**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.

![](media/image1.png){width="2.5034722222222223in"
height="0.5805555555555556in"}

David L. Bennett

Director, Office of Airport Safety and Standards

#   {#section .AC-header1}

#  {#section-1 .AC-header1}

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TABLE OF CONTENTS

[CHAPTER 1. INTRODUCTION 1](#chapter-1.-introduction)

[1.0 BACKGROUND 1](#__RefHeading___Toc146697831)

[1.1 DEVELOPMENT OF A STANDARDIZED METHOD
1](#__RefHeading___Toc146697832)

[**a.** **Definition of ACN** 1](#__RefHeading___Toc146697833)

[**b.** **Definition of PCN** 1](#__RefHeading___Toc146697834)

[**c.** **System Methodology** 1](#__RefHeading___Toc146697835)

[1.2 APPLICATION 1](#__RefHeading___Toc146697836)

[1.3 LIMITATIONS OF THE ACN-PCN SYSTEM 1](#__RefHeading___Toc146697837)

[CHAPTER 2. DETERMINATION OF AIRCRAFT CLASSIFICATION NUMBER
3](#chapter-2.-determination-of-aircraft-classification-number)

[2.0 DETERMINATION OF THE ACN. 3](#__RefHeading___Toc146697839)

[2.1 SUBGRADE CATEGORY. 3](#__RefHeading___Toc146697840)

[**Table 2-1. Standard Subgrade Support Conditions for Rigid Pavement
ACN Calculation**
3](#table-2-1.-standard-subgrade-support-conditions-for-rigid-pavement-acn-calculation)

[**Table 2-2. Standard Subgrade Support Conditions for Flexible Pavement
ACN Calculation**
3](#table-2-2.-standard-subgrade-support-conditions-for-flexible-pavement-acn-calculation)

[2.2 OPERATIONAL FREQUENCY. 3](#__RefHeading___Toc146697843)

[2.3 RIGID PAVEMENT ACN. 4](#__RefHeading___Toc146697844)

[2.4 FLEXIBLE PAVEMENT ACN. 4](#__RefHeading___Toc146697845)

[2.5 ACN CALCULATION. 4](#__RefHeading___Toc146697846)

[2.6 VARIABLES INVOLVED IN DETERMINATION OF ACN VALUES.
4](#__RefHeading___Toc146697847)

[CHAPTER 3. DETERMINATION OF ACN VALUES USING COMFAA
5](#chapter-3.-determination-of-acn-values-using-comfaa)

[3.0 ANNOUNCEMENT OF COMFAA SOFTWARE APPLICATION.
5](#__RefHeading___Toc146697849)

[3.1 ORIGIN OF THE COMFAA PROGRAM. 5](#__RefHeading___Toc146697850)

[3.2 COMFAA PROGRAM. 5](#__RefHeading___Toc146697851)

[**a.** **ACN Computation Mode** 5](#__RefHeading___Toc146697852)

[**b.** **Pavement Thickness Mode:** 5](#b.-pavement-thickness-mode)

[3.3 INTERNAL AIRPLANE LIBRARY. 5](#__RefHeading___Toc146697854)

[3.4 EXTERNAL AIRPLANE LIBRARY. 6](#__RefHeading___Toc146697855)

[3.5 USING THE COMFAA PROGRAM. 6](#__RefHeading___Toc146697856)

[**Figure 3-1. Operation of the COMFAA Program in ACN Mode**
7](#figure-3-1.-operation-of-the-comfaa-program-in-acn-mode)

[**Figure 3-2. Operation of the COMFAA Program in Pavement Thickness
Mode**
8](#figure-3-2.-operation-of-the-comfaa-program-in-pavement-thickness-mode)

[CHAPTER 4. DETERMINATION OF PCN NUMERICAL VALUE
9](#chapter-4.-determination-of-pcn-numerical-value)

[4.0 PCN CONCEPT. 9](#__RefHeading___Toc146697861)

[4.1 DETERMINATION OF NUMERICAL PCN VALUE.
9](#__RefHeading___Toc146697862)

[4.2 Using Airplane Method to Determine PCN.
9](#__RefHeading___Toc146697863)

[**a.** **Assumptions of the Using Airplane Method.**
9](#__RefHeading___Toc146697864)

[**b.** **Inaccuracies of the Using Airplane Method.**
9](#__RefHeading___Toc146697865)

[4.3 Technical Evaluation Method to Determine PCN.
9](#__RefHeading___Toc146697866)

[**a.** **Determination of the PCN Value.**
10](#__RefHeading___Toc146697867)

[**b.** **Concept of Equivalent Traffic.**
10](#__RefHeading___Toc146697868)

[**c.** **Counting Airplane Operations.**
10](#__RefHeading___Toc146697869)

[4.4 LIMITATIONS OF THE PCN. 11](#__RefHeading___Toc146697870)

[4.5 REPORTING THE PCN. 11](#__RefHeading___Toc146697871)

[**a.** **Numerical PCN Value.** 11](#__RefHeading___Toc146697872)

[**b.** **Pavement Type.** 11](#__RefHeading___Toc146697873)

[**Table 4-1. Pavement Codes for Reporting PCN**
11](#table-4-1.-pavement-codes-for-reporting-pcn)

[**i)** **Flexible Pavement.** 11](#__RefHeading___Toc146697875)

[**ii)** **Rigid Pavement.** 11](#__RefHeading___Toc146697876)

[**iii)** **Composite Pavement.** 12](#__RefHeading___Toc146697877)

[**c.** **Subgrade Strength Category** 12](#__RefHeading___Toc146697878)

[**d.** **Allowable Tire Pressure.** 12](#__RefHeading___Toc146697879)

[**TABLE 4-2. Tire Pressure Codes for Reporting PCN**
12](#table-4-2.-tire-pressure-codes-for-reporting-pcn)

[**i)** **Tire Pressures on Rigid Pavements.**
12](#__RefHeading___Toc146697881)

[**ii)** **Tire Pressures on Flexible Pavements**
12](#__RefHeading___Toc146697882)

> [e. Method Used to Determine PCN. 13](#__RefHeading___Toc146697883)
>
> [f. Example PCN Reporting. 13](#__RefHeading___Toc146697884)
>
> [g. Report PCN Values to the FAA. 13](#__RefHeading___Toc146697885)

[APPENDIX 1. EQUIVALENT TRAFFIC 1](#appendix-1.-equivalent-traffic)

[1.0 Equivalent Traffic. 1](#__RefHeading___Toc146697887)

[1.1 Equivalent Traffic Terminology. 1](#__RefHeading___Toc146697888)

[**a.** **Arrival (Landing) and Departure (Takeoff).**
1](#__RefHeading___Toc146697889)

[**b.** **Pass.** 2](#__RefHeading___Toc146697890)

[**Figure A1-1. Traffic Load Distribution Patterns**
2](#figure-a1-1.-traffic-load-distribution-patterns)

[**c.** **Coverage.** 3](#__RefHeading___Toc146697892)

[**d.** **Operation.** 4](#__RefHeading___Toc146697893)

[**e.** **Traffic Cycle and Traffic Cycle Ratio.**
4](#__RefHeading___Toc146697894)

[**Table A1-1. TC/C Ratio for Flexible Pavements -- Additional Fuel Not
Obtained**
5](#table-a1-1.-tcc-ratio-for-flexible-pavements-additional-fuel-not-obtained)

[**Table A1-2. TC/C Ratio for Flexible Pavements -- Additional Fuel
Obtained**
5](#table-a1-2.-tcc-ratio-for-flexible-pavements-additional-fuel-obtained)

[**Table A1-3. TC/C Ratio for Rigid Pavements -- Additional Fuel Not
Obtained**
6](#table-a1-3.-tcc-ratio-for-rigid-pavements-additional-fuel-not-obtained)

[**Table A1-4. TC/C Ratio for Rigid Pavements -- Additional Fuel
Obtained**
6](#table-a1-4.-tcc-ratio-for-rigid-pavements-additional-fuel-obtained)

[1.2 Equivalent Traffic Based on Gear Type.
6](#__RefHeading___Toc146697899)

[**Table A1-5. Conversion Factors to Convert from One Landing Gear Type
to Another**
7](#table-a1-5.-conversion-factors-to-convert-from-one-landing-gear-type-to-another)

[**Table A1-6. Equivalency Conversion to a Dual Tandem (2D) Gear Type**
8](#table-a1-6.-equivalency-conversion-to-a-dual-tandem-2d-gear-type)

[**Table A1-7. Equivalency Conversion to a Dual Gear (D) Type**
8](#table-a1-7.-equivalency-conversion-to-a-dual-gear-d-type)

[1.3 Equivalent Traffic Based on Load Magnitude.
9](#__RefHeading___Toc146697903)

[**Table A1-8. Equivalent Traffic Cycles Based on Load Magnitude**
10](#table-a1-8.-equivalent-traffic-cycles-based-on-load-magnitude)

[APPENDIX 2. PCN DETERMINATION EXAMPLES
1](#appendix-2.-pcn-determination-examples)

[1.0. The Using Airplane Method. 1](#__RefHeading___Toc146697906)

[1.1 Using Airplane Example for Flexible Pavements
2](#__RefHeading___Toc146697907)

[**Table A2-1. Using Airplane and Traffic for a Flexible Pavement**
2](#table-a2-1.-using-airplane-and-traffic-for-a-flexible-pavement)

[1.2 Using Airplane Example for Rigid Pavements.
3](#__RefHeading___Toc146697909)

[**Table A2-2. Using Airplane and Traffic for a Rigid Pavement**
3](#table-a2-2.-using-airplane-and-traffic-for-a-rigid-pavement)

[2.0 The Technical Evaluation Method. 4](#__RefHeading___Toc146697911)

[2.1 Technical Evaluation for Flexible Pavements
4](#__RefHeading___Toc146697912)

[2.2 Technical Evaluation Examples for Flexible Pavements.
7](#__RefHeading___Toc146697913)

[**a.** **Flexible Pavement Example 1.**
7](#__RefHeading___Toc146697914)

[**Figure A2‑1. Flexible Pavement Example Cross-Section**
7](#figure-a21.-flexible-pavement-example-cross-section)

[**Table A2-3. Technical Evaluation Critical Airplane Determination**
8](#table-a2-3.-technical-evaluation-critical-airplane-determination)

[**Table A2-4. Equivalent Annual Departures of the Critical Airplane**
9](#table-a2-4.-equivalent-annual-departures-of-the-critical-airplane)

[**b.** **Flexible Pavement Example 2.**
10](#__RefHeading___Toc146697918)

[**c.** **Flexible Pavement Example 3.**
10](#__RefHeading___Toc146697919)

[**d.** **Flexible Pavement Example 4.**
11](#__RefHeading___Toc146697920)

[2.3 Technical Evaluation for Rigid Pavements.
11](#__RefHeading___Toc146697921)

[2.4 Technical Evaluation Examples for Rigid Pavements
13](#__RefHeading___Toc146697922)

[**a.** **Rigid Pavement Example 1.** 13](#__RefHeading___Toc146697923)

[**Figure A2-2. Rigid Pavement Example Cross-Section**
14](#figure-a2-2.-rigid-pavement-example-cross-section)

[**Table A2-5. Rigid Pavement Technical Evaluation Traffic**
14](#table-a2-5.-rigid-pavement-technical-evaluation-traffic)

[**Table A2-6. Technical Evaluation Critical Airplane Determination**
15](#table-a2-6.-technical-evaluation-critical-airplane-determination)

[**Table A2-7. Equivalent Annual Departures of the Critical Airplane**
15](#table-a2-7.-equivalent-annual-departures-of-the-critical-airplane)

[**b.** **Rigid Pavement Example 2.** 16](#__RefHeading___Toc146697928)

[**c.** **Rigid Pavement Example 3.** 17](#__RefHeading___Toc146697929)

[APPENDIX 3. Pavement Overload Evaluation by the ACN-PCN system
1](#appendix-3.-pavement-overload-evaluation-by-the-acn-pcn-system)

[1.0 ICAO Pavement Overload Evaluation Guidance.
1](#__RefHeading___Toc146697931)

[1.1 Overload Guidance. 1](#__RefHeading___Toc146697932)

[1.2 Adjustments for Flexible Pavement Overloads.
2](#__RefHeading___Toc146697933)

[**a.** **Flexible Pavement Overload Illustration 1.**
3](#__RefHeading___Toc146697934)

[**b.** **Flexible Pavement Overload Illustration 2.**
3](#__RefHeading___Toc146697935)

[**Figure A3-1. B747-400 Flexible Pavement ACN Versus Gross Weight**
3](#figure-a3-1.-b747-400-flexible-pavement-acn-versus-gross-weight)

[**Figure A3-2. B747-400 Flexible Pavement Life Versus ACN**
4](#figure-a3-2.-b747-400-flexible-pavement-life-versus-acn)

[**Table A3-1. Data for Constructing Flexible Pavement Life Curves for
B747-400**
5](#table-a3-1.-data-for-constructing-flexible-pavement-life-curves-for-b747-400)

[**Figure A3-3. B747-400 Flexible Pavement Life**
6](#figure-a3-3.-b747-400-flexible-pavement-life)

[**c.** **Flexible Pavement Overload Illustration 3.**
7](#__RefHeading___Toc146697940)

[**Table A3-2. Flexible Pavement Overload Airplane Added**
7](#table-a3-2.-flexible-pavement-overload-airplane-added)

[**Table A3-3. Flexible Pavement New Airplane Equivalent Traffic**
8](#table-a3-3.-flexible-pavement-new-airplane-equivalent-traffic)

[1.3 Adjustments for Rigid Pavement Overloads
8](#__RefHeading___Toc146697943)

[**a.** **Rigid Overload Illustration 1.**
8](#__RefHeading___Toc146697944)

[**Figure A3-4. B747-400 Rigid Pavement ACN Versus Gross Weight**
9](#figure-a3-4.-b747-400-rigid-pavement-acn-versus-gross-weight)

[**Figure A3-5. B747-400 Rigid Pavement Life Versus ACN**
10](#figure-a3-5.-b747-400-rigid-pavement-life-versus-acn)

[**Table A3-4. Data for Constructing Rigid Pavement Life Curves for
B747-400**
10](#table-a3-4.-data-for-constructing-rigid-pavement-life-curves-for-b747-400)

[**Figure A3-6. B747-400 Rigid Pavement Life**
12](#figure-a3-6.-b747-400-rigid-pavement-life)

[**b.** **Rigid Pavement Overload Illustration 2.**
13](#__RefHeading___Toc146697949)

[**Table A3-5. Rigid Pavement Overload Example with New Airplane**
13](#table-a3-5.-rigid-pavement-overload-example-with-new-airplane)

[**Table A3-6 Equivalent Annual Departures of the Critical Airplane**
14](#table-a3-6-equivalent-annual-departures-of-the-critical-airplane)

[APPENDIX 4. RELATED READING MATERIAL
1](#appendix-4.-related-reading-material)

#   {#section-2 .AC-header1}

#  {#section-3 .AC-header1}

#  {#section-4 .AC-header1}

# This page intentionally left blank {#this-page-intentionally-left-blank .AC-header1}

#  {#section-5 .AC-header1}

# CHAPTER 1. INTRODUCTION {#chapter-1.-introduction .AC-header1}

#  {#section-6 .AC-header1}

[]{#__RefHeading___Toc146697831 .anchor}**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).

[]{#__RefHeading___Toc146697832 .anchor}**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).

[]{#__RefHeading___Toc146697833 .anchor}**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.

[]{#__RefHeading___Toc146697834 .anchor}**b. Definition of PCN.** PCN is
a number that expresses the load-carrying capacity of a pavement for
unrestricted operations.

[]{#__RefHeading___Toc146697835 .anchor}**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.

[]{#__RefHeading___Toc146697836 .anchor}**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.

[]{#__RefHeading___Toc146697837 .anchor}**1.3 LIMITATIONS OF THE ACN-PCN
SYSTE**M**.** 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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# CHAPTER 2. DETERMINATION OF AIRCRAFT CLASSIFICATION NUMBER {#chapter-2.-determination-of-aircraft-classification-number .AC-header1}

[]{#__RefHeading___Toc146697839 .anchor}**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.

[]{#__RefHeading___Toc146697840 .anchor}**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 {#table-2-1.-standard-subgrade-support-conditions-for-rigid-pavement-acn-calculation .AC-header3}

+----------------+-----------------+----------------------+-----------+
| Subgrade       | Subgrade        | Represents\          | Code      |
| Strength       | Support\        | pci (MN/m^3^)        | De        |
| Category       | k-Value         |                      | signation |
|                |                 |                      |           |
|                | pci (MN/m^3^)   |                      |           |
+----------------+-----------------+----------------------+-----------+
| High           | 552.6 (150)     | k [\>]{.underline}   | A         |
|                |                 | 442 (                |           |
|                |                 | [\>]{.underline}120) |           |
+----------------+-----------------+----------------------+-----------+
| Medium         | 294.7 (80)      | 221\<k\<442          | B         |
|                |                 | (60\<k\<120)         |           |
+----------------+-----------------+----------------------+-----------+
| Low            | 147.4 (40)      | 92\<                 | C         |
|                |                 | k[\<]{.underline}221 |           |
|                |                 | (25\<                |           |
|                |                 | k[\<]{.underline}60) |           |
+----------------+-----------------+----------------------+-----------+
| Ultra Low      | 73.7 (20)       | k[\<]{.underline}92  | D         |
|                |                 | ([\<]{.underline}25) |           |
+----------------+-----------------+----------------------+-----------+

# Table 2-2. Standard Subgrade Support Conditions for Flexible Pavement ACN Calculation {#table-2-2.-standard-subgrade-support-conditions-for-flexible-pavement-acn-calculation .AC-header3}

  ---------------- ------------------ ------------------------- -------------
  Subgrade         Subgrade Support\  Represents                Code
  Strength         CBR-Value                                    Designation
  Category                                                      

  High             15                 CBR [\>]{.underline} 13   A

  Medium           10                 8\<CBR\<13                B

  Low              6                  4\<CBR[\<]{.underline}8   C

  Ultra Low        3                  CBR[\<]{.underline}4      D
  ---------------- ------------------ ------------------------- -------------

[]{#__RefHeading___Toc146697843 .anchor}**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 full-load
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.

[]{#__RefHeading___Toc146697844 .anchor}**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).

[]{#__RefHeading___Toc146697845 .anchor}**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.

[]{#__RefHeading___Toc146697846 .anchor}**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).

[]{#__RefHeading___Toc146697847 .anchor}**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.

# CHAPTER 3. DETERMINATION OF ACN VALUES USING COMFAA {#chapter-3.-determination-of-acn-values-using-comfaa .AC-header1}

[]{#__RefHeading___Toc146697849 .anchor}**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.

[]{#__RefHeading___Toc146697850 .anchor}**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.

[]{#__RefHeading___Toc146697851 .anchor}**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.

[]{#__RefHeading___Toc146697852 .anchor}**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: {#b.-pavement-thickness-mode .AC-header3}

> • 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.

[]{#__RefHeading___Toc146697854 .anchor}**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 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.

[]{#__RefHeading___Toc146697855 .anchor}**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-to-coverage (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.

[]{#__RefHeading___Toc146697856 .anchor}**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.

![](media/image4.png){width="4.664583333333334in"
height="4.039583333333334in"}

# Figure 3-1. Operation of the COMFAA Program in ACN Mode  {#figure-3-1.-operation-of-the-comfaa-program-in-acn-mode .AC-header3}

![](media/image5.png){width="5.179861111111111in"
height="4.465972222222222in"}

#  {#section-7 .AC-header1}

#  {#section-8 .AC-header1}

# Figure 3-2. Operation of the COMFAA Program in Pavement Thickness Mode  {#figure-3-2.-operation-of-the-comfaa-program-in-pavement-thickness-mode .AC-header3}

# CHAPTER 4. DETERMINATION OF PCN NUMERICAL VALUE {#chapter-4.-determination-of-pcn-numerical-value .AC-header1}

[]{#__RefHeading___Toc146697861 .anchor}**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.

[]{#__RefHeading___Toc146697862 .anchor}**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.

[]{#__RefHeading___Toc146697863 .anchor}**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.

[]{#__RefHeading___Toc146697864 .anchor}**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.

[]{#__RefHeading___Toc146697865 .anchor}**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.

[]{#__RefHeading___Toc146697866 .anchor}**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 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.

[]{#__RefHeading___Toc146697867 .anchor}**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.

[]{#__RefHeading___Toc146697868 .anchor}**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.

[]{#__RefHeading___Toc146697869 .anchor}**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.

[]{#__RefHeading___Toc146697870 .anchor}**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.

[]{#__RefHeading___Toc146697871 .anchor}**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.

[]{#__RefHeading___Toc146697872 .anchor}**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.

[]{#__RefHeading___Toc146697873 .anchor}**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 {#table-4-1.-pavement-codes-for-reporting-pcn .AC-header3}

  ------------------------------------ ----------------------------------
  Pavement Type                        Pavement Code

  Flexible                             F

  Rigid                                R
  ------------------------------------ ----------------------------------

[]{#__RefHeading___Toc146697875 .anchor}**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.

[]{#__RefHeading___Toc146697876 .anchor}**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
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.

[]{#__RefHeading___Toc146697877 .anchor}**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.

[]{#__RefHeading___Toc146697878 .anchor}**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.

[]{#__RefHeading___Toc146697879 .anchor}**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 {#table-4-2.-tire-pressure-codes-for-reporting-pcn .AC-header3}

  ------------- ----------- ---------------------------------------------
  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)
  ------------- ----------- ---------------------------------------------

[]{#__RefHeading___Toc146697881 .anchor}**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.

[]{#__RefHeading___Toc146697882 .anchor}**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 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.

[]{#__RefHeading___Toc146697883 .anchor}**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.

[]{#__RefHeading___Toc146697884 .anchor}**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.

[]{#__RefHeading___Toc146697885 .anchor}**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.

This page intentionally left blank

# APPENDIX 1. EQUIVALENT TRAFFIC {#appendix-1.-equivalent-traffic .AC-header1}

[]{#__RefHeading___Toc146697887 .anchor}**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.

[]{#__RefHeading___Toc146697888 .anchor}**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:

[]{#__RefHeading___Toc146697889 .anchor}**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.

[]{#__RefHeading___Toc146697890 .anchor}**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.

![](media/image6.wmf){width="6.2868055555555555in"
height="1.5159722222222223in"}

**Figure A1-1a. Runway with Parallel Taxiway**

![](media/image7.wmf){width="6.2868055555555555in"
height="1.5972222222222223in"}

**Figure A1-1b. Runway with Central Taxiway**

# Figure A1-1. Traffic Load Distribution Patterns {#figure-a1-1.-traffic-load-distribution-patterns .AC-header3}

**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.

**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.

[]{#__RefHeading___Toc146697892 .anchor}**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 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.

[]{#__RefHeading___Toc146697893 .anchor}**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.

[]{#__RefHeading___Toc146697894 .anchor}**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):

$\text{TC}/C = P/C \div P/\text{TC}$ (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 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 {#table-a1-1.-tcc-ratio-for-flexible-pavements-additional-fuel-not-obtained .AC-header3}

  ----------------- --------------- ------------------ ------------------
  Taxiway Type      Typical Dual    Typical Dual       Typical Triple
                    Gear (D)        Tandem Gear (2D)   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 {#table-a1-2.-tcc-ratio-for-flexible-pavements-additional-fuel-obtained .AC-header3}

  ----------------- --------------- ------------------ ------------------
  Taxiway Type      Typical Dual    Typical Dual       Typical Triple
                    Gear (D)        Tandem Gear (2D)   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.

# Table A1-3. TC/C Ratio for Rigid Pavements -- Additional Fuel Not Obtained {#table-a1-3.-tcc-ratio-for-rigid-pavements-additional-fuel-not-obtained .AC-header3}

  ----------------- --------------- ------------------ ------------------
  Taxiway Type      Typical Dual    Typical Dual       Typical Triple
                    Gear (D)        Tandem Gear (2D)   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 {#table-a1-4.-tcc-ratio-for-rigid-pavements-additional-fuel-obtained .AC-header3}

  ----------------- --------------- ------------------ ------------------
  Taxiway Type      Typical Dual    Typical Dual       Typical Triple
                    Gear (D)        Tandem Gear (2D)   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
  ----------------- --------------- ------------------ ------------------

[]{#__RefHeading___Toc146697899 .anchor}**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.

# Table A1-5. Conversion Factors to Convert from One Landing Gear Type to Another {#table-a1-5.-conversion-factors-to-convert-from-one-landing-gear-type-to-another .AC-header3}

  -------------------------- ------------------ --------------------------
  To Convert From Gear Type  To Gear Type (M)   Multiply Traffic Cycles By
  (N)                                           

  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:

$0\text{.}8^{(M - N)}$ (Equation A1-2)

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.

# Table A1-6. Equivalency Conversion to a Dual Tandem (2D) Gear Type {#table-a1-6.-equivalency-conversion-to-a-dual-tandem-2d-gear-type .AC-header3}

  ---------------- ----------- -------------- --------------- -------------
  Airplane         Gear Type   Annual Traffic Conversion      Total
                               Cycles (TC)    Factor          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 {#table-a1-7.-equivalency-conversion-to-a-dual-gear-d-type .AC-header3}

  --------------- ----------- --------------- ------------- --------------
  Airplane        Gear Type   Annual Traffic  Conversion    Total
                              Cycles (TC)     Factor        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
  --------------- ----------- --------------- ------------- --------------

[]{#__RefHeading___Toc146697903 .anchor}**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:

$\text{Log R}_{1} = \text{ Log R}_{2} \times \sqrt{\frac{W_{2}}{W_{1}}}$
or $R_{1} = \left( R_{2} \right)^{\sqrt{W_{2}/W_{1}}}$ (Equation A1-3)

Where:

R~1~ = Equivalent traffic cycles of the critical airplane

R~2~ = Traffic cycles of a given airplane expressed

in terms of the critical airplane landing gear

W~1~ = Wheel load of the critical airplane

W~2~ = 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 R~2~ column is from Table A1-6.

# Table A1-8. Equivalent Traffic Cycles Based on Load Magnitude {#table-a1-8.-equivalent-traffic-cycles-based-on-load-magnitude .AC-header3}

+----------+--------+----------+-------+--------+---------+-----------+
| Airplane | Ope    | (W~2~)   | (     | (W~2   | (R~1~)  |           |
|          | rating |          | R~2~) | ~/W~1~ |         |           |
|          | Weight | Single   |       | )^1/2^ | Equ     |           |
|          | lb     | Wheel    | (2D)  |        | ivalent |           |
|          |        | Load, lb | TC    | Wheel  | B       |           |
|          |        |          |       | Load   | 747-400 |           |
|          |        |          |       | Ratio  | TC      |           |
+----------+--------+----------+-------+--------+---------+-----------+
| 727-200  | 1      | 43,938   | 256   | 0.95   | 194     |           |
|          | 85,000 |          |       |        |         |           |
+----------+--------+----------+-------+--------+---------+-----------+
| 737-300  | 1      | 30,875   | 3,840 | 0.796  | 716     |           |
|          | 30,000 |          |       |        |         |           |
+----------+--------+----------+-------+--------+---------+-----------+
| A319-100 | 1      | 34,438   | 768   | 0.841  | 268     |           |
|          | 45,000 |          |       |        |         |           |
+----------+--------+----------+-------+--------+---------+-----------+
| B747-400 | 8      | 48,688   | 3,000 | 1.000  | 3,000   | (Critical |
|          | 20,000 | (W1)     |       |        |         | Airplane) |
+----------+--------+----------+-------+--------+---------+-----------+
| B7       | 3      | 43,938   | 2,000 | 0.950  | 1,368   |           |
| 67-200ER | 70,000 |          |       |        |         |           |
+----------+--------+----------+-------+--------+---------+-----------+
| DC8-63   | 3      | 39,188   | 800   | 0.897  | 403     |           |
|          | 30,000 |          |       |        |         |           |
+----------+--------+----------+-------+--------+---------+-----------+
| A300-B4  | 3      | 43,938   | 1,500 | 0.950  | 1,041   |           |
|          | 70,000 |          |       |        |         |           |
+----------+--------+----------+-------+--------+---------+-----------+
| B777-200 | 6      | 47,500   | 468   | 0.988  | 434     |           |
|          | 00,000 |          |       |        |         |           |
+----------+--------+----------+-------+--------+---------+-----------+
|          |        |          | 1     |        | 7,424   |           |
|          |        |          | 2,632 |        |         |           |
+----------+--------+----------+-------+--------+---------+-----------+

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.

# APPENDIX 2. PCN DETERMINATION EXAMPLES {#appendix-2.-pcn-determination-examples .AC-header1}

[]{#__RefHeading___Toc146697906 .anchor}**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*.*

[]{#__RefHeading___Toc146697907 .anchor}**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 {#table-a2-1.-using-airplane-and-traffic-for-a-flexible-pavement .AC-header3}

  ------------- ------------- ------------- ------------ ------ ------------
  Airplane      Operating     Tire Pressure \% Gross     ACN    Annual
                Weight, lbs   (psi)         Weight on    F/B    Departures
                                            Main Gear           
                                            for ACN             

  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
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.

[]{#__RefHeading___Toc146697909 .anchor}**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 {#table-a2-2.-using-airplane-and-traffic-for-a-rigid-pavement .AC-header3}

  ------------- ------------- ------------- ----------- ------ ------------
  Airplane      Operating     Tire Pressure \% Gross    ACN    Annual
                Weight, lbs   (psi)         Weight on   R/C    Departures
                                            Main Gear          
                                            for ACN            

  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.

-   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.

[]{#__RefHeading___Toc146697911 .anchor}**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.

[]{#__RefHeading___Toc146697912 .anchor}**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.

-   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 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.

[]{#__RefHeading___Toc146697913 .anchor}**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.

[]{#__RefHeading___Toc146697914 .anchor}**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.

+-----------------------------------------------------------------------+
| ![](media/image8.emf)                                                 |
+-----------------------------------------------------------------------+
| # Figure A2‑1. Flexible Pavement Example Cross-Secti                  |
| on {#figure-a21.-flexible-pavement-example-cross-section .AC-header3} |
+-----------------------------------------------------------------------+

# Table A2-3. Technical Evaluation Critical Airplane Determination {#table-a2-3.-technical-evaluation-critical-airplane-determination .AC-header3}

+------------+---------+--------+------+----------+-------+---------+
| Airplane   | Op      | Tire   | ACN  | Annual   | Fle   | R       |
|            | erating | Pr     | F/B  | De       | xible | equired |
|            | Weight, | essure |      | partures |       | Thi     |
|            | (lbs)   | (psi)  |      |          | \*    | ckness, |
|            |         |        |      |          | \*P/C | (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.

# Table A2-4. Equivalent Annual Departures of the Critical Airplane {#table-a2-4.-equivalent-annual-departures-of-the-critical-airplane .AC-header3}

+----------+-------+------+-----+-------+-------+---------+----------+
| Airplane | A     | Gear | G   | (     | (     | (W~1~)  | (R~1~)   |
|          | nnual | Type | ear | R~2~) | W~2~) |         |          |
|          | D     |      | T   |       |       | B       | B747-400 |
|          | epart |      | ype | E     | Wheel | 747-400 | Equiv.   |
|          | -ures |      | Fac | quiv. | Load  | Wheel   | Ann.     |
|          |       |      | tor | (2D)  | (lbs) | Load    | De       |
|          |       |      |     | D     |       | (lbs)   | partures |
|          |       |      |     | epart |       |         |          |
|          |       |      |     | -ures |       |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
| B727-200 | 400   | D    | 0   | 256   | 4     | 48,688  | 194      |
|          |       |      | .64 |       | 3,938 |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
| B737-300 | 6,000 | D    | 0   | 3,840 | 3     | 48,688  | 716      |
|          |       |      | .64 |       | 0,875 |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
| A319-100 | 1,200 | D    | 0   | 768   | 3     | 48,688  | 268      |
|          |       |      | .64 |       | 4,438 |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
| B747-400 | 3,000 | 2D   | 1   | 3,000 | 4     | 48,688  | 3,000    |
|          |       | /2D2 | .00 |       | 8,688 |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
| B7       | 2,000 | 2D   | 1   | 2,000 | 4     | 48,688  | 1368     |
| 67-300ER |       |      | .00 |       | 3,938 |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
| DC8-63   | 800   | 2D   | 1   | 800   | 3     | 48,688  | 403      |
|          |       |      | .00 |       | 9,188 |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
| A300-B4  | 1,500 | 2D   | 1   | 1,500 | 4     | 48,688  | 1,041    |
|          |       |      | .00 |       | 3,938 |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
| B777-200 | 300   | 3D   | 1   | 468   | 4     | 48,688  | 434      |
|          |       |      | .56 |       | 7,500 |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+
|          | 1     |      |     |       |       |         | 7,424    |
|          | 5,200 |      |     |       |       |         |          |
+----------+-------+------+-----+-------+-------+---------+----------+

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 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.

[]{#__RefHeading___Toc146697918 .anchor}**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.

[]{#__RefHeading___Toc146697919 .anchor}**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

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.

[]{#__RefHeading___Toc146697920 .anchor}**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).

[]{#__RefHeading___Toc146697921 .anchor}**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 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.

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.

[]{#__RefHeading___Toc146697922 .anchor}**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.

[]{#__RefHeading___Toc146697923 .anchor}**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.

![](media/image9.emf)

# Figure A2-2. Rigid Pavement Example Cross-Section {#figure-a2-2.-rigid-pavement-example-cross-section .AC-header3}

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 {#table-a2-5.-rigid-pavement-technical-evaluation-traffic .AC-header3}

+------------+-----------+--------+------+------+----------+---------+
| Airplane   | Operating | Tire   | ACN  | \*\* | Annual   | Co      |
|            | Weight,   | Pr     | (    |      | De       | verages |
|            | lbs       | essure | R/C) | P/C  | partures |         |
|            |           | (psi)  |      |      |          |         |
+------------+-----------+--------+------+------+----------+---------+
| 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.

# Table A2-6. Technical Evaluation Critical Airplane Determination {#table-a2-6.-technical-evaluation-critical-airplane-determination .AC-header3}

  ------------------------ -------------------- --------------------------
  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 {#table-a2-7.-equivalent-annual-departures-of-the-critical-airplane .AC-header3}

+-----------+----------+-------+----------+------+-------+----------+
| Airplane  | Annual   | Gear  | (R~2~)   | (W   | (     | (R~1~)   |
|           | De       | Type  |          | ~2~) | W~1~) |          |
|           | partures |       | Equiv.   |      |       | B747-400 |
|           |          |       | (2D)     | W    | B74   | Eq       |
|           |          |       | De       | heel | 7-400 | uiv.Ann. |
|           |          |       | partures | Load | Wheel | De       |
|           |          |       |          |      | Load  | partures |
+-----------+----------+-------+----------+------+-------+----------+
| B727-200  | 400      | D     | 256      | 43   | 4     | 194      |
|           |          |       |          | ,938 | 8,688 |          |
+-----------+----------+-------+----------+------+-------+----------+
| B737-300  | 6,000    | D     | 3,840    | 30   | 4     | 716      |
|           |          |       |          | ,875 | 8,688 |          |
+-----------+----------+-------+----------+------+-------+----------+
| A319-100  | 1,200    | D     | 768      | 34   | 4     | 268      |
|           |          |       |          | ,438 | 8,688 |          |
+-----------+----------+-------+----------+------+-------+----------+
| B747-400  | 3,000    | 2     | 3,000    | 48   | 4     | 3,000    |
|           |          | D/2D2 |          | ,688 | 8,688 |          |
+-----------+----------+-------+----------+------+-------+----------+
| B         | 2,000    | 2D    | 2,000    | 43   | 4     | 1,368    |
| 767-200ER |          |       |          | ,938 | 8,688 |          |
+-----------+----------+-------+----------+------+-------+----------+
| DC8-63    | 800      | 2D    | 800      | 39   | 4     | 403      |
|           |          |       |          | ,188 | 8,688 |          |
+-----------+----------+-------+----------+------+-------+----------+
| A300-B4   | 1,500    | 2D    | 1,500    | 43   | 4     | 1,041    |
|           |          |       |          | ,938 | 8,688 |          |
+-----------+----------+-------+----------+------+-------+----------+
| B777-200  | 300      | 3D    | 468      | 47   | 4     | 434      |
|           |          |       |          | ,500 | 8,688 |          |
+-----------+----------+-------+----------+------+-------+----------+
|           | 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

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.

[]{#__RefHeading___Toc146697928 .anchor}**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 accommodate the existing traffic
within its design life, and no adjustments to the pavement cross-section
or life will have to be made.

[]{#__RefHeading___Toc146697929 .anchor}**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---

$\text{TC}/C = 3\text{.}\text{46} \div 2 = 1\text{.}\text{73}$

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.

This page intentionally left blank

# APPENDIX 3. Pavement Overload Evaluation by the ACN-PCN system {#appendix-3.-pavement-overload-evaluation-by-the-acn-pcn-system .AC-header1}

[]{#__RefHeading___Toc146697931 .anchor}**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.

[]{#__RefHeading___Toc146697932 .anchor}**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 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.

[]{#__RefHeading___Toc146697933 .anchor}**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.

[]{#__RefHeading___Toc146697934 .anchor}**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.

[]{#__RefHeading___Toc146697935 .anchor}**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.

![](media/image10.wmf){width="4.654861111111111in"
height="3.8645833333333335in"}

# Figure A3-1. B747-400 Flexible Pavement ACN Versus Gross Weight {#figure-a3-1.-b747-400-flexible-pavement-acn-versus-gross-weight .AC-header3}

-   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.

![](media/image11.wmf){width="5.082638888888889in"
height="3.604861111111111in"}

# Figure A3-2. B747-400 Flexible Pavement Life Versus ACN {#figure-a3-2.-b747-400-flexible-pavement-life-versus-acn .AC-header3}

# Table A3-1. Data for Constructing Flexible Pavement Life Curves for B747-400 {#table-a3-1.-data-for-constructing-flexible-pavement-life-curves-for-b747-400 .AC-header3}

+-----------+-------------+------------+--------+-----------+-------+
| B747-400  | Coverages   | T=32       | T=32   | T-36      | T=36  |
| Annual    | at (P/C =   |            |        |           |       |
| D         | 1.73)       | Gross      | ACN    | Gross     | ACN   |
| epartures |             | Weight     |        | Weight    |       |
+-----------+-------------+------------+--------+-----------+-------+
| 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.

![Figure A3-3. B747-400 Flexible Pavement
Life](media/image12.wmf){width="5.125196850393701in"
height="3.3751968503937007in"}![Figure A3-3. B747-400 Flexible Pavement
Life](media/image13.wmf){width="5.25in" height="4.932677165354331in"}

#  {#section-9 .AC-header3}

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# Figure A3-3. B747-400 Flexible Pavement Life {#figure-a3-3.-b747-400-flexible-pavement-life .AC-header3}

[]{#__RefHeading___Toc146697940 .anchor}**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 {#table-a3-2.-flexible-pavement-overload-airplane-added .AC-header3}

+------------+---------+--------+------+----------+-------+---------+
| Airplane   | Op      | Tire   | ACN  | Annual   | Fle   | R       |
|            | erating | Pr     | F/B  | De       | xible | equired |
|            | Weight, | essure |      | partures |       | t,      |
|            | (lbs)   | (psi)  |      |          | \*    | (in.)   |
|            |         |        |      |          | \*P/C |         |
+------------+---------+--------+------+----------+-------+---------+
| 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.

# Table A3-3. Flexible Pavement New Airplane Equivalent Traffic {#table-a3-3.-flexible-pavement-new-airplane-equivalent-traffic .AC-header3}

+-----------+----------+-------+----------+------+--------+----------+
| Airplane  | Annual   | Gear  | (R~2~)   | (W   | (W~1~) | (R~1~)   |
|           | De       | Type  |          | ~2~) |        |          |
|           | partures |       | Equiv.   |      | B7     | B747-400 |
|           |          |       | (2D)     | W    | 47-400 | Equiv.   |
|           |          |       | De       | heel | Wheel  | Ann.     |
|           |          |       | partures | Load | Load   | De       |
|           |          |       |          | (    | (lbs)  | partures |
|           |          |       |          | lbs) |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| B727-200  | 400      | D     | 256      | 43   | 48,688 | 194      |
|           |          |       |          | ,938 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| B737-300  | 6,000    | D     | 3,840    | 30   | 48,688 | 716      |
|           |          |       |          | ,875 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| A319-100  | 1,200    | D     | 768      | 34   | 48,688 | 268      |
|           |          |       |          | ,438 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| B747-400  | 3,000    | 2     | 3,000    | 48   | 48,688 | 3,000    |
|           |          | D/2D2 |          | ,688 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| B         | 2,000    | 2D    | 2,000    | 43   | 48,688 | 1,368    |
| 767-300ER |          |       |          | ,938 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| DC8-63    | 800      | 2D    | 800      | 39   | 48,688 | 403      |
|           |          |       |          | ,188 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| A300-B4   | 1,500    | 2D    | 1,500    | 43   | 48,688 | 1,041    |
|           |          |       |          | ,938 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| B777-200  | 300      | 3D    | 468      | 47   | 48,688 | 434      |
|           |          |       |          | ,500 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
| L1011-500 | 760      | 2D    | 760      | 54   | 48,688 | 510      |
|           |          |       |          | ,981 |        |          |
+-----------+----------+-------+----------+------+--------+----------+
|           | 15,960   |       |          |      |        | 7,934    |
+-----------+----------+-------+----------+------+--------+----------+

[]{#__RefHeading___Toc146697943 .anchor}**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.

[]{#__RefHeading___Toc146697944 .anchor}**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 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.

![](media/image14.wmf){width="4.151388888888889in" height="3.8125in"}

# Figure A3-4. B747-400 Rigid Pavement ACN Versus Gross Weight {#figure-a3-4.-b747-400-rigid-pavement-acn-versus-gross-weight .AC-header3}

![](media/image15.wmf){width="5.0159722222222225in"
height="4.092361111111111in"}

# Figure A3-5. B747-400 Rigid Pavement Life Versus ACN {#figure-a3-5.-b747-400-rigid-pavement-life-versus-acn .AC-header3}

# Table A3-4. Data for Constructing Rigid Pavement Life Curves for B747-400 {#table-a3-4.-data-for-constructing-rigid-pavement-life-curves-for-b747-400 .AC-header3}

+----------+--------------+------------+--------+------------+--------+
| Annual   | Load         | T=14 gross | T=14   | T-16       | T=16   |
| Traffic  | Repetitions  | weight     | ACN    |            | ACN    |
| Cycles   |              | (lbs)      |        | gross      |        |
|          | (coverages)  |            |        | weight     |        |
|          | (P/C = 3.46) |            |        | (lbs)      |        |
+----------+--------------+------------+--------+------------+--------+
| 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   |
+----------+--------------+------------+--------+------------+--------+

> 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.

![Figure A3-6. B747-400 Rigid Pavement
Life](media/image16.wmf){width="5.125196850393701in"
height="4.3125984251968505in"}![Figure A3-6. B747-400 Rigid Pavement
Life](media/image17.wmf){width="5.05in" height="3.6692913385826773in"}

# Figure A3-6. B747-400 Rigid Pavement Life  {#figure-a3-6.-b747-400-rigid-pavement-life .AC-header3}

[]{#__RefHeading___Toc146697949 .anchor}**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 {#table-a3-5.-rigid-pavement-overload-example-with-new-airplane .AC-header3}

+------------+----------+---------+------+------+----------+----------+
| **         | **O      | **Tire  | *    | **\* | **Annual | **Load   |
| Airplane** | perating | P       | *ACN | \*** | Depa     | Repet    |
|            | Weight,  | ressure | (R/  |      | rtures** | itions** |
|            | lbs**    | (psi)** | C)** | **P  |          |          |
|            |          |         |      | /C** |          |          |
+------------+----------+---------+------+------+----------+----------+
| 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.

# Table A3-6 Equivalent Annual Departures of the Critical Airplane {#table-a3-6-equivalent-annual-departures-of-the-critical-airplane .AC-header3}

+-----------+----------+-------+----------+------+---------+----------+
| **A       | **Annual | *     | **       | **   | **(     | **       |
| irplane** | Depa     | *Gear | (R~2~)** | (W~2 | W~1~)** | (R~1~)** |
|           | rtures** | T     |          | ~)** |         |          |
|           |          | ype** | **Equiv. |      | **B     | **       |
|           |          |       | (2D)     | **W  | 747-400 | B747-400 |
|           |          |       | Depa     | heel | Wheel   | Eq       |
|           |          |       | rtures** | Lo   | Load**  | uiv.Ann. |
|           |          |       |          | ad** |         | Depa     |
|           |          |       |          |      |         | rtures** |
+-----------+----------+-------+----------+------+---------+----------+
| 727-200   | 400      | D     | 256      | 43   | 48,688  | 194      |
|           |          |       |          | ,938 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
| 737-300   | 6,000    | D     | 3,840    | 30   | 48,688  | 716      |
|           |          |       |          | ,875 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
| A319-100  | 1,200    | D     | 768      | 34   | 48,688  | 268      |
|           |          |       |          | ,438 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
| B747-400  | 3,000    | 2     | 3,000    | 48   | 48,688  | 3,000    |
|           |          | D/2D2 |          | ,688 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
| B         | 2,000    | 2D    | 2,000    | 43   | 48,688  | 1,368    |
| 767-200ER |          |       |          | ,938 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
| DC8-63    | 800      | 2D    | 800      | 39   | 48,688  | 403      |
|           |          |       |          | ,188 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
| A300-B4   | 1,500    | 2D    | 1,500    | 43   | 48,688  | 1,041    |
|           |          |       |          | ,938 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
| B777-200  | 300      | 3D    | 468      | 47   | 48,688  | 434      |
|           |          |       |          | ,500 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
| A300-600R | 760      | 2D    | 760      | 42   | 48,688  | 510      |
|           |          |       |          | ,988 |         |          |
+-----------+----------+-------+----------+------+---------+----------+
|           | 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.

# APPENDIX 4. RELATED READING MATERIAL {#appendix-4.-related-reading-material .AC-header1}

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.