport if the data is available; otherwise, use an estimate or the 

published maximum allowable gross weight of the airplane in question. The airplane with the 

highest ACN that regularly uses the pavement, is the critical airplane. 

4. The PCN is simply the ACN of the critical airplane, with appropriate tire pressure and 

evaluation codes added. The numerical value of the PCN may be adjusted up or down at the 

preference of the airport authority. Reasons for adjustment include local restrictions, 

allowances for certain airplanes, or pavement conditions. 

5. The tire pressure code (W, X, Y, or Z) should represent the highest tire pressure of the 

airplane fleet currently using the runway. For flexible pavements, code X should be used if 

no higher tire pressure is evident from among the existing traffic. It is commonly understood 

that concrete can tolerate substantially higher tire pressures, so the rigid pavement rating 

should normally be given as W. 

6. The evaluation method for the using airplane method is reported as U.

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AC 150/5335-5A - APPENDIX 2 09/28/06 

1.1 USING AIRPLANE EXAMPLE FOR FLEXIBLE PAVEMENTS. The following 

example illustrates the using airplane PCN process for flexible pavements: 

An airport has a flexible (asphalt-surfaced) pavement runway with a subgrade strength of CBR 9 

and traffic having the operating gross weights and ACNs shown in Table A2-1. 

Table A2-1. Using Airplane and Traffic for a Flexible Pavement 

 

Airplane 

Operating 

Weight, lbs 

Tire Pressure 

(psi) 

% Gross 

Weight on 

Main Gear 

for ACN 

ACN 

F/B 

Annual 

Departures 

B727-200 185,000 148 96.00 48 400 

B737-300 130,000 195 90.86 32 6,000 

A319-100 145,000 196 92.60 34 1,200 

B747-400 820,000 200 93.32 59 3,000 

B767-300ER 370,000 190 92.40 50 2,000 

DC8-63 330,000 194 96.12 53 800 

A300-B4 370,000 205 94.00 57 1,500 

B777-200 600,000 215 95.42 52 300 

• Since this is a flexible pavement, the pavement type code is F, (Table 4-1). 

• The subgrade strength under the pavement is CBR 9, or Medium category, so the 

appropriate code is B (Table 2-2). 

• The highest tire pressure of any airplane in the traffic mix is 215 psi, so the tire 

pressure code is X (Table 4-2). 

• From the above list, the critical airplane is the B747-400, because it has the highest 

ACN of the group at the operational weights shown (59/F/B). Additionally, it has 

regular service as compared to the rest of the traffic, which qualifies it as a possible 

critical airplane. 

• Since there was no engineering analysis done in this example, and the rating was 

determined simply by examination of the current airplanes using the runway, the 

evaluation code from Paragraph 4.5e is U. 

• Based on the results of the previous steps, the pavement should tentatively be rated as 

PCN 59/F/B/X/U, assuming that the pavement is performing satisfactorily under the 

current traffic. 

If the pavement shows obvious signs of distress, this rating may need to be adjusted downward at 

the discretion of the airport authority. If the rating is lowered, then one or more of the airplanes 

will have ACNs that exceed the assigned rating. This may require the airport to restrict the 

A2-2 
09/28/06 AC 150/5335-5A - APPENDIX 2 

allowable gross weight for those airplanes or consider pavement strengthening. The rating could 

also be adjusted upward, depending on the performance of the pavement under the current traffic. 

1.2 USING AIRPLANE EXAMPLE FOR RIGID PAVEMENTS. The following example 

illustrates the using airplane PCN process for rigid pavements: 

An airport has a rigid (concrete-surfaced) pavement runway with a subgrade modulus strength of 

k=200 pci and traffic having the operating gross weights and ACNs shown in Table A2-2. 

Table A2-2. Using Airplane and Traffic for a Rigid Pavement 

Airplane 

Operating 

Weight, lbs 

Tire Pressure 

(psi) 

% Gross 

Weight on 

Main Gear 

for ACN 

ACN 

R/C 

Annual 

Departures 

B727-200 185,000 148 96.00 56 400 

B737-300 130,000 195 90.86 38 6,000 

A319-100 145,000 196 92.60 42 1,200 

B747-400 820,000 200 93.32 68 3,000 

B767-300ER 370,000 190 92.40 58 2,000 

DC8-63 330,000 194 96.12 62 800 

A300-B4 370,000 205 94.00 67 1,500 

B777-200 600,000 215 95.42 77 300 

• Since this is a rigid pavement, the pavement type code is R, (Table 4-1). 

• The subgrade strength under the pavement is k=200 pci, which is Low category, so the 

appropriate code is C (Table 2-1). 

• The highest tire pressure of any airplane in the traffic mix is 215 psi, so the tire 

pressure code is X, as found in Table 4-2. However, since concrete can normally 

tolerate substantially higher tire pressures, the code W should be assigned. 

• From the above list, the critical airplane is the B777-200, because it has the highest 

ACN of the group at the operational weights shown (77/R/C). However, the critical 

airplane could also be the A300-B4 at ACN 67/R/C or the B747-400 at ACN 68/R/C 

since these airplanes have higher frequencies than the B777-200. 

• Since there was no engineering analysis done in this example, and the rating was 

determined simply by examination of the current airplanes using the runway, the 

evaluation code from Paragraph 4.5e is U. 

• Based on these steps, the pavement should tentatively be rated as PCN 77/R/C/W/U in 

order to accommodate all of the current traffic. 

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AC 150/5335-5A - APPENDIX 2 09/28/06 

• If the pavement shows obvious signs of distress, this rating may need to be adjusted 

downward at the discretion of the airport authority. If the rating is lowered, then one 

or more of the airplanes will have ACNs that exceed the assigned rating. This may 

require the airport to restrict the allowable gross weight for those airplanes or 

consideration of pavement strengthening. The rating could also be adjusted upward, 

depending on the performance of the pavement under the current traffic. 

2.0 THE TECHNICAL EVALUATION METHOD. The technical evaluation method of 

determining PCN should be used when there is reliable knowledge of the existing traffic and 

pavement characteristics. Although the technical evaluation provides a good representation of 

existing conditions, the airport authority should still be somewhat flexible in its application since 

there are many variables in the pavement structure as well as the method of analysis itself. The 

objective of the technical method is to determine a critical airplane allowable gross weight in 

order to assess the PCN. 

2.1 TECHNICAL EVALUATION FOR FLEXIBLE PAVEMENTS. The following list 

summarizes the steps for using the technical evaluation method for flexible pavements: 

• Determine the traffic volume in terms of type of airplane and number of operations of 

each airplane that the pavement will experience over its life. 

• Convert that traffic into a single critical airplane equivalent. 

• Determine pavement characteristics, including the subgrade CBR and pavement thickness. 

• Calculate the maximum allowable gross weight of the critical airplane on that pavement. 

• Calculate the ACN of the critical airplane at its maximum allowable gross weight. 

• Assign the PCN to be the ACN of the critical airplane. 

These steps are explained in greater detail below. Several examples at the end of this section 

further illustrate the process. 

1. Determine the traffic volume in terms of traffic cycles for each airplane that has used or is 

planned to use the airport during the pavement life period. Record all significant traffic, 

including non-scheduled, charter, and military, as accurately as possible. This includes 

traffic that has occurred since the original construction or last overlay and traffic that will 

occur before the next planned overlay or reconstruction. If the pavement life is unknown 

or undetermined, assume that it will include a reasonable period of time. The normal 

design life for pavement is 20 years. However, the expected life can vary depending on 

the existing pavement conditions, climatic conditions, and maintenance practices. 

The information necessary for the traffic volume process is— 

 Past, current, and forecasted traffic cycles of each significant airplane. 

 Operational or maximum gross weights. 

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09/28/06 AC 150/5335-5A - APPENDIX 2 

 Typical airplane weight distribution on the main and nose gear. If unknown, AC 

150/5320-6 assumes 95 percent weight on the main gear. 

 Main gear type (dual, dual tandem, etc.). 

 Main gear tire pressure. 

 The pass-to-coverage (P/C) ratio of each airplane that might be considered 

as the critical airplane. 

 Fuel-loading practices of airplanes at the airport. 

 Type of taxiway system – parallel or central. 

2. Determine which airplane in the traffic mix from step 1 is critical or the most significant. 

This is required because the ACN procedure implemented in the COMFAA program is 

able to accommodate only one airplane at a time. The critical airplane is the one that has 

the greatest pavement thickness requirements based on its individual gross weight, traffic 

volume, P/C ratio, and tire pressure; and it is not necessarily the one with the highest ACN 

or the highest gross weight. 

3. The COMFAA program calculates pavement thickness requirements based on coverages 

rather than traffic cycles or passes. It is therefore a requirement to convert these types of 

frequencies to coverages by using a pass-to-coverage ratio. Airplane-specific P/C ratios 

on flexible pavement can be calculated in the COMFAA program. 

4. Using the conversion factors in Table A1-5, convert the traffic volume of each airplane in 

the traffic mix to the critical airplane equivalent based on gear configuration differences. 

For example, if the critical airplane has a dual tandem gear, then all single wheel, dual 

wheel, and triple dual tandem wheel gears need to be converted into the dual tandem gear 

equivalent. 

5. Determine the critical airplane equivalent traffic cycles based on the single wheel load 

magnitude of each airplane in the traffic mix. These calculations should be based on 

Equation A1-3. 

6. Calculate the critical airplane TC/C ratio from Equation A1-1 for the type of taxiway and 

the fuel-loading method. This will allow the COMFAA program to determine coverages 

from the critical airplane equivalent traffic cycles determined in Step 5. 

7. From field data or construction drawings, document the average CBR of the subgrade soil. 

Alternatively, conduct field or laboratory tests of the subgrade soil in order to determine 

the CBR. Accurate portrayal of the subgrade CBR value is vital to the technical method 

because a small variation in CBR could result in a disproportionately large variation in the 

critical airplane allowable gross weight and the corresponding PCN. 

8. Determine the total pavement thickness and cross-sectional properties. The thickness of 

the pavement section under consideration must be referenced to a standard pavement 

section for evaluation purposes. The standard section is the total thickness requirement 

calculated by the COMFAA program assuming minimum layer thickness for the asphalt 

surface, minimum base layer thickness of material with a CBR 80 or higher, and a 

variable subbase layer with a CBR 20 or greater. If the pavement has excess material or 

improved materials, the total pavement thickness may be increased according to the 

 A2-5 
AC 150/5335-5A - APPENDIX 2 09/28/06 

methods described in paragraph 321 of AC 150/5320-6D. The pavement is considered to 

have excess asphalt, which can be converted to extra equivalent thickness, when the 

asphalt thickness is greater than the minimum thickness of asphalt surfaced required for 

the critical airplane. Minimum asphalt surface course thickness requirements are 4 inches 

for standard body jet transport airplanes and 5 inches for widebody airplanes. The 

pavement may also be considered to have excess base thickness when the cross-section 

has a base thickness greater than the minimum specified in Table 3-4 of AC 150/5320-6D 

or when other improved materials such as asphalt stabilization or cement treated materials, 

are present. Likewise, additional subbase thickness or improved subbase materials may 

also be converted to additional total pavement thickness. 

9. Using the equivalent traffic and TC/C ratio of the critical airplane, the equivalent 

pavement thickness, and the average CBR of the subgrade, compute the maximum 

allowable gross weight of the critical airplane using the COMFAA program in the 

pavement design mode. 

10. Assign the subgrade CBR strength found in Step 7 to the appropriate standard ACN-PCN 

subgrade code as given in Table 2-2. 

11. The ACN of the critical airplane may now be determined from the COMFAA program 

using the ACN mode. Enter the allowable gross weight of the critical airplane, and 

calculate the ACN based on the standard subgrade code of Step 10. Alternatively, consult 

an “ACN versus Gross Weight” chart as published in the manufacturer’s ACAP manuals. 

12. Assign the tire pressure code based on the highest tire pressure in the traffic mix from 

Table 4-2. Keep in mind the quality of the asphalt surface layer, as discussed in Section 

2.1, when assigning this code. 

13. As the evaluation method is technical, assign the code of T, as described in paragraph 

4.5e. 

14. The numerical value of the PCN is the same as the numerical value of the ACN of the 

critical airplane calculated in Step 11. 

15. If the calculated allowable gross weight of Step 11 is equal to or greater than the critical 

airplane operational gross weight required for the desired pavement life, then the 

pavement is capable of handling the predicted traffic for the time period established in the 

traffic forecast. Accordingly, the assigned PCN determined in Step 14 is sufficient. If the 

allowable gross weight from Step 11 is less than the critical airplane gross weight required 

for the desired pavement life, then the pavement may be assigned a PCN equal to the 

ACN of the critical airplane at that gross weight, but with a lower expected pavement life. 

Additionally, it may then be necessary to develop a relationship of allowable gross weight 

based on the assigned PCN versus pavement life. Any overload should be treated in terms 

of ACN and equivalent critical airplane operations per individual operation. Allowance 

for the overload should be negotiated with the airport authority since pre-approval cannot 

be assumed. Specific procedures on how to relate pavement life and gross weight for 

flexible pavements are found in Appendix 3 of this document. 

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09/28/06 AC 150/5335-5A - APPENDIX 2 

2.2 TECHNICAL EVALUATION EXAMPLES FOR FLEXIBLE PAVEMENTS. The 

following four examples help demonstrate the technical evaluation method of determining a PCN 

for flexible pavements. The first example is for an under-strength pavement with a traffic volume 

that has increased to such a level that pavement life is reduced from the original design. The 

second example pavement has more than adequate strength to handle the forecasted traffic. The 

third example pavement is the same as the second, except that the runway has a central rather 

than a parallel taxiway. Example 4 discusses the effect on pavement life of a higher PCN rather 

than a reduced allowable gross weight. 

 a. Flexible Pavement Example 1. An airport has a flexible (asphalt-surfaced) 

runway pavement with a subgrade CBR of 9 and a total thickness of 32.0 inches, as shown in 

Figure A2-1 (5 inch minimum asphalt surface layer, 8 minimum base layer and variable subbase 

layer). Additional fuel is generally obtained at the airport before departure, and the runway has a 

parallel taxiway. The pavement was designed for a life of 20 years. It is assumed for the 

purposes of this example that the traffic level is constant over the 20-year time period. The traffic 

is shown in Table A2-3, which is similar to Table A2-1 but with additional information added. 

5 in. asphalt surface course

subgrade

19 in. crushed gravel and sand

8 in. crushed gravel

Figure A2-1. Flexible Pavement Example Cross-Section 

 A2-7 
AC 150/5335-5A - APPENDIX 2 09/28/06 

Table A2-3. Technical Evaluation Critical Airplane Determination 

Airplane 

Operating 

Weight, 

(lbs) 

Tire 

Pressure 

(psi) 

ACN 

F/B 

Annual 

Departures 

Flexible 

**P/C 

Required 

Thickness, 

(in.) 

B727-200 185,000 148 48 400 2.92 22.6 

B737-300 130,000 195 35 6,000 3.79 22.7 

A319-100 145,000 196 35 1,200 3.18 20.3 

B747-400 820,000 200 59 3,000 1.73 30.9 

B767-300ER 370,000 190 52 2,000 1.80 27.9 

DC8-63 330,000 194 52 800 1.68 26.6 

A300-B4 370,000 205 57 1,500 1.75 29.1 

B777-200 600,000 215 51 300 1.42 28.0 

** P/C determined at 95 percent of gross load on main gear 

The required total pavement thickness results are shown in Table A2-3 for each airplane. 

The B747-400 airplane has the greatest individual pavement thickness requirement (30.9 inches) 

for its total traffic over 20 years; it is therefore the critical airplane. Note the thickness 

requirements for each individual airplane are less than the existing pavement thickness of 32 

inches. 

Table A2-4 shows the conversion of departures of the other traffic to the critical airplane (B747-

400) equivalent. Gear configuration conversion factors from Table A1-5 were used to determine 

the equivalent dual tandem gear departures. The B747-400 equivalent annual departures were 

calculated by using Equation A1-3. Although the B747-400 had only 3,000 annual departures, 

the effect of the other traffic has increased the number to an equivalent 7,424. 

 

Note the equivalent annual departure total shown would also be the same for the B767-300ER 

and the A300-B4 because the assumed wheel loads are the same as that of the critical airplane. 

This would not be true, however, for the B777-200 because of the different gear configuration. 

Note also the effect of wheel load on the critical airplane equivalent annual departures. Wheel 

loads of the individual airplanes that are greater than the critical airplane wheel load add to the 

critical airplane equivalent departures by a factor greater than one, while wheel loads that are less 

add by a factor less than one. This relationship indicates the need to carefully consider the 

loading of each airplane in the traffic mix when determining equivalent traffic. 

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09/28/06 AC 150/5335-5A - APPENDIX 2 

Table A2-4. Equivalent Annual Departures of the Critical Airplane 

 

Airplane 

Annual 

Departures 

Gear 

Type 

Gear 

Type 

Factor

(R2) 

Equiv. 

(2D) 

Departures 

(W2) 

Wheel 

Load 

(lbs) 

(W1) 

B747-400 

Wheel 

Load (lbs) 

(R1) 

B747-400 

Equiv. Ann. 

Departures 

B727-200 400 D 0.64 256 43,938 48,688 194 

B737-300 6,000 D 0.64 3,840 30,875 48,688 716 

A319-100 1,200 D 0.64 768 34,438 48,688 268 

B747-400 3,000 2D/2D2 1.00 3,000 48,688 48,688 3,000 

B767-300ER 2,000 2D 1.00 2,000 43,938 48,688 1368 

DC8-63 800 2D 1.00 800 39,188 48,688 403 

A300-B4 1,500 2D 1.00 1,500 43,938 48,688 1,041 

B777-200 300 3D 1.56 468 47,500 48,688 434 

 15,200 7,424 

With the total equivalent traffic of the critical airplane known, the traffic cycle ratio for the 

taxiway and fuel situation can be calculated. Using a critical airplane P/C ratio of 1.73 (Table 

A2-3) and a P/TC ratio of 1 for a parallel taxiway (Table A1-2), the traffic cycle to coverage ratio 

can be calculated with Equation A1-1: 

 TC/C = 1.73 ÷ 1 = 1.73 

It is now possible to calculate the maximum allowable gross weight of the B747-400 critical 

airplane on this pavement. The input parameters to the COMFAA program are as follows: 

 Critical airplane B747-400 

 Pavement thickness 32.0 inches 

 Subgrade CBR 9.0 (code B) 

 Tire pressure 200 psi (code X) 

 Percent weight on the main gear 95.0 percent 

 TC/C ratio 1.73 

 Pavement life 20 years 

 Annual equivalent departures 7,424 

 Total coverages (TC/1.73) x 20 85,827 

For these conditions, the COMFAA program calculates an allowable gross weight for the B747-

400 of 797,500 pounds. Further, the program determines the B747-400 ACN at this weight to be 

56.4/F/B, for a recommended pavement rating of PCN 56/F/B/X/T. 

Referring to Table A2-3, it can be seen that the B747-400 and the A300-B4 airplanes would be 

restricted in their operations on this runway due to their ACNs of 59/F/B and 57/F/B, 

respectively—both of which are higher than the recommended PCN of 56/F/B. It is apparent the 

pavement is not adequate to accommodate the existing traffic, and either the operating weights 

 A2-9 
AC 150/5335-5A - APPENDIX 2 09/28/06 

will have to be restricted or pavement life will be less than originally expected. An analysis of 

this situation and the requirements for adjustments is provided in Appendix 3. 

 b. Flexible Pavement Example 2. This second example has the same input 

parameters as the first, except the pavement cross-section is increased to 36 inches. 

The input parameters to the COMFAA program for this example are: 

 Critical airplane B747-400 

 Pavement thickness 36.0 inches 

 Subgrade CBR 9.0 (code B) 

 Tire pressure 200 psi (code X) 

 Percent weight on the main gear 95.0 percent 

 TC/C ratio 1.73 

 Pavement life 20 years 

 Annual equivalent departures 7,424 

 Total coverages (TC/1.73) x 20 85,827 

For these conditions, the calculated allowable gross weight of the B747-400 is 923,000 pounds. 

The COMFAA program determines the B747-400 ACN at this weight is 69.3/F/B, for a 

recommended rating of PCN 69/F/B/X/T. 

An examination of Table A2-3 shows that the traffic has ACNs that are less than the 

recommended PCN. It can therefore be safely assumed that the pavement will adequately handle 

the existing traffic within its design life, and no adjustments to the pavement cross-section or life 

will have to be made. Note that the addition of 4 inches in pavement thickness from Example 1 

has resulted in a net increase in PCN of 13.0. 

 c. Flexible Pavement Example 3. The only change in this example from the second 

example is that the taxiway is a central configuration rather than parallel, such as that shown in 

Figure A1-1b. Using data from Table A1-2, the P/TC ratio changes from 1 to 2. From Equation 

A1-1, the TC/C ratio for the critical B747-400 airplane becomes— 

 TC/C = 1.73 ÷ 2 = 0.865 

The input parameters to the COMFAA program are: 

 Critical airplane B747-400 

 Pavement thickness 36.0 inches 

 Subgrade CBR 9.0 (code B) 

 Tire pressure 200 psi (code X) 

 Percent weight on the main gear 95.0 percent 

 TC/C ratio 0.86 

 Pavement life 20 years 

 Annual equivalent departures 7,424 

 Total coverages (TC/0.865) x 20 171,653 

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09/28/06 AC 150/5335-5A - APPENDIX 2 

For these conditions, the calculated allowable gross weight of the B747-400 is 875,000 pounds. 

The COMFAA program determines the B747-400 ACN at this weight to be 64.2/F/B, for a 

recommended runway rating of PCN 64/F/B/X/T. The net effect of the change in taxiway 

configuration from that of Example 2 is the reduction in PCN by 5. 

 d. Flexible Pavement Example 4. As an alternate way of looking at the effect of a 

parallel versus central taxiway effects, consider how the pavement life would change instead of 

the PCN. If the PCN from Example 2 were to remain at 69/F/B/X/T, which is equivalent to a 

B747-400 critical airplane allowable gross weight of 923,000 pounds, then the pavement life 

would be reduced from 20 to 10 years. This is due to the change in the TC/C ratio from 1.73 to 

0.86. A similar effect would be noticed if fuel was not obtained at the airport, (it was obtained in 

the first flexible pavement example case). 

2.3 TECHNICAL EVALUATION FOR RIGID PAVEMENTS. The following list 

summarizes the steps for using the technical evaluation method for rigid pavements: 

• Determine the traffic volume in terms of type of airplane and number of operations of 

each airplane that the pavement will experience over its life. 

• Convert that traffic into a single critical (design) airplane equivalent. 

• Determine the pavement characteristics; including subgrade soil modulus, k, and the 

concrete thickness and elastic modulus. 

• Calculate the maximum allowable gross weight of the critical airplane on that pavement. 

• Look up or calculate the ACN of the critical airplane at its maximum allowable gross 

weight, as determined in the previous step. 

• Assign the PCN to be the ACN just calculated. 

The above steps are explained in greater detail: 

1. Determine the traffic volume in the same fashion as noted in paragraph A2-2.1 for 

flexible pavements 

2. Determine which airplane in the traffic mix from step 1 is critical or the most 

significant. The critical airplane is the one that has the greatest pavement thickness 

requirements based on its individual gross weight, traffic volume, P/C ratio, and tire 

pressure; it is not necessarily the one with the highest ACN or the highest gross 

weight. 

3. The rigid design procedure implemented in the COMFAA program calculates 

pavement thickness requirements based on the concrete edge stress, which is in turn 

dependent on load repetitions of the total traffic mix. It is therefore a requirement to 

convert traffic cycles or passes to load repetitions by using a pass-to-load repetition 

 A2-11 
AC 150/5335-5A - APPENDIX 2 09/28/06 

ratio. P/C ratios for any airplane on rigid pavement are calculated in the COMFAA 

program. 

4. Using the conversion factors in Table A1-5, convert the traffic volume of each 

airplane in the traffic mix to the critical airplane equivalent based on gear 

configuration differences. 

5. Determine the critical airplane equivalent traffic cycles based on the single wheel 

loads of each airplane in the traffic mix using Equation A1-3. 

6. Calculate the critical airplane TC/C ratio from Equation A1-1 for the type of taxiway 

and the fuel-loading method. 

7. Using the critical airplane equivalent traffic cycles from Step 5 and the TC/C ratio 

from Step 6, calculate the equivalent load repetitions of the critical airplane based on 

the life expectation of the pavement. 

8. Obtain the pavement characteristics including the concrete slab thickness, the concrete 

modulus of rupture, and average modulus, k, of the subgrade. Concrete elastic 

modulus is set at 4,000,000 psi and Poisson’s ratio is set at 0.15 in the COMFAA 

program. Accurate subgrade modulus determination is important to the technical 

method, but small variations in the modulus will not affect the PCN results in a 

disproportionate manner. This is in contrast to flexible pavement subgrade modulus in 

which strength variations have a significant effect on PCN. If the pavement has a 

subbase course and/or stabilized subbase layers, then the subgrade modulus is adjusted 

upwards in the rigid design procedure to an equivalent value in order to account for 

the improvement in support. Subgrade modulus adjustments are made from Figures 2-

4 and 3-16 of AC 150/5320-6D. 

9. Using the known slab thickness, subgrade modulus, and airplane parameters compute 

the maximum allowable gross weight of the critical airplane using the COMFAA 

program in the pavement design mode. By setting the airplane total coverages, gross 

airplane weight can be adjusted until the known pavement thickness is achieved. 

10. Assign the subgrade modulus (k-value) to the nearest standard ACN-PCN subgrade 

code. The k-value to be reported for PCN purposes is the improved k-value seen at 

the top of all improved layers. Subgrade codes for k-value ranges are found in Table 

2-1. 

11. The ACN of the critical airplane may now be determined from the COMFAA 

program. Enter the allowable gross weight of the critical airplane from Step 9, and 

calculate the ACN for the standard subgrade code of Step 10. Alternatively, consult 

an “ACN versus Gross Weight” chart as published in the manufacturer’s ACAP 

manual. 

12. Assign the tire pressure code based on the highest tire pressure in the traffic mix from 

Table 4-2. As discussed previously, rigid pavements are typically able to handle high 

tire pressures, so code W can usually be assigned. 

A2-12 
09/28/06 AC 150/5335-5A - APPENDIX 2 

13. The evaluation method is technical, so the code T will be used as discussed in 

paragraph 4.5e. 

14. The numerical value of the PCN is the same as the numerical value of the ACN of the 

critical airplane just calculated in Step 11. 

15. If the allowable gross weight of Step 11 is equal to or greater than the critical airplane 

operational gross weight required for the desired pavement life, then the pavement is 

capable of handling the predicted traffic for the time period established in the traffic 

forecast. Accordingly, the assigned PCN determined in Step 12 is sufficient. If the 

allowable gross weight from Step 11 is less than the critical airplane gross weight 

required for the desired pavement life, then the pavement may be assigned a PCN 

equal to the ACN of the critical airplane at that gross weight, but with a reduced 

pavement life. Additionally, it may then be necessary to develop a relationship of 

allowable gross weight based on the assigned PCN versus pavement life. Appendix 3 

provides procedures on how to relate pavement life and gross weight for rigid 

pavements in terms of PCN. Any overload should be treated in terms of ACN and 

equivalent critical airplane operations per individual operation. Allowance for the 

overload should be negotiated with the airport authority, since pre-approval cannot be 

assumed. Appendix 3 provides specific procedures on how to relate pavement life and 

gross weight for rigid pavements. 

2.4 TECHNICAL EVALUATION EXAMPLES FOR RIGID PAVEMENTS. The following 

three examples help explain the technical evaluation method of determining a PCN for rigid 

pavements. The first example pavement is under-designed and the traffic volume has increased to 

such a level that pavement life is reduced from the original design. The second pavement has 

more than adequate strength to handle the forecasted traffic. The third example pavement is the 

same as number two, except that the airplanes generally do not obtain fuel at the airport. 

 a. Rigid Pavement Example 1. An airport has a rigid (concrete-surfaced) runway 

pavement with an effective subgrade k-value of 200 pci and a slab thickness of 14 inches, as 

shown in Figure A2-2. The concrete has a modulus of rupture of 700 psi, an elastic modulus of 

4,000,000 psi, and a Poisson’s ratio of 0.15. The runway has a parallel taxiway, and additional 

fuel is generally obtained at the airport before departure. The pavement life is estimated to be 20 

years from the original construction. The traffic shown in Table A2-5 is basically the same as in 

Table A2-1, but with P/C ratios and life-load repetitions added. 

 A2-13 
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8 in. subbase

subgrade

14 in. concrete slab

Figure A2-2. Rigid Pavement Example Cross-Section 

The critical airplane will be the one with the highest required thickness for its load magnitude and 

frequency. The thickness required for each airplane is determined with the COMFAA program in 

the pavement design mode. The load repetitions must first be calculated for each airplane by 

using Equation A1-1 and then converted to coverages for use in the COMFAA program. Since 

additional fuel is generally obtained at the airport, and there is a parallel taxiway, so— 

 P/TC = 1 

 TC/C = P/C 

 Coverages (C) = annual departures * 20 years ÷ TC/C 

The resulting coverages are listed for each airplane in Table A2-5. The thickness of rigid 

pavement required for each airplane at the operating weight and frequency are shown in Table 

A2-6. 

Table A2-5. Rigid Pavement Technical Evaluation Traffic 

Airplane 

Operating 

Weight, lbs 

Tire 

Pressure 

(psi) 

ACN 

(R/C) 

** 

P/C 

Annual 

Departures Coverages 

B727-200 185,000 148 55 2.92 400 2,740 

B737-300 130,000 195 38 3.79 6,000 31,662 

A319-100 145,000 173 42 3.18 1,200 7,547 

B747-400 820,000 200 68 3.46 3,000 17,341 

B767-300ER 370,000 190 58 3.60 2,000 11,111 

DC8-63 330,000 194 62 3.35 800 4,776 

A300-B4 370,000 205 67 3.49 1,500 8,595 

B777-200 600,000 215 77 4.25 300 1,412 

 ** Rigid P/C determined at 95 percent of gross load on main gear 

Table A2-6 shows that the critical airplane is the B747-400, based on its required thickness. 

However, the A300-B4 should also be given consideration as critical in that its required thickness 

is very close to that of the B747-400. In this example, the B777-200 is not the critical airplane, 

even though it has the highest ACN, because of the comparatively low number of coverages. 

A2-14 
09/28/06 AC 150/5335-5A - APPENDIX 2 

Table A2-6. Technical Evaluation Critical Airplane Determination 

Airplane 

Operating 

Weight, lb 

 Required 

Thickness (in.) 

B727-200 185,000 13.0 

B737-300 130,000 13.2 

A319-100 145,000 11.1 

B747-400 820,000 14.1 

B767-300ER 370,000 12.8 

DC8-63 330,000 12.5 

A300-B4 370,000 13.6 

B777-200 600,000 11.5 

Table A2-7. Equivalent Annual Departures of the Critical Airplane 

 

Airplane 

Annual 

Departures 

Gear 

Type 

(R2) 

Equiv. (2D) 

Departures 

(W2) 

Wheel 

Load 

(W1) 

B747-

400 

Wheel 

Load 

(R1) 

B747-400 

Equiv.Ann. 

Departures 

B727-200 400 D 256 43,938 48,688 194 

B737-300 6,000 D 3,840 30,875 48,688 716 

A319-100 1,200 D 768 34,438 48,688 268 

B747-400 3,000 2D/2D2 3,000 48,688 48,688 3,000 

B767-200ER 2,000 2D 2,000 43,938 48,688 1,368 

DC8-63 800 2D 800 39,188 48,688 403 

A300-B4 1,500 2D 1,500 43,938 48,688 1,041 

B777-200 300 3D 468 47,500 48,688 434 

15,200 7,424 

All departures of the other traffic must be converted to the B747-400 equivalent as shown in 

Table A2-7. Note that this table is identical to Table A2-4 for the flexible pavement examples. 

Before the maximum allowable gross weight of the critical airplane can be determined from the 

COMFAA program, the anticipated traffic reported in annual departures must be converted to 

total coverages (lifetime coverages). As stated previously, since additional fuel is generally 

obtained at the airport, and there is a parallel taxiway, the following should be used: 

 P/TC = 1 

 P/C = 3.46 

 TC/C = 3.46 

 A2-15 
AC 150/5335-5A - APPENDIX 2 09/28/06 

 Coverages = 7,424 * 20 years ÷ 3.46 = 42,913 

The input parameters to the COMFAA program (pavement design mode) are— 

 Critical airplane B747-400 

 Coverages 42,913 

 Percent weight on the main gear 95.0 percent 

 Tire pressure 200 psi (code X) tire contact area 260.4 sq. in. 

 Slab thickness 14.0 inches 

 Slab flexural strength 700 psi 

 Effective subgrade k-value 200 pci (code C) 

For these conditions, the COMFAA program can be used to iterate to a solution by adjusting the 

gross airplane weight until the known pavement thickness is obtained. For this example, the 

calculated allowable gross weight of the B747-400 is 762,000 pounds. By switching the 

COMFAA program back to the ACN mode and entering in the allowable gross weight, an ACN 

of 61.3/R/C is obtained for the B747-400. The final recommended runway rating is PCN 

61/R/C/W/T. As mentioned in Paragraph 4.5d(i), even though none of the airplanes in this 

example have tire pressures that exceed the limits of code X, the code for rigid pavement should 

normally be W. 

Based on the ACNs in Table A2-5, it can be seen that the B747-400, the DC8-63, the A300-B4, 

and the B777-200 airplane would be restricted in their operations on this runway due to their 

respective ACNs of 68/R/C, 62/R/C, 67/R/C and 77/R/C; all of which are higher than the derived 

PCN of 61/R/C. It is apparent the pavement is not adequate to accommodate the existing traffic, 

and either the operating weights will have to be restricted or pavement life will be less than 

originally expected. An analysis of this situation and the requirements for adjustments are 

discussed in Appendix 3. 

 b. Rigid Pavement Example 2. This second example has the same input parameters 

as the first, except the slab thickness is increased to 16 inches. The input parameters to the 

COMFAA program are— 

 Critical airplane B747-400 

 Coverages 42,913 

 Percent weight on the main gear 95.0 percent 

 Tire pressure 200 psi (code X) tire contact area 260.4 sq. in. 

 Slab thickness 16.0 inches 

 Effective subgrade k-value 200 pci (code C) 

 Concrete flexural strength 700 psi 

For these conditions, the calculated allowable gross weight of the B747-400 is 890,000 pounds. 

The B747-400 ACN is 76.2/R/C, for a recommended runway rating of PCN 76/R/C/W/T. 

It can be seen from Table A2-5 that all of the traffic have ACNs that are less than the 

recommended PCN. It can therefore be safely assumed that the pavement will adequately 

A2-16 
09/28/06 AC 150/5335-5A - APPENDIX 2 

accommodate the existing traffic within its design life, and no adjustments to the pavement crosssection or life will have to be made. 

 c. Rigid Pavement Example 3. The only change in this example from the second 

example is that the airplanes generally do not obtain fuel at the airport. Using Table A1-3, the 

P/TC ratio changes from 1 to 2. Using Equation A1-2, the TC/C ratio for the critical B747-400 

airplane becomes— 

=÷= 73.1246.3CTC

 Where: 

 P/TC = 2 

 P/C = 3.46 

Therefore, lifetime coverages = 7,424 * 20 years ÷ 1.73 = 85,827 

The input parameters to the COMFAA program in pavement thickness mode are— 

 Critical airplane B747-400 

 Coverages 85,827 

 Percent weight on the main gear 95.0 percent 

 Tire pressure 200 psi (code X) tire contact area 260.4 sq. in. 

 Slab thickness 16.0 inches 

 Subgrade k-value 200 (code C) 

 Concrete flexural strength 700 psi 

For these conditions, the calculated allowable gross weight of the B747-400 is 851,000 pounds. 

The B747-400 ACN is 71.6/R/C, for a recommended runway rating of PCN 72/R/C/W/T. This 

rating would require small weight restrictions on the B777 airplane. However since additional 

fuel is not obtained, the landing weight is most likely already below the possible restriction. 

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A2-18 
09/28/06 AC 150/5335-5A - APPENDIX 3 

APPENDIX 3. PAVEMENT OVERLOAD EVALUATION BY THE ACN-PCN SYSTEM 

1.0 ICAO PAVEMENT OVERLOAD EVALUATION GUIDANCE. In the life of a 

pavement, it is possible that either the current or the future traffic will load the pavement in such a 

manner that the assigned pavement rating is exceeded. ICAO provides a simplified method to 

account for minor pavement overloading in which the overloading may be adjusted by applying a 

fixed percentage to the existing PCN. 

The ICAO procedure for overload operations is based on minor or limited traffic having ACNs 

that exceed the reported PCN. Loads that are larger than the defined PCN will shorten the 

pavement design life, while smaller loads will use up the life at a reduced rate. With the 

exception of massive overloading, pavements in their structural behavior do not suddenly or 

catastrophically fail. As a result, occasional minor airplane overloading is acceptable with only 

limited loss of pavement life expectancy and relatively small acceleration of pavement 

deterioration. For those operations in which the magnitude of overload and/or frequency does not 

justify a detailed (technical) analysis, the following criteria are suggested. 

• For flexible pavements, occasional traffic cycles by airplanes with an ACN not exceeding 

10 percent above the reported PCN should not adversely affect the pavement. 

• For rigid or composite pavements, occasional traffic cycles by airplanes with an ACN not 

exceeding 5 percent above the reported PCN should not adversely affect the pavement. 

• The annual number of overload traffic cycles should not exceed approximately 5 percent 

of the total annual airplane traffic cycles. 

• Overloads should not normally be permitted on pavements exhibiting signs of distress, 

during periods of thaw following frost penetration, or when the strength of the pavement 

or its subgrade could be weakened by water. 

• Where overload operations are conducted, the airport authority should review the relevant 

pavement condition on a regular basis and should also review the criteria for overload 

operations periodically, since excessive repetition of overloads can cause severe 

shortening of pavement life or require major rehabilitation of the pavement. 

However, these criteria give little guidance to the airport authority as to the impact of these 

overload operations on the pavement in terms of pavement life reduction or increased 

maintenance requirements. This appendix discusses methods for making overload allowances for 

both flexible and rigid pavements that will clearly indicate these effects and will give the 

authority the ability to determine the impact both economically and in terms of pavement life. 

1.1 OVERLOAD GUIDANCE. The overload evaluation guidance in this appendix applies 

primarily to flexible and rigid pavements that have PCN values that were established by the 

technical method. Pavements that have ratings determined by the using airplane method can use 

the overload guidelines provided by ICAO. The procedures presented here rely on the COMFAA 

program. 

The adjustments for pavement overloads start with the assumption that some of the airplanes in 

the traffic mix have ACNs that exceed the PCN. If the steps outlined in Appendix 2 have been 

 A3-1 
AC 150/5335-5A - APPENDIX 3 09/28/06

followed for the technical method, then most of the necessary data already exists to perform an 

examination of overloading. 

For flexible pavement, it was found in the first example of Appendix 2 that the B747-400 and 

A300-B4 airplanes have ACNs that exceed the recommended runway rating. Likewise, for the 

first rigid pavement example, the ACNs of the B747-400, A300B4, DC8-63, and B777-200 

exceed the recommended runway rating. Individually, none of the airplanes in the traffic mix 

have requirements that exceed the existing pavement thickness requirements. However, even 

though each of these airplanes were included in the derivation of the allowable gross weight of 

the critical airplane, the recommended PCN is not adequate for the larger airplanes. To resolve 

these kinds of problems the airport authority has three options when making a pavement strength 

rating selection: 

1. Let the PCN remain as derived from the technical evaluation method, but retain local 

knowledge that there are some airplanes in the traffic mix that can be allowed to operate 

with ACNs that exceed the published PCN or at a reduced weight to not exceed the PCN. 

2. Provide for an increased PCN by either adding an overlay or by reconstructing to 

accommodate airplanes with the higher ACNs. 

3. Adjust the PCN upward to that of the airplane with the highest ACN, but recognize the 

need to expect possible severe maintenance. This will result in earlier than planned 

reconstruction or overlay due to reduced pavement life. 

The first option requires that the airport authority be constantly aware of the composition of the 

entire traffic mix in terms of operating gross weights and loading frequency. If the traffic mix has 

changes that affect the factors involved in developing a technically based PCN, then the PCN will 

need to be adjusted to reflect the changes. The airport authority will also have to internally make 

allowance for or prevent airplane operations that exceed the PCN. The difficulty in doing this is 

that the magnitude of the PCN is out of step with the ACNs of some of the traffic. 

The second option alleviates the problems discussed for the first option, but it does require 

additional expense to bring the pavement up to the strength required by the combination of 

airplanes in the traffic mix. However, providing the pavement strengthening will allow 

operations at the required strength and for the desired pavement life. 

The third option has the benefit of allowing all airplanes in the traffic mix to operate as necessary. 

However, by increasing the PCN, which implies higher pavement strength, the pavement life will 

be reduced unless an increase in thickness is provided. 

Each of these options is considered in the following discussion on pavement overloading—first 

for flexible pavement and then for rigid pavement. 

1.2 ADJUSTMENTS FOR FLEXIBLE PAVEMENT OVERLOADS. It is most efficient to 

describe the procedures for flexible pavement overloading by referencing flexible pavement 

technical evaluation example 1 in Appendix 2 (Paragraph 2.2a). In this example, two airplanes of 

the traffic mix were found to exceed the pavement capability. The derived rating was found to be 

PCN 56/F/B/X/T, with the traffic of Table A2-3 operating on the runway. 

A3-2 
09/28/06 AC 150/5335-5A - APPENDIX 3 

 a. Flexible Pavement Overload Illustration 1. Table A2-3 indicates that the 

B747-400 was operating at a gross weight of 820,000 pounds, with an ACN of 59/F/B. Likewise, 

the A300-B4 had a gross weight of 370,000 pounds and an ACN of 57/F/B. Reduction of the 

gross weights to the rated PCN of 56/F/B/X/T would result in a gross weight for the B747-400 of 

797,500 pounds and a gross weight of 366,500 pounds for the A300-B4. Although these limited 

operating weights would solve the problem of pavement loading, they have the disadvantage of 

restricting airline operations. Additionally, new traffic with airplanes having ACNs exceeding 

the PCN would also have to be restricted. 

 b. Flexible Pavement Overload Illustration 2. Rather than restricting operating 

weights, the airport could refurbish the pavement by adding an overlay. The computer steps for 

determining such a flexible pavement overlay are as follows: 

 Construct an ACN versus gross weight diagram, such as that shown in Figure A3-1, 

for the B747-400 critical airplane at the subgrade code previously determined. Data 

for this chart may be obtained from the COMFAA program by calculating ACN at 

various gross weights. Note in this figure that the relationship of ACN and gross 

weight is not a straight line, but is slightly curved because the line was derived by 

calculating the ACN at a series of gross weights, rather than just connecting the 

minimum and maximum values. 

30

40

50

60

70

80

90

400 500 600 700 800 900 1000

Gross Weight, 1000 lb

ACN Value .

747-400

Flexible pavement

Code B subgrade

COMFAA program

Figure A3-1. B747-400 Flexible Pavement ACN Versus Gross Weight 

 A3-3 
AC 150/5335-5A - APPENDIX 3 09/28/06

 Use the COMFAA program to develop data of pavement life versus ACN, such as 

shown in Figure A3-2. This chart is similar to that found in Section 7 of the 

manufacturer’s ACAP manuals, except that subgrade CBR and pavement thickness are 

not shown because they are already fixed. For example, there are four basic 

parameters involved in pavement design: 

• Subgrade CBR 

• Pavement thickness 

• Airplane gross weight 

• Traffic volume and pavement life 

Of these four, the only variables are gross weight and pavement life in terms of annual traffic 

cycles. By relating gross weight to ACN (as was done in Figure A3-1), ACN can be substituted 

on the abscissa of Figure A3-2. For each pavement life number, a gross weight is found that 

satisfies the subgrade CBR and pavement thickness, which is then converted to ACN. Table A3-

1 contains part of the data used in the COMFAA program to construct the curves of Figure A3-2 

for a B747-400 airplane with a subgrade CBR of 9.0. 

30

40

50

60

70

80

90

100 1000 10000 100000

Total Traffic Cycles

ACN 

T = 32 in.

T = 36 in. 

747-400

Subgrade CBR = 9

20 year pavement life

COMFAA program

500 1500

Figure A3-2. B747-400 Flexible Pavement Life Versus ACN 

A3-4 
09/28/06 AC 150/5335-5A - APPENDIX 3 

Table A3-1. Data for Constructing Flexible Pavement Life Curves for B747-400 

B747-400 

Annual 

Departures 

Coverages at 

(P/C = 1.73) 

T=32 

Gross 

Weight 

T=32 

ACN 

T-36 

Gross 

Weight 

T=36 

ACN 

500 5,780 1,014,000 79.4 -- -- 

1,200 13,873 926,000 69.6 1,075,000 86.8 

2,400 27,746 875,000 64.2 1,013,000 79.3 

3,000 34,682 858,000 62.5 994,500 77.2 

5,000 57,803 822,500 58.9 953,400 72.6 

7,424 85,827 797,500 56.4 923,000 69.3 

20,000 231,214 738,500 50.7 855,000 62.2 

50,000 578,035 690,400 46.3 800,000 56.6 

86,500 1,000,000 664,000 43.9 768,000 53.5 

Note: Pass-to-coverage ratio determined for airplane configurations reported by airplane 

manufacturers when calculating ACN value (gross load, center of gravity, tire pressure). 

It is now possible to relate the effects of gross weight, ACN, and pavement life by combining the 

two charts, as shown in Figure A3-3. The left-hand side of this figure is the chart of Figure A3-1, 

while the right-hand chart is that of Figure A3-2. It can now be seen how the critical airplane 

gross weight of 797,500 pounds (PCN 56/F/B/X/T) equates to 7,424 equivalent B747-400 traffic 

cycles per year for 20 years. If the PCN were increased to 69/F/B/X/T to accommodate the 

higher gross weights, the allowable traffic cycles of the critical airplane at 923,000 pounds gross 

weight would decrease to 1,254 per year for the 20-year time period. This effectively reduces the 

pavement life from 20 years to just over 3 years (1,254 x 20 ÷ 7,424 = 3.38). 

This example shows that a pavement with a thickness of 32 inches is under-designed for the 

traffic expected over the next 20 years. It is therefore reasonable to expect that an overlay to 

bring the effective thickness to 36 inches will be required if the pavement is to last for the 

required 7,424 annual departures for 20 years. This can be seen graphically in Figure A3-3. 

Figure A3-3 also shows that for any combination of critical airplane gross weight in terms of 

ACN, the pavement life is known. Thus, the airport authority can determine from this type of 

chart the allowances to be made for traffic overloading. The airport authority also now has the 

information necessary to make a decision on the assignment of a PCN. If the PCN is raised to a 

level to permit all current traffic, the required pavement overlay can be determined. Furthermore, 

the impact of the higher ACN airplane can be determined in the requirements for overlay 

thickness. It may be necessary to repeat this process if new airplanes are added to the traffic mix 

since their effects are not accounted for in the above calculations. Likewise, if there are any other 

significant changes in the traffic mix, the rating should be reviewed. 

This example is only intended to illustrate the effect of pavement thickness on the PCN rating. 

Overlay thickness requirements for pavement design purposes should be determined using AC 

150/5320-6. 

 A3-5 
AC 150/5335-5A - APPENDIX 3 09/28/06 Annual Traffic Cycles Gross Weight, 1,000 lb 1000 10000 100000 T = 36 in. Flexible Pavement Subgrade CBR = 9 20 year pavement life Parallel taxiway Fuel obtained at airport COMFAA program T = 32 in. 400 500 600 700 800 900 100090807060504030 747-400 Flexible pavement Code B subgrade COMFAA program

ACN 

Figure A3-3. B747-400 Flexible Pavement Life 

A3-6 
09/28/06 AC 150/5335-5A - APPENDIX 3 

 c. Flexible Pavement Overload Illustration 3. This example will illustrate the 

effect of ICAO allowable overloading in which the ACN is no more than 10 percent above the 

PCN and the number of traffic cycles does not exceed 5 percent of the total annual traffic. 

Table A2-3 is repeated here as Table A3-2, but with a new airplane added to the traffic mix with 

an ACN that is 10 percent above the rated PCN of 56/F/B/X/T. The total annual departures, as 

shown in Table A2-4, is 15,200, of which 760 is 5 percent of the total. This amount is shown in 

Table A3-3. 

Table A3-2. Flexible Pavement Overload Airplane Added 

Airplane 

Operating 

Weight, 

(lbs) 

Tire 

Pressure 

(psi) 

ACN 

F/B 

Annual 

Departures 

Flexible 

**P/C 

Required 

t, (in.) 

B727-200 185,000 148 48 400 2.92 22.6 

B737-300 130,000 195 35 6,000 3.79 22.7 

A319-100 145,000 196 35 1,200 3.18 20.3 

B747-400 820,000 200 59 3,000 1.73 30.9 

B767-300ER 370,000 190 52 2,000 1.80 27.9 

DC8-63 330,000 194 52 800 1.68 26.6 

A300-B4 370,000 205 57 1,500 1.75 29.3 

B777-200 600,000 215 51 300 1.42 28.0 

L1011-500 463,000 184 62 760 1.80 28.6 

 ** Flexible P/C determined at 95 percent of gross load on main gear 

The end result on the critical airplane calculation is that for an equivalent annual departure level 

of 7,934, the allowable gross weight is reduced from 797,500 to 793,500 pounds for an ACN of 

56.0/F/B. Alternately, for the same allowable gross weight of 797,500 pounds and an ACN of 

56.4/F/B, the pavement thickness would have to be increased to 32.13 inches from the current 

32.0 inches. 

This example shows the impact both on required pavement thickness and on PCN of a new 

airplane that is within the ICAO guidelines of no more than 10 percent overload and no more than 

5 percent traffic increase. Knowing the impact of new airplanes on pavement thickness 

requirements, the airport authority can make a decision as to the relative effects. 

Although these examples were for specific conditions as described, the methods can also be 

applied to any other traffic overloading condition. 

 A3-7 
AC 150/5335-5A - APPENDIX 3 09/28/06

Table A3-3. Flexible Pavement New Airplane Equivalent Traffic 

Airplane 

Annual 

Departures 

Gear 

Type 

(R2) 

Equiv. (2D) 

Departures 

(W2) 

Wheel 

Load 

(lbs) 

(W1) 

B747-400 

Wheel 

Load 

(lbs) 

(R1) 

B747-400 

Equiv. Ann. 

Departures 

B727-200 400 D 256 43,938 48,688 194 

B737-300 6,000 D 3,840 30,875 48,688 716 

A319-100 1,200 D 768 34,438 48,688 268 

B747-400 3,000 2D/2D2 3,000 48,688 48,688 3,000 

B767-300ER 2,000 2D 2,000 43,938 48,688 1,368 

DC8-63 800 2D 800 39,188 48,688 403 

A300-B4 1,500 2D 1,500 43,938 48,688 1,041 

B777-200 300 3D 468 47,500 48,688 434 

L1011-500 760 2D 760 54,981 48,688 510 

 15,960 7,934 

1.3 ADJUSTMENTS FOR RIGID PAVEMENT OVERLOADS. As was done for the 

flexible pavement overload illustration, the procedures for rigid pavement overloading can best be 

explained by continuing the first rigid pavement technical evaluation example in Appendix 2 

(Paragraph 2.4a). In this example, for which the derived PCN was 61/R/C/W/T, the B747-400, 

A300-B4, B777-200, and DC863 were found to exceed the pavement capability, as shown in 

Table A2-5. This requires that adjustments be made to allow these airplanes to operate at their 

desired gross weight. These adjustments take the form of either a reduced pavement life or an 

overlay to increase the pavement strength. 

A second overload illustration examines the effect of occasional traffic of airplanes with ACNs 

that exceeds the PCN. 

 a. Rigid Overload Illustration 1. Evaluation of rigid pavement overload is similar 

to that of flexible pavement. It is necessary to develop the pavement life variables first and then 

examine the results with the COMFAA program. The necessary steps for determining rigid 

pavement overloading effects are— 

1. Construct an ACN versus gross weight diagram such as that shown in Figure A3-4, 

for the B747-400 critical airplane at the subgrade code previously determined. Note 

that the line relating ACN and gross weight is not straight because it was constructed 

using a selection of many points rather than just connecting the minimum and 

maximum values. Figure A3-4 can be generated in the COMFAA program by 

calculating ACN values at various gross weights. 

2. Construct an ACN versus pavement life chart, as shown in Figure A3-5. Data for 

Figure A3-5 can be generated by the COMFAA program by first entering the load 

repetitions (coverages) in the pavement thickness mode and adjusting the gross 

A3-8 
09/28/06 AC 150/5335-5A - APPENDIX 3 

weight until the desired thickness is achieved. Then switch, to the ACN mode, and 

enter the allowable gross weight to obtain the ACN value. It is possible to develop a 

chart such as Figure A3-5 because the parameters of subgrade modulus and the 

pavement thickness are already known. This reduces the variables to the 

relationship of pavement life and allowable gross weight. By relating ACN to gross 

weight, as in Figure A3-4, ACN can be used in place of gross weight on the ordinate 

of the Figure A3-5 chart. Each of these steps will be illustrated by using data from 

the first rigid pavement example in Appendix 2. 

30

40

50

60

70

80

90

100

400 500 600 700 800 900 1000

Gross Weight, 1000 lb

ACN

747-400

Rigid pavement

Code C subgrade

COMFAA program

Figure A3-4. B747-400 Rigid Pavement ACN Versus Gross Weight 

 A3-9 
AC 150/5335-5A - APPENDIX 3 09/28/06

30

40

50

60

70

80

90

100

100 1000 10000 100000 1000000

Annual Traffic Cycles

ACN

747-400

Rigid pavement

Subgrade modulus = 200 pci

COMFAA program

Figure A3-5. B747-400 Rigid Pavement Life Versus ACN 

Table A3-4. Data for Constructing Rigid Pavement Life Curves for B747-400 

Annual 

Traffic 

Cycles 

Load 

Repetitions 

(coverages) 

(P/C = 3.46) 

T=14 

gross 

weight 

(lbs) 

T=14 

ACN 

T-16 

gross 

weight 

(lbs) 

T=16 

ACN 

500 2,890 910,000 78.6 1,063,000 97.7 

1,200 6,936 871,000 73.9 1,015,000 91.6 

2,400 13,873 827,100 68.8 962,000 85.0 

3,000 17,341 813,500 67.2 947,000 83.1 

5,000 28,902 783,900 63.8 913,500 79.0 

7,424 42,913 762,000 61.3 890,000 76.2 

20,000 115,607 714,500 56.1 835,000 69.7 

50,000 289,017 675,000 51.8 786,250 64.1 

86,500 500,000 654,000 49.6 760,000 61.1 

129,750 750,000 638,500 48.0 742,000 59.1 

A3-10 
09/28/06 AC 150/5335-5A - APPENDIX 3 

3. It is now possible to relate the effects of gross weight, ACN, and pavement life by 

combining these two charts, as shown in Figure A3-6. The left hand side of this 

figure is the chart of Figure A3-4, while the right had chart is that of Figure A3-5. It 

can now be seen that the rating of PCN 61/R/C/W/T on a 14-inch pavement equates 

to 7,424 traffic cycles per year of a B747-400 at 762,000 pounds. 

4. The line for a thickness of 16 inches in Figures A3-5 and A3-6 shows how pavement 

life is increased by the addition of two inches of concrete. This line is included, not 

to imply that an overlay of two inches is recommended, but to show the effect of 

increased thickness. It can be seen that the 16-inch pavement will accommodate a 

B747-400 with a gross weight of 890,000 pounds. Alternately, at a gross weight of 

762,000 pounds, the B747-400 can be accommodated on the thicker pavement to 

about 85,000 annual traffic cycles. Not shown directly in Figure A3-6 is that a 15-

inch pavement (one additional inch) will accommodate 25,000 annual traffic cycles 

of a B747-400 at 762,000 pounds. 

 A3-11 
AC 150/5335-5A - APPENDIX 3 09/28/06

Figure A3-6. B747-400 Rigid Pavement Life 

ACN 

Annual Traffic Cycles Gross Weight, 1,000 lb 

400 500 600 700 800 900 1000

7060504030

1000 10000 100000 1000000

1009080

Rigid pavementSubgrade modulus = 200 pciParallel TaxiwayFuel obtained at airportCOMFAA 

T = 14 in.

T = 16 in.

ram

g ro

p

747-40093.32% wt. on main gear200 psi tire pressureCode C subgradeCOMFAA program

A3-12 
09/28/06 AC 150/5335-5A - APPENDIX 3 

 b. Rigid Pavement Overload Illustration 2. This example illustrates the effect of 

ICAO allowable overloading in which the ACN is no more than 5 percent above the PCN and the 

number of traffic cycles does not exceed 5 percent of the total annual traffic. 

Table A2-5 is repeated here as Table A3-5, but with a new airplane added to the traffic mix. The 

ACN of the new airplane is 5 percent above the rated PCN of 61/R/C/W/T. The 760 annual 

departures of the new airplane represents 5 percent of the 15,200 total annual departures, as 

shown in Table A2-4. 

Table A3-5. Rigid Pavement Overload Example with New Airplane 

Airplane 

Operating 

Weight, 

lbs 

Tire 

Pressure 

(psi) 

ACN 

(R/C) 

** 

P/C 

Annual 

Departures 

Load 

Repetitions 

B727-200 185,000 148 55 2.92 400 2,740 

B737-300 130,000 195 38 3.79 6,000 31,662 

A319-100 145,000 173 42 3.18 1,200 7,547 

B747-400 820,000 200 68 3.46 3,000 17,341 

B767-300ER 370,000 190 58 3.60 2,000 11,111 

DC8-63 330,000 194 62 3.35 800 4,776 

A300-B4 370,000 205 67 3.49 1,500 8,596 

B777-200 600,000 215 77 4.25 300 1,412 

A300-600R 362,250 231 64 3.39 760 4,484 

** Rigid P/C determined at 95 percent of gross load on main gear and at manufacturer’s recommended 

operating characteristics for ACN calculation 

It is next necessary to determine the new total departures of the critical B747-400 airplane. To do 

so, Table A2-7 is shown here as Table A3-6 with the new A300-600R airplane included. As seen 

from this table, the number of B747-400 equivalent annual departures has increased to 7,934 from 

7,424. The new equivalent departures are 7,934, which convert to 45,861 lifetime load repetitions 

(7,934 * 20 ÷3.46 = 45,861). From the COMFAA program, the new allowable B747-400 gross 

weight is calculated to be 758,000 pounds and the ACN at this weight is 60.9/R/C. 

 A3-13 
AC 150/5335-5A - APPENDIX 3 09/28/06

Table A3-6 Equivalent Annual Departures of the Critical Airplane 

Airplane 

Annual 

Departures 

Gear 

Type 

(R2) 

Equiv. (2D) 

Departures 

(W2) 

Wheel 

Load 

(W1) 

B747-400 

Wheel 

Load 

(R1) 

B747-400 

Equiv.Ann. 

Departures 

727-200 400 D 256 43,938 48,688 194 

737-300 6,000 D 3,840 30,875 48,688 716 

A319-100 1,200 D 768 34,438 48,688 268 

B747-400 3,000 2D/2D2 3,000 48,688 48,688 3,000 

B767-200ER 2,000 2D 2,000 43,938 48,688 1,368 

DC8-63 800 2D 800 39,188 48,688 403 

A300-B4 1,500 2D 1,500 43,938 48,688 1,041 

B777-200 300 3D 468 47,500 48,688 434 

A300-600R 760 2D 760 42,988 48,688 510 

15,960 7,934 

The new recommended PCN would then be 61/R/C/W/T. Note this new PCN is the same as the 

existing PCN due to rounding. Alternatively, the effect on pavement thickness can be seen by 

keeping the critical airplane gross weight the same at 762,000 pounds. The resulting required 

concrete slab thickness is 14.04 inches, which is a 0.04-inch increase which not practical to 

measure or attempt to construct. 

A3-14 
09/28/06 AC 150/5335-5A APPENDIX 4 

APPENDIX 4. RELATED READING MATERIAL 

The following publications were used in the development of this AC: 

a. AC 150/5320-6, Airport Pavement Design and Evaluation. This publication is 

available free of charge from the Department of Transportation, Section, M-442.32, Washington, 

DC, 20590. 

c. ICAO Bulletin, Official Magazine of International Civil Aviation, Airport Technology, 

Volume 35, No. 1, Montreal, Quebec, Canada H3A 2R2, January l980. 

 A4-1