U.S. Department 

of Transportation 

Federal Aviation 

Administration

Advisory 

Circular

Subject: AIRPORT PAVEMENT DESIGN AND Date: April 30, 2004 AC No: 150/5320-6D

EVALUATION Initiated by: AAS-100 Change: 3 

1. PURPOSE. Advisory Circular (AC) 150/5320-6D, Airport Pavement Design and Evaluation, has been revised to

incorporate the contents of AC 150/5320-16, Airport Pavement Design for the Boeing 777 Airplane, and to announce

design software for Chapters 3 and 4. 

2. PRINCIPAL CHANGES. This document makes three principal changes to AC 150/5320-6D: 

a. A new Chapter 7, Layered Elastic Pavement Design, incorporates the contents of AC 150/5320-16, which 

is cancelled. The user’s manual previously published as an appendix to 150/5320-16 is now available as a help file to

the LEDFAA design program described in Chapter 7. 

b. The layered elastic design method can now be used as an alternate design method to the procedures 

described in Chapters 3 and 4. Layered elastic design procedures were previously reserved for use only when the 

Boeing 777 aircraft was in the anticipated traffic mixture. 

c. A new Appendix 5, Airfield Pavement Design Software, announces the availability of Microsoft Excel®

spreadsheets for the design procedures described in Chapters 3 and 4. The appendix explains the purpose of the 

spreadsheets and describes how to access both the spreadsheets (F806faa.xls and R805faa.xls) and the associated user’s 

manuals. 

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DAVID L. BENNETT 

Director of Airport Safety and Standards 

4/30/2004 AC 150/5320-6D Change 3 

605. APPLICATION OF RIGID PAVEMENT EVALUATION PROCEDURES.................139 

606. USE OF RESULTS ..........................................................................................................139 

607. REPORTING PAVEMENT STRENGTH.......................................................................139 

 

CHAPTER 7. LAYERED ELASTIC PAVEMENT DESIGN 

SECTION 1. DESIGN CONSIDERATIONS ...........................................................................141 

 701. PURPOSE .......................................................................................................................141 

 702. APPLICATION................................................................................................................141 

 703. BACKGROUND..............................................................................................................141 

704. COMPUTER PROGRAM................................................................................................141 

705. PAVEMENT DESIGN CONSIDERATIONS.................................................................142 

706. FLEXIBLE PAVEMENT DESIGN.................................................................................142 

707. RIGID PAVEMENT DESIGN ........................................................................................143 

708. LAYERED ELASTIC OVERLAY DESIGN ..................................................................144 

TABLE 7-1. RIGID PAVEMENT DISTRESS TYPES USED TO CALCULATE 

THE STRUCTURAL CONDITION INDEX, SCI .........................................................145 

APPENDIX 1. ECONOMIC ANALYSIS .................................................................................... 1 

 1. BACKGROUND................................................................................................................... 1 

2. ANALYSIS METHOD ......................................................................................................... 1 

3. STEP BY STEP PROCEDURE.............................................................................................1 

4. EXAMPLE PROBLEM - LIGHT-LOAD GENERAL AVIATION AIRPORT ..................2 

TABLE 1. COSTS OF REHABILITATION ACTIVITIES.............................................. 4 

 TABLE 3. SUMMARY OF ALTERNATIVES .................................................................5 

 TABLE 4. COMPARATIVE RANKING OF ALTERNATIVES......................................5 

 5. SUMMARY .......................................................................................................................... 5 

APPENDIX 2. DEVELOPMENT OF PAVEMENT DESIGN CURVES ................................ 1 

 1. BACKGROUND................................................................................................................... 1 

 TABLE 1. SINGLE WHEEL ASSEMBLY........................................................................1 

TABLE 2. DUAL WHEEL ASSEMBLY.......................................................................... 1 

 TABLE 3. DUAL TANDEM ASSEMBLY........................................................................1 

2. RIGID PAVEMENTS............................................................................................................1

 TABLE 4. PASS-TO-COVERAGE RATIOS FOR RIGID PAVEMENTS .....................2 

 FIGURE 1. ASSEMBLY POSITIONS FOR RIGID PAVEMENT ANALYSIS................3 

 FIGURE 2. PERCENT THICKNESS VS. COVERAGES ................................................. 4 

3. FLEXIBLE PAVEMENTS ....................................................................................................5 

 TABLE 5. PASS-TO-COVERAGE RATIOS FOR FLEXIBLE PAVEMENTS...............5 

 FIGURE 3. LOAD REPETITION FACTOR VS. COVERAGES ......................................6 

APPENDIX 3. DESIGN OF STRUCTURES FOR HEAVY AIRCRAFT.................................1 

 1. BACKGROUND....................................................................................................................1 

2. RECOMMENDED DESIGN PARAMETERS......................................................................1 

FIGURE 1. TYPICAL GEAR CONFIGURATIONS FOR DESIGN OF 

 STRUCTURES.......................................................................................................................2 

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

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AC 150/5320-6D Change 3 4/30/2004 

APPENDIX 5. AIRFIELD PAVEMENT DESIGN SOFTWARE ............................................. 1 

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4/30/2004 AC 150/5320-6D Change 3 

FOREWORD 

This AC provides guidance on the structural design and evaluation of airport pavements. 

Although aircraft landing gears play a role in airport pavement design and evaluation, this AC does not dictate any facet

of landing gear design. In 1958, the FAA adopted a policy of limiting maximum Federal participation in airport 

pavements to a pavement section designed to serve a 350,000-pound (159 000 kg) aircraft with a DC-8-50 series

landing gear configuration. The intent of the policy was to ensure that future aircraft were equipped with landing gears 

that would not stress pavements more than the referenced 350,000-pound (159 000 kg) aircraft. 

Throughout the 20th century, aircraft manufacturers accepted and followed the 1958 policy and designed aircraft 

landing gears that conformed to it—even though aircraft gross weights have long exceeded 350,000 pounds (159 000 

kg). Despite the greater weights, manufacturers were able to conform to the policy by increasing the number and 

spacing of landing gear wheels. This AC does not affect the 1958 policy with regard to landing gear design. 

The pavement design guidance presented in Chapter 3 is based on methods of analysis that resulted from experience and

past research. The methods employed in Chapter 3 were adopted in 1978 to exploit advances in pavement technology

and thus provide better performing pavements and easier-to-use design curves than were previously available. 

Generally speaking, the Chapter 3 guidance requires somewhat thicker pavement sections than were required prior to

1978. 

Chapter 6 presents the pavement evaluation portion of this AC. It relates back to the previous FAA method of design to

ensure continuity. An aircraft operator could be penalized unfairly if an existing facility was evaluated using a method

different from that employed in the original design. A slight change in pavement thickness can have a dramatic effect on 

the payload or range of an aircraft. Since the new pavement design methodology might produce different pavement

thicknesses, an evaluation of an existing pavement using the new methodology could result in incompatible results. To

avoid this situation, the evaluation should be based whenever possible on the same methodology as was used for the 

design. 

Where new aircraft have been added to the traffic mixture at an existing facility, it may not be possible to evaluate the 

pavement with the original design procedure. For example, when a triple dual tandem (TDT) gear aircraft is added to

the traffic mixture at a facility originally designed in accordance with Chapter 3, it will be impossible to assess the 

impact of the new aircraft using the procedures in Chapter 3. In instances where it is not appropriate to evaluate the 

pavement with the original design procedure, the pavement must be evaluated with the newer design procedures. 

The pavement design guidance presented in Chapter 7 implements layered elastic theory based design procedures. The 

FAA adopted this methodology to address the impact of new landing gear configurations such as the TDT gear, which

aircraft manufacturers developed and implemented in the early 1990s. The TDT gear produces an unprecedented 

airport pavement loading configuration, which appears to exceed the capability of the previous methods of design. 

Previous methods incorporated some empiricism and have limited capacity for accommodating new gear and wheel 

arrangements. 

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4/30/2004 AC 150/5320-6D Change 3 

CHAPTER 3. PAVEMENT DESIGN 

SECTION 1. DESIGN CONSIDERATIONS 

300. SCOPE. This chapter covers pavement design for airports serving aircraft with gross weights of 30,000

pounds (13 000 kg) or more. Chapter 5 discusses the design of pavements serving lighter aircraft with gross weights

under 30,000 pounds (13 000 kg).

301. DESIGN PHILOSOPHY. The Foreword of this AC describes the FAA policy of treating the design of 

aircraft landing gear and the design and evaluation of airport pavements as separate entities. The design of airport 

pavements is a complex engineering problem that involves a large number of interacting variables. The design curves

presented in this chapter are based on the California Bearing Ratio (CBR) method of design for flexible pavements and 

a jointed edge stress analysis for rigid pavements. Other design procedures, such as those based on layered elastic

analysis and those developed by the Asphalt Institute and the Portland Cement Association may be used to determine 

pavement thicknesses when approved by the FAA. These procedures will yield slightly different pavement thicknesses 

due to different basic design assumptions. 

All manual and electronic pavement designs should be summarized on FAA Form 5100-1, Airport Pavement Design,

which is considered part of the Engineer’s Report. The Engineer’s Report should be submitted for FAA review and

approval along with initial plans and specifications. 

Because of thickness variations, the evaluation of existing pavements should be performed using the same method 

employed for design. Chapter 6 describes in detail procedures to use when evaluating pavements. Details on the 

development of the FAA method of design are as follows: 

a. Flexible Pavements. The flexible pavement design curves presented in this chapter are based on the 

CBR method of design. The CBR design method is basically empirical; however, a great deal of research has been

done with the method, resulting in the development of reliable correlations. Gear configurations are considered using 

theoretical concepts as well as empirically developed data. The design curves provide the required total thickness of

flexible pavement (surface, base, and subbase) needed to support a given weight of aircraft over a particular subgrade. 

The curves also show the required surface thickness. Minimum base course thicknesses are given in a separate table. A 

more detailed discussion of CBR design is presented in Appendix 2.

b. Rigid Pavements. The rigid pavement design curves in this chapter are based on the Westergaard

analysis of edge loaded slabs. The edge loading analysis has been modified to simulate a jointed edge condition.

Pavement stresses are higher at the jointed edge than at the slab interior. Experience shows practically all load-induced

cracks develop at jointed edges and migrate toward the slab interior. Design curves are furnished for areas where traffic 

will travel primarily parallel or perpendicular to joints and where traffic is likely to cross joints at an acute angle. The 

thickness of pavement determined from the curves is for slab thickness only. Subbase thicknesses are determined 

separately. A more detailed discussion of the basis for rigid pavement design is presented in Appendix 2. 

302. BACKGROUND. An airfield pavement and the aircraft that operate on it represent an interactive system that 

must be addressed in the pavement design process. Design considerations associated with both the aircraft and the 

pavement must be recognized in order to produce a satisfactory design. Producing a pavement that will achieve the 

intended design life will require careful construction control and some degree of maintenance. Pavements are designed

to provide a finite life, and fatigue limits are anticipated. Poor construction and a lack of preventative maintenance will 

usually shorten the service life of even the best-designed pavement. 

a. Variables. The determination of pavement thickness requirements is a complex engineering problem. 

Pavements are subject to a wide variety of loading and climatic effects. The design process involves a large number of

interacting variables, which are often difficult to quantify. Despite considerable research on this subject, it has been

impossible to arrive at a direct mathematical solution for thickness requirements. For this reason, pavement engineers

must base pavement thickness on a theoretical analysis of load distribution through pavements and soils, the analysis of

experimental pavement data, and a study of the performance of pavements under actual service conditions. The FAA 

developed the thickness curves presented in this chapter by correlating the data obtained from these sources. Pavements 

designed in accordance with these standards should have a structural life of 20 years. In addition, as long as there are 

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AC 150/5320-6D Change 3 4/30/2004 

no major changes in forecast traffic, the pavements should not require any major maintenance. It is likely, however, 

that rehabilitation of surface grades and renewal of skid-resistant properties will be needed before 20 years because of 

destructive climatic effects and the deteriorating effects of normal usage. 

b. Structural Design. The structural design of airport pavements requires determining both the overall

pavement thickness and the thickness of the component parts of the pavement. There are a number of factors that

influence the thickness of pavement required to provide satisfactory service. These include the magnitude and character 

of the aircraft loads to be supported, the volume of traffic, the concentration of traffic in certain areas, and the quality of

the subgrade soil and materials that make up the pavement structure. 

303. AIRCRAFT CONSIDERATIONS.

a. Load. The pavement design method is based on the gross weight of the aircraft. The pavement

should be designed for the maximum anticipated takeoff weight of the aircraft. The design procedure assumes 95

percent of the gross weight is carried by the main landing gears and 5 percent is carried by the nose gear. AC 150/5300-13, 

Airport Design, lists the weight of many civil aircraft. The FAA recommends using the maximum anticipated takeoff 

weight, which provides some degree of conservatism in the design. This will allow for changes in operational use and 

forecast traffic, which is approximate at best. The conservatism will be offset somewhat by ignoring arriving traffic. 

b. Landing Gear Type and Geometry. Gear type and configuration dictate how aircraft weight is 

distributed to a pavement and how the pavement will respond to aircraft loadings. Because of this, separate design 

curves would be necessary for each type of aircraft unless some valid assumptions could be made to reduce the number 

of variables. However, examination of gear configuration, tire contact areas, and tire pressure in common use indicate 

that these factors follow a definite trend related to aircraft gross weight. Therefore, reasonable assumptions can be 

made, the variables reduced, and design curves constructed from the assumed data. These assumed data are as follows: 

(1) Single Gear Aircraft. No special assumptions needed. 

(2) Dual Gear Aircraft. A study of the spacing between dual wheels for these aircraft indicated

the following design values are appropriate: a dimension of 20 inches (0.51 m) between the centerline of the tires for 

lighter aircraft and a dimension of 34 inches (0.86 m) between the centerline of the tires for heavier aircraft. 

(3) Dual Tandem Gear Aircraft. The study indicated the following design values are

appropriate: a dual wheel spacing of 20 inches (0.51m) and a tandem spacing of 45 inches (1.14 m) for lighter aircraft

and a dual wheel spacing of 30 inches (0.76 m) and a tandem spacing of 55 inches (1.40 m) for heavier aircraft. 

(4) Wide Body Aircraft. Aircraft such as the B-747, B-767, DC-10, and L-1011 have large 

spaced dual tandem gear geometries, which represent a radical departure from the geometry assumed for dual tandem

aircraft described in paragraph 303b(3) above. Due to the large differences in gross weights and gear geometries, 

separate design curves are provided for these aircraft. The term wide body was originally applied to these aircraft 

because of their width compared to other contemporary aircraft. 

(5) Triple Dual tandem Gear Aircraft. Aircraft such as the B-777 and A-380 have landing

gears with three rows of dual wheels. Pavement design requirements for traffic mixtures containing triple dual tandem

aircraft are discussed in Chapter 7.

c. Tire Pressure. Tire pressure varies between 75 and 200 psi (515 to 1 380 kPa), depending on gear

configuration and gross weight. It should be noted that tire pressure asserts less influence on pavement stresses as gross

weight increases, and the assumed maximum of 200 psi (1 380 kPa) may be safely exceeded if other parameters are not

exceeded and a high-stability surface course is used. 

d. Traffic Volume. Forecasts of annual departures by aircraft type are needed for pavement design. 

Information on aircraft operations is available from Airport Master Plans, Terminal Area Forecasts, the National Plan of 

Integrated Airport Systems, Airport Activity Statistics, and FAA Air Traffic Activity Reports. Pavement engineers 

should consult these publications when developing forecasts of annual departures by aircraft type. 

304. DETERMINATION OF DESIGN AIRCRAFT. The forecast of annual departures by aircraft type will 

result in a list of several different aircraft. The required pavement thickness for each aircraft type in the forecast should 

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4/30/2004 AC 150/5320-6D Change 3 

be checked using the appropriate design curve and the forecast number of annual departures for that aircraft. The 

design aircraft is the aircraft type that produces the greatest pavement thickness. It will not necessarily be the heaviest

aircraft in the forecast. 

305. DETERMINATION OF EQUIVALENT ANNUAL DEPARTURES BY THE DESIGN AIRCRAFT. 

a. Conversions. Since the traffic forecast is a mixture of aircraft having different landing gear types and 

different weights, the effects of all traffic must be accounted for in terms of the design aircraft. First, all aircraft must be 

converted to the same landing gear type as the design aircraft. The FAA has established factors to accomplish this

conversion. These factors are constant and apply to both flexible and rigid pavements. They represent an 

approximation of the relative fatigue effects of different gear types. Much more precise and theoretically rigorous

factors could be developed for different types and thicknesses of pavement. However, at this stage of the design

process, such precision is not warranted and would be impractical for hand calculation since design changes would

require numerous iterations and adjustments. 

The following conversion factors should be used to convert from one landing gear type to another: 

 

To Convert From To Multiply 

Departures By

single wheel dual wheel 0.8 

single wheel dual tandem 0.5 

dual wheel single wheel 1.3 

dual wheel dual tandem 0.6 

dual tandem single wheel 2.0 

dual tandem dual wheel 1.7 

double dual tandem dual tandem 1.0 

double dual tandem dual wheel 1.7 

After the aircraft have been grouped into the same landing gear configuration, the following formula should be used to

convert to equivalent annual departures of the design aircraft:

W

W Log R Log R

1

2

1 = 2 ×

where: 

R1 = equivalent annual departures by the design aircraft

R2 = annual departures expressed in design aircraft landing gear 

W1 = wheel load of the design aircraft 

W2 = wheel load of the aircraft in question 

This computation assumes that 95 percent of the gross weight of the aircraft is carried by the main landing gears. The 

procedure discussed above is a relative rating that compares different aircraft to a common design aircraft. Since wide 

body aircraft have significantly different landing gear assembly spacing than other aircraft, special considerations are 

needed to maintain the relative effects. This is done by treating each wide body as a 300,000-pound (136 100 kg) dual

tandem aircraft when computing equivalent annual departures. Wide body aircraft should be treated this way in every

instance—even when the design aircraft is a wide body. After the equivalent annual departures are determined, the 

design should proceed using the appropriate design curve for the design aircraft. For example, if a wide body is the 

design aircraft, all equivalent departures should be calculated as described above; then the design curve for the wide 

body should be used with the calculated equivalent annual departures. 

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AC 150/5320-6D Change 3 4/30/2004 

b. Example. Assume an airport pavement is to be designed for the following forecast traffic.

Aircraft Gear Type

Average Annual 

Departures 

Maximum Takeoff 

Weight (lbs.)

Maximum Takeoff 

Weight (kg) 

727-100 dual 3,760 160,000 72 580

727-200 dual 9,080 190,500 86 410

707-320B dual tandem 3,050 327,000 148 330

DC-9-30 dual 5,800 108,000 49 000

CV-880 dual tandem 400 184,500 83 690

737-200 dual 2,650 115,500 52 390

L-1011-100 dual tandem 1,710 450,000 204 120

747-100 double dual tandem 85 700,000 317 520 

(1) Determine Design Aircraft. A pavement thickness is determined for each aircraft in the 

forecast with the appropriate design curves. The pavement input data, CBR, k value, flexural strength, etc. should be the 

same for all aircraft. Aircraft weights and departure levels must correspond to the particular aircraft in the forecast. In 

this example, the 727-200 requires the greatest pavement thickness and thus is the design aircraft. 

(2) Group Forecast Traffic into Landing Gear of Design Aircraft. In this example, the 

design aircraft is equipped with a dual wheel landing gear, so all traffic must be grouped into the dual wheel

configuration. 

(3) Convert Aircraft to Equivalent Annual Departures of the Design Aircraft. After the 

aircraft mixture has been grouped into a common landing gear configuration, the equivalent annual departures of the 

design aircraft can be calculated. 

Wheel Load Wheel Load of Design 

Aircraft Aircraft 

Equiv. Dual

Gear 

Departs. (lbs) (kg) (lbs) (kg)

Equiv. Annual

Departures Design

Aircraft 

727-100 3,760 38,000 17 240 45,240 20 520 1,891 

727-200 9,080 45,240 20 520 45,240 20 520 9,080 

707-320B 5,185 38,830 17 610 45,240 20 520 2,764 

DC-9-30 5,800 25,650 11 630 45,240 20 520 682 

CV-880 680 21,910 9 940 45,240 20 520 94

737-200 2,650 27,430 12 440 45,240 20 520 463 

L-1011-100 2,907 35,6251 16 160 45,240 20 520 1,184 

747-100 145 35,6251 16 160 45,240 20 520 83

 Total = 16,241 1 Wheel loads for wide body aircraft are taken as the wheel load for a 300,000-pound (136 100 kg) dual 

tandem aircraft for equivalent annual departure calculations.

(4) Final Result. In this example, the pavement would be designed for 16,241 annual

departures of a dual wheel aircraft weighing 190,500 pounds (86 410 kg). The design, however, should provide for the 

heaviest aircraft in the traffic mixture, the B747-100, when considering depth of compaction, thickness of asphalt

surface, drainage structures, etc. 

c. Other Methods. More refined methods of considering mixed traffic are possible. These refined

methods might consider variations in material properties due to climatic effects, takeoff versus landing loads, aircraft

tread dimensions, etc. The FAA allows the use of these refined methods under the conditions given in paragraph 301.

306. TRAFFIC DISTRIBUTION. Research studies have shown that aircraft traffic is distributed laterally across 

runways and taxiways according to statistically normal (bell-shaped) distribution. FAA Report No. FAA-RD-36, Field 

Survey and Analysis of Aircraft Distribution on Airport Pavements, dated February 1975, contains research information

on traffic distribution. The design procedures presented in this AC incorporate the statistically normal distribution in

the departure levels. In addition to the lateral distribution of traffic across pavements, it also considers traffic 

distribution and the nature of loadings for aprons and high-speed turnoffs.

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4/30/2004 AC 150/5320-6D Change 3 

SECTION 3. RIGID PAVEMENT DESIGN

324. GENERAL. Rigid pavements for airports are composed of Portland cement concrete placed on a granular or 

treated subbase course that is supported on a compacted subgrade. Under certain conditions, a subbase is not required 

(see paragraph 326). 

325. CONCRETE PAVEMENT. The concrete surface must provide a nonskid surface, prevent the infiltration of 

surface water into the subgrade, and provide structural support to the aircraft. The quality of the concrete, acceptance

and control tests, methods of construction and handling, and quality of workmanship are covered in Item P-501, 

Portland Cement Concrete Pavement. 

326. SUBBASE. The purpose of a subbase under a rigid pavement is to provide uniform stable support for the 

pavement slabs. A minimum thickness of 4 inches (100 mm) of subbase is required under all rigid pavements, except as 

shown in Table 3-10 below:

TABLE 3-10. CONDITIONS WHERE NO SUBBASE IS REQUIRED

Soil Good Drainage Poor Drainage

Classification No Frost Frost No Frost Frost

GW X X X X

GP X X X

GM X

GC X

SW X

Note: X indicates conditions where no subbase is required. 

327. SUBBASE QUALITY. The standard FAA subbase for rigid pavements is 4 inches (100 mm) of Item P-154, 

Subbase Course. In some instances, it may be desirable to use higher-quality materials or thicknesses of P-154 greater 

than 4 inches (100 mm). The following materials are acceptable for use as subbase under rigid pavements: 

 

Item P-154 – Subbase Course

Item P-208 – Aggregate Base Course 

Item P-209 – Crushed Aggregate Base Course

Item P-211 – Lime Rock Base Course 

Item P-304 – Cement Treated Base Course 

Item P-306 – Econocrete Subbase Course 

Item P-401 – Plant Mix Bituminous Pavements 

Materials of higher quality than P-154 and/or greater thicknesses of subbase are considered in the design process 

through the foundation modulus (k value). The costs of providing the additional thickness or higher-quality subbase 

should be weighed against the savings in concrete thickness. 

328. STABILIZED SUBBASE. Stabilized subbase is required for all new rigid pavements designed to

accommodate aircraft weighing 100,000 pounds (45 400 kg) or more. Stabilized subbases are as follows: 

 

Item P-304 – Cement Treated Base Course 

Item P-306 – Econocrete Subbase Course 

Item P-401 – Plant Mix Bituminous Pavements 

The structural benefit imparted to a pavement section by a stabilized subbase is reflected in the modulus of subgrade 

reaction assigned to the foundation. Exceptions to the policy of using stabilized subbase are the same as those given in

paragraph 320.

329. SUBGRADE. As with a flexible pavement, the subgrade materials under a rigid pavement should be 

compacted to provide adequate stability and uniform support; however, the compaction requirements for rigid

pavements are not as stringent as for flexible pavement because of the relatively lower subgrade stress. For cohesive 

soils used in fill sections, the top 6 inches (150 mm) must be compacted to 90 percent maximum density. Fill depths 

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AC 150/5320-6D Change 3 4/30/2004 

greater than 6 inches (150 mm) must be compacted to 90 percent maximum density or meet the requirements of Table 

3-2. For cohesive soils in cut sections, the top 6 inches (150 mm) of the subgrade must be compacted to 90 percent

maximum density. For noncohesive soils used in fill sections, the top 6 inches (150 mm) of fill must be compacted to

100 percent maximum density, and the remainder of the fill must be compacted to 95 percent maximum density or meet 

the requirements of Table 3-2. For cut sections in noncohesive soils, the top 6 inches (150 mm) of subgrade must be 

compacted to 100 percent maximum density and the next 18 inches (460 mm) of subgrade must be compacted to 95

percent maximum density. Swelling soils require special considerations. Paragraph 314 contains guidance on the 

identification and treatment of swelling soils. 

a. Contamination. In rigid pavement systems, repeated loading might cause intermixing of soft

subgrade soils and aggregate base or subbase. This mixing can create voids below the pavement in which moisture can 

accumulate, causing pumping to occur. Chemical and mechanical stabilization of the subbase or subgrade can 

effectively reduce aggregate contamination (see paragraph 207). Geotextiles have been found to be effective at 

providing separation between fine-grained subgrade soils and pavement aggregates (FHWA-HI-90-001 Geotextile 

Design and Construction Guidelines). Geotextiles should be considered for separation between fine-grained soils and 

overlying pavement aggregates. In this application, the geotextile is not considered to act as a structural element within 

the pavement. Therefore, the modulus of the base or subbase is not increased when a geotextile is used for stabilization. 

For separation applications, the geotextile is designed based on survivability properties. FHWA-HI-90-001 contains

additional information about design and construction using separation geotextiles. 

330. DETERMINATION OF FOUNDATION MODULUS (k VALUE) FOR RIGID PAVEMENT. In

addition to the soils survey and analysis and classification of subgrade conditions, rigid pavement design also requires 

the determination of the foundation modulus. The k value should be assigned to the material directly beneath the 

concrete pavement. However, the FAA recommends that a k value be established for the subgrade and then corrected to

account for the effects of the subbase. 

a. Determination of k Value for Subgrade. The preferred method of determining the subgrade 

modulus is by testing a limited section of embankment that has been constructed to the required specifications. The 

plate bearing test procedures are given in AASHTO T 222, Nonrepetitive Static Plate Load Test of Soils and Flexible 

Pavement Components for Use in Evaluation and Design of Airport and Highway Pavements. If the construction and

testing of a test section of embankment is impractical, the values listed in Table 2-3 may be used. The values in Table

2-3, however, are approximate, and engineering judgment should be used when selecting a design value. Fortunately,

rigid pavement is not overly sensitive to k value, and an error in estimating k will not have a large impact on rigid

pavement thickness. 

b. Determination of k Value for Granular Subbase. It is usually not practical to determine a 

foundation modulus on top of a subbase by testing, at least in the design phase. Usually, the embankment and subbase 

will not be in place in time to perform any field tests, so the k value will have to be assigned without the benefit of 

testing. The probable increase in k value associated with various thicknesses of different subbase materials is shown in 

Figure 2-4. The upper graph in Figure 2-4 should be used when the subbase is composed of well-graded crushed 

aggregate, such as P-209. The lower graph in Figure 2-4 applies to bank-run sand and gravel, such as P-154. Both

curves in Figure 2-4 apply to unstabilized granular materials. Values shown in Figure 2-4 are guides and can be 

tempered by local experience. 

c. Determination of k Value for Stabilized Subbase. As with granular subbase, the effect of stabilized 

subbase is reflected in the foundation modulus. Figure 3-16 shows the probable increase in k value with various 

thicknesses of stabilized subbase located on subgrades of varying moduli. Figure 3-16 is applicable to cement stabilized

(P-304), Econocrete (P-306), and bituminous stabilized (P-401) layers. Figure 3-16 assumes a stabilized layer is twice 

as effective as well-graded crushed aggregate in increasing the subgrade modulus. Stabilized layers of lesser quality 

than P-304, P-306, or P-401 should be assigned somewhat lower k values. After a k value is assigned to the stabilized

subbase, the concrete slab thickness design procedure is the same as that described in paragraph 331. 

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 CBR 3 4 5 6 7 8 9 11 13 16 20 

FAA F10 F9 F8 F7 F6 F5 F4 F3 F2 F1 Fa

SUBGRADE CLASS 

FIGURE 6-1. CBR–FAA SUBGRADE CLASS COMPARISONS 

 (1) Subbase and Base Equivalencies. For evaluation purposes, the FAA recommends the 

equivalency factor ranges shown in Tables 3-6 through 3-9 for subbase and base. The actual value selected will depend

on the composition, quality, and condition of the layer. If experience or physical test results show that other values are 

valid, they may be used in lieu of the values recommended here. Subbase or base courses should not be assigned a

higher equivalency factor than any layer above it in the pavement structure. The FAA does not permit the conversion of 

material to a higher classification, such as subbase to base, except where excess stabilized base course (P-401 or P-304) 

exists immediately under a flexible surface, in which case the stabilized material may be counted as an equal thickness 

of surface. 

 (2) Surfacing. Broken hot mix asphalt surface course (shrinkage cracks due to age and 

weathering, without evidence of base failure) must be evaluated as an equal thickness of nonstabilized base. A hot mix 

asphalt surface, well maintained and with limited cracking, might justify use of an equivalency between the limits

noted. 

603. APPLICATION OF FLEXIBLE PAVEMENT EVALUATION PROCEDURES. After all of the 

evaluation parameters of the existing flexible pavement have been established using the guidance given in the above 

paragraphs, the evaluation process is essentially the reverse of the design procedure. The design curves presented in

Chapters 3 or 5 are used to determine the load carrying capacity of the existing pavement. Required inputs are subgrade

and subbase CBR values, thicknesses of surfacing, base and subbase courses, and an annual departure level. Several

checks must be performed to determine the load carrying capacity of a flexible pavement. The calculation that yields 

the lowest allowable load will control the evaluation. 

a. Total Pavement Thickness. Enter the lower abscissa of the appropriate design curve in Chapters 3 

or 5 with the total pavement thickness of the existing pavement. Make a vertical projection to the annual departure level

line. For light load pavements, described in Chapter 5, a single pivot line is used. At the point of intersection between 

the vertical projection and the departure level line, or single pivot line in the case of light load pavements, make a 

horizontal projection across the design curve. Enter the upper abscissa with the CBR value of the subgrade. Make a 

vertical projection downward until it intersects the horizontal projection made previously. The point of intersection of

these two projections will be in the vicinity of the load lines on the design curves. An allowable load is read by noting 

where the intersection point falls in relation to the load lines. 

b. Thickness of Surfacing and Base. The combined thickness of surfacing and base must also be

checked to establish the load carrying capacity of an existing flexible pavement. This calculation requires the CBR of

the subbase, the combined thickness of surfacing and base, and the annual departure level as inputs. The procedure is

the same as that described in subparagraph a above, except that the subbase CBR and combined thickness of surfacing

and base are used to enter the design curves. 

c. Minimum Base Course Thickness. The thickness of the existing base course should be compared 

with the minimum base course thicknesses in Table 3-4 or Figure 5-2. Notice that the minimum base course thickness 

is 4 inches (100 mm) for heavy load pavements and 3 inches (75 mm) for light load pavements. If there is a deficiency

in the thickness of the existing base course, the pavement should be closely monitored for signs of distress. The FAA 

recommends that overlaying the pavement to correct the deficiency be considered. 

d. Minimum Surface Thickness. The thickness of the existing surface course should be compared with 

that shown on the appropriate design curve. If the existing surface course is thinner than that given on the design curve, 

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the pavement should be closely observed for surface failures. The FAA recommends that correction of the deficiency in

surfacing thickness be considered. 

604. RIGID PAVEMENTS. Evaluation of rigid pavements requires, at a minimum, the determination of the

thickness of the component layers, the flexural strength of the concrete, and the modulus of subgrade reaction. 

a. Layer Thicknesses. The thickness of the component layers is sometimes available from construction 

records. Where information is not available or of questionable accuracy, thicknesses may be determined by borings or 

test pits in the pavement. 

b. Concrete Flexural Strength. The flexural strength of the concrete is most accurately determined 

from test beams sawed from the existing pavement and tested in accordance with ASTM C 78. However, this method is 

often impractical, as sawed beams are expensive to obtain and costs incurred in obtaining sufficient numbers of beams 

to establish a representative sample are prohibitive. Construction records, if available, may be used as a source of 

concrete flexural strength data, but the construction data will probably have to be adjusted for age, as concrete strength 

increases with time. 

 (1) Correlations with Other Strength Tests. Correlations between concrete flexural strength 

and other concrete strength tests are available, but the correlations between flexural strength and other strength tests are 

approximate, and considerable variations are likely. 

 (i) Tensile Split Strength. An approximate relationship between concrete flexural

strength and tensile splitting strength (ASTM C 496) exists and can be computed with the following formula:

R = 1.02( T ) + 117 

 where:

R = flexural strength, psi 

T = tensile split strength, psi 

Note: For conversions in metric units, the above formula remains the same—except 

the 117 psi constant should be changed to 0.81 MPa. 

This equation can be used with 85 percent confidence that the estimated flexural strength is at least as strong as the strength

derived from the original regression analysis detailed in Concrete Strength Relationships, Miscellaneous Report Number S74-30, published by the US Army Engineer Waterways Experiment Station, December 1974. 

 

 (ii) Compressive Strength. Flexural strength can be estimated from compressive 

strength (ASTM C 39) using the formula below:

'

c R = 9 f

 where:

 R = flexural strength

= compressive strength '

c f

 

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4/30/2004 AC 150/5320-6D Change 3 

c. Modulus of Subgrade Reaction. The modulus of subgrade reaction is determined by plate bearing 

tests performed on the subgrade. These tests should be made in accordance with the procedures established in

AASHTO T 222. An important part of the test procedure for determining the subgrade reaction modulus is the 

correction for soil saturation that is contained in the prescribed standard. The normal application utilizes a correction 

factor determined by the consolidation testing of samples at in situ and saturated moisture content. For evaluation of

older pavement where evidence exists that the subgrade moisture has stabilized or varies through a limited range, the

correction for saturation is not necessary. If a field plate bearing test is not practical, the modulus of subgrade reaction 

may be determined by nondestructive testing or estimated by using Table 2-3 in Chapter 2 of this AC. Fortunately, 

pavement evaluation is not too sensitive to the modulus of subgrade reaction. 

 (1) Adjustment for Subbase. An adjustment to the modulus of subgrade reaction will be 

required if a subbase exists beneath the existing pavement. The thickness of the subbase is required to calculate an 

adjusted k value. The subbase thickness can be determined from construction records or from borings. The guidance 

contained in Chapter 3, Section 3, should be used in assigning a k value to a subbase. 

605. APPLICATION OF RIGID PAVEMENT EVALUATION PROCEDURES. The evaluation of rigid 

pavements for aircraft weighing more than 30,000 pounds (13 600 kg) requires concrete flexural strength, k value of the 

foundation, slab thickness, and annual departure level as inputs. The rigid pavement design curves in Chapter 3 are

used to establish load carrying capacity. The design curves are entered on the left ordinate with the flexural strength of

the concrete. A horizontal projection is made to the k value of the foundation. At the point of intersection of the 

horizontal projection and the k line, a vertical projection is made into the vicinity of the load lines. The slab thickness is

entered on the appropriate departure level scale on the right side of the chart. A horizontal projection is made from the 

thickness scale until it intersects the previous vertical projection. The point of intersection of these projections will be

in the vicinity of the load lines. The load carrying capacity is read by noting where the intersection point falls in 

relation to the load lines. 

606. USE OF RESULTS. If the evaluation is being used for planning purposes and the existing pavement is found

to be deficient in accordance with the design standards given in Chapters 3 or 5, the sponsor should be notified as to the 

deficiency and should consider corrective action. If the evaluation is being used as part of the design for a project to 

reconstruct or upgrade the facility, the procedures given in Chapters 3, 4, or 5 should be used to design the 

reconstruction or overlay project. In the latter case, the main concern is not the load carrying capacity, but the 

difference between the existing pavement structure and the section that is needed to support forecast traffic. 

607. REPORTING PAVEMENT STRENGTH. The International Civil Aviation Organization (ICAO) 

developed a standardized method of reporting airport pavement strength known as the Aircraft Classification 

Number/Pavement Classification Number (ACN/PCN). This method is based on the concept of reporting strength in

terms of a standardized equivalent single wheel load. This method of reporting pavement strength is discussed in FAA

AC 150/5335-5, Standardized Method of Reporting Airport Pavement Strength–PCN. 

608. EVALUATIONS OF EXISTING PAVEMENT WITH LAYERED ELASTIC PROCEDURES. When an

existing pavement must be evaluated for use by new aircraft not covered in the nomographs provided in Chapter 3, the 

layered elastic design procedures described in Chapter 7 may be used. However, an evaluation with these procedures

might present unique difficulties when trying to correlate results to original thickness requirements. In many cases, the 

original traffic mixture might not be known and only the design aircraft gross weight and annual departures be

available. Since the evaluation is for current usage, the existing and forecasted traffic mixture should be used to

evaluate the pavement requirements. 

Existing pavements may also be evaluated for use by new aircraft with the ACN/PCN system described in AC 150/5335-5. 

The ACN value of new aircraft can be compared to the existing PCN value of the pavement to determine whether 

restricted or unrestricted operations of the new aircraft should be permitted. 

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CHAPTER 7. LAYERED ELASTIC PAVEMENT DESIGN

701. PURPOSE. The design procedure presented in this chapter provides a method of design based on layered

elastic analysis developed to calculate design thicknesses for airfield pavements. Layered elastic design theory was

adopted to address the impact of new gear and wheel arrangements such as the triple dual tandem (TDT) main gear. 

The Boeing 777 and the Airbus A-380 are examples of aircraft that utilize this gear geometry. The TDT gear produces 

an airport pavement loading configuration that appears to exceed the capability of the previous methods of design, 

which incorporate some empiricism and have limited capacity for accommodating new gear and wheel arrangements. 

This design method is computationally intense and is thus in the form of a computer program called LEDFAA. 

702. APPLICATION. The procedures and design software identified in this chapter are primarily intended to 

provide pavement thickness design standards for airfield pavement intended to serve aircraft traffic mixtures that

include aircraft utilizing TDT or other complex main gear configurations. The pavement design procedures presented in

this chapter may also be used as a design alternate for the design procedures presented in Chapters 3 and 4. In instances 

where newer aircraft gear configuration, tire pressures, and/or wheel loads are not appropriate for the nomographs 

presented in Chapters 3 and 4, the FAA recommends using the layered elastic design procedures. Pavement designs for

traffic mixes that do not include aircraft with the TDT main gear must conform to the minimum pavement sections 

defined in Chapters 3, and 4. A companion design prepared in accordance with Chapters 3 and 4 must be submitted 

with the layered elastic design for FAA approval. To aid in the design review, the summary information from the 

design software should be printed and included with the pavement design submittal. 

The LEDFAA program may not be used to compare individual aircraft pavement thickness requirements for pavement

designs, in accordance with Chapters 3 and 4, to individual aircraft thickness requirements for pavement designs based

on the layered elastic design methodology. The program may not be used to evaluate existing pavement structures 

designed in accordance with Chapters 3 and 4 against single aircraft thickness requirements using the layered elastic 

design methodology. Any comparison between the design methodology of LEDFAA and Chapters 3 and 4 must be 

performed utilizing the entire traffic mixture (see paragraph 704e). 

703. BACKGROUND. The TDT main landing gear is unique in that it has six wheels arranged as three pairs of 

wheels in a row. When the TDT main gear assembly is analyzed using the conventional FAA design methodology, the 

pavement thickness requirements are considered to be unduly conservative. This is particularly noticeable for flexible 

pavements. In 1995, the FAA adopted a layered elastic airport pavement design methodology for the Boeing 777 to

reduce some of the conservatism experienced with the methods presented in Chapters 3 and 4 and to phase in a more 

mechanistic approach. The U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS, originally

developed the layered elastic method of design for flexible and rigid pavements and overlays of rigid pavements. 

Verification of the new flexible pavement design procedure was performed by full-scale pavement testing at the 

National Airfield Pavement Test Facility, William J. Hughes Technical Center, Atlantic City, NJ. 

704. COMPUTER PROGRAM. The design method is computer-based and is called LEDFAA. The core 

program is LEAF, a layered elastic computational program implemented as a Microsoft Windows

TM ActiveX dynamic 

link library written in Visual BasicTM 6.0. The remainder of the program is written in Visual BasicTM and operates 

under Microsoft Windows

TM. 

a. Aircraft Considerations. A wide variety of aircraft with pertinent pavement design characteristics 

are stored in the program library. The designer has considerable latitude in selecting and adjusting aircraft parameters. 

b. Metric Units. The program may be operated with U.S. customary or metric dimensions.

c. Availability. LEDFAA can be downloaded from the Office of Airport Safety and Standards website

(http://www.faa.gov/arp/). 

d. Related Reference Material. The internal help file for LEDFAA contains a user’s manual, which 

provides detailed information on proper execution of the program. The manual also contains additional technical 

references for specific details of the LEDFAA design procedure. 

e. Aircraft Traffic Mixture. LEDFAA was developed and calibrated specifically to produce pavement

thickness designs consistent with previous methods based on a mixture of different aircraft rather than individual

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aircraft. If a single aircraft is used for design, a warning will appear in the Aircraft Window indicating a non-standard

aircraft list is used in the design. This warning is intended to alert the user that the program was intended for use with a 

mixture of different aircraft types. Nearly any traffic mix can be developed from the aircraft in the program library. 

Solution times are a function of the number of aircraft in the mix. The LEDFAA design procedure deals with mixed 

traffic differently than did previous design methods. Determination of a design aircraft is not required to operate 

LEDFAA. Instead, the program calculates the damaging effects of each aircraft in the traffic mix. The damaging 

effects of all aircraft are summed in accordance with Miner's Law. When the cumulative damage factor (CDF) sums to 

a value of 1.0, the design conditions have been satisfied. 

705. PAVEMENT DESIGN CONSIDERATIONS. There are distinct differences between the design 

methodology presented in Chapters 3 and 4 and the methodology contained in LEDFAA. These differences, along with

some common design assumptions between the two methods, are discussed below. 

 

a. Design Life. The FAA design standard for pavements is based on a 20-year design life. The 

computer program is capable of considering other design life time frames, but the use of a design life other than 20

years constitutes a deviation from FAA standards. 

b. Traffic Mix. As noted in paragraph 704e, traffic mix considerations are handled differently by the 

layered elastic design method. The procedures described in Chapters 3 and 4 require the traffic mixture to be converted 

into a single design aircraft and all annual departures converted to equivalent annual departures of the design aircraft. 

The design aircraft is determined by selecting the most damaging aircraft based on the anticipated gross weight and the 

number of departures for each aircraft. The layered elastic design program does not convert the traffic mixture; instead, 

it analyzes the damage to the pavement section for each aircraft and determines a final thickness for the total cumulative 

damage. LEDFAA considers the placement of each aircraft’s main gear in relationship to the pavement centerline. It 

also allows the pavement damage associated with a particular aircraft to be completely isolated from one or more of the

other aircraft in the traffic mixture. 

 

c. Design Reliability. The reliability of the layered elastic method of design should be approximately 

the same as the reliability of the CBR method of pavement design. 

 

d. Materials. In the layered elastic design procedure, pavement materials are characterized by 

thickness, elastic moduli, and Poisson's ratio. Layer thicknesses can be varied, except where minimum thicknesses are 

required. Elastic moduli are either fixed or variable, depending on the material. The permissible range of variability for 

elastic moduli is fixed to ensure reasonable values. Poisson's ratio for all material is fixed. Materials are identified with

their corresponding FAA specification designations; for example, crushed stone base course is identified as Item P-209. 

The list of materials contains an undefined layer with variable properties. If an undefined layer is used, a warning will 

appear in the Structure Window stating that a non-standard material has been selected and its use in the structure will 

require FAA approval. 

e. Minimum Layer Thickness. When the layered elastic design procedure is used in lieu of the design

procedures in Chapters 3 and 4, LEDFAA will not automatically establish the minimum layer thickness for each layer, 

as required. The user must consult paragraphs 706, 707, and 708 below to assure the minimum thickness requirements 

are obtained. 

 

706. FLEXIBLE PAVEMENT DESIGN. The design process considers two modes of failure for flexible

pavement: vertical strain in the subgrade and horizontal strain in the asphalt layer. Limiting vertical strain in the 

subgrade is intended to preclude failure by subgrade rutting. Limiting horizontal strain at the bottom of the asphalt 

surfacing layer guards against pavement failure initiated by cracking of the asphalt surface layer. 

 

a. Hot Mix Asphalt Surfacing. Hot mix asphalt surfacing should meet the requirements of FAA Item

P-401. A minimum thickness of 5 inches (127 mm) of hot mix surfacing is required for traffic mixes that include 

aircraft with the TDT gear. The minimum thickness of hot mix surfacing, as shown in Figures 3-2 through 3-15, is 

required for traffic mixes that do not include aircraft with the TDT gear. A fixed modulus value for hot mix surfacing is

set in the program at 200,000 psi (1 380 MPa). This modulus value was chosen to produce results that closely matched 

thickness requirements for pavements designed with the CBR methodology.

 

b. Base Course. A minimum 5-inch-thick (127 mm) stabilized base course is required for pavements 

serving aircraft with the TDT gear. LEDFAA includes two stabilized flexible base options, designated P-401 and 

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Variable. The word flexible is used to indicate that these bases have a higher Poisson's ratio (0.35), act as flexible 

layers as opposed to rigid layers, and are not likely to crack. P-401 is the standard FAA Item P-401 bituminous base, 

which has a fixed modulus of 400,000 psi (2 760 MPa). The variable stabilized flexible base can be used to characterize 

a bound base, which does not conform to the properties of P-401. It has a variable modulus ranging from 150,000 to

400,000 psi (1 035 to 2 760 MPa). This modulus range was selected to closely duplicate results obtained from using an 

equivalency factor range of 1.2 to 1.6 in the CBR method of design. Stabilized (rigid) bases, P-304, and P-306 may also 

be used as base course; although, they are subject to cracking and can induce reflection cracking in the hot mix asphalt

surfacing. Item P-301, Soil Cement Base, is not acceptable for use as a base course for these pavements. 

When the layered elastic design procedure is used in lieu of the design procedures in Chapters 3 and 4, the minimum

base thickness will be determined in a manner similar to the procedure described in paragraph 319b. The minimum

base thickness must be determined by increasing the subgrade CBR value to 20 and removing all subbase layers. The 

minimum thickness required will be the greater of the resulting base thickness from the LEDFAA program or the 

minimum base thickness in Table 3-4. 

For traffic mixtures with aircraft exceeding 100,000 lbs (45 350 kg) but not containing a TDT gear aircraft, a stabilized 

base course is required, as noted in paragraph 320. When using the LEDFAA design procedure, the minimum stabilized

base thickness is 6 inches (150 mm). 

 

c. Subbase Course. Subbases may be aggregate or bound materials. The minimum thickness of

subbase for structural purposes is 3 inches (76 mm). Additional thickness might be required for practical construction

limitations. Acceptable aggregate materials are P-209, Crushed Aggregate Base Course; P-208 Aggregate Base Course; 

or P-154, Subbase Course. Acceptable bound materials are P-401, P-304, and P-306. Use of Item P-301 is limited to 

locations not subject to freeze-thaw cycles. More than one layer of subbase material may be used, i.e., P-209 over a

layer of P-154. Layering must be done so as not to produce a sandwich (granular layer between two bound layers) 

section and to assure that material quality increases toward the top of the pavement section. 

For traffic mixtures with aircraft exceeding 100,000 lbs (45 350 kg), a stabilized subbase is required, as noted in

paragraph 320. Acceptable materials are specified in Tables 3-6 and 3-7.

 

d. Subgrade. The subgrade is assumed to be infinite in thickness and is characterized by either a 

modulus or CBR value. LEDFAA converts CBR to modulus by multiplying it by 1,500. Subgrade compaction and 

embankment construction should be in accordance with Table 3-2. 

 

e. Seasonal Frost and Permafrost. Seasonal frost and permafrost effects should be considered by 

applying the techniques in Chapter 2. 

 

707. RIGID PAVEMENT DESIGN. The design process considers one mode of failure for rigid pavement, 

cracking of the concrete slab. Failure of subbase and subgrade layers is not considered. Limiting the horizontal stress

at the bottom surface of the concrete surfacing layer guards against failure by cracking of the surface layer. LEDFAA 

iterates on the concrete layer thickness until the CDF reaches a value of 1.0. Once a CDF of 1.0 is achieved, the section 

satisfies the design conditions. 

a. Concrete Pavement Surfacing. Concrete pavement surfacing should meet the requirements of 

Item P-501. The minimum concrete surfacing thickness is 6 inches (152 mm). 

b. Subbase Course. When the layered elastic design procedure is used in lieu of the design procedures

in Chapters 3 and 4, the subbase must meet the requirement of paragraphs 326, 327, and 328. 

c. Stabilized Subbase Course. Bound materials are required for subbase under rigid pavements serving 

aircraft with the TDT gear. Acceptable bound materials are P-304, P-306, and P-401, variable stabilized rigid and 

variable stabilized flexible. The minimum thickness of subbase is 4 inches (102 mm). More than one layer of subbase 

may be used, i.e., P-306 over a layer of P-209. Layering must be done so as not to produce a sandwich (granular layer 

between two bound layers) section. 

d. Subgrade. The subgrade is assumed to be infinite in thickness and is characterized by either a 

modulus or k value. The computer converts k to modulus by using the logarithmic relationship log E = 1.415 + 1.284 

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log k. Subgrade compaction requirements and embankment construction should be in accordance with Chapter 3 and

AC 150/5370-10. 

e. Seasonal Frost and Permafrost. Seasonal frost and permafrost effects should be considered by 

applying the techniques in Chapter 2. 

f. Jointing Details. Jointing details for rigid pavements are presented in Chapter 3. The limitations on

jointing of rigid pavements for wide body aircraft also apply to pavements designed to serve aircraft with the TDT gear. 

708. LAYERED ELASTIC OVERLAY DESIGN. Layered system design permits a direct design approach for 

overlays. The overlay design method in Chapter 4 relies on an empirically based thickness deficiency approach. The

layered system design calculates the thickness of overlay required to provide a 20-year life, which satisfies the layered

elastic failure criteria for limiting stress or strain. The 20-year life thickness is defined as the design thickness. Dr. R.

S. Rollings designed the design method for overlays of rigid pavement through an FAA-funded research effort, as listed 

in Appendix 4. Overlay pavements are grouped into four different types as follows: 

• Hot Mix Asphalt Overlay of Existing Flexible Pavement

• Concrete Overlay of Existing Flexible Pavement 

• Hot Mix Asphalt Overlay of Existing Rigid Pavement

• Concrete Overlay of Existing Rigid Pavement

a. Overlays of Existing Flexible Pavements. The design of an overlay for an existing flexible 

pavement is essentially the same as designing a new pavement. The existing flexible pavement is characterized by

assigning the appropriate thicknesses and moduli of the existing layers. A qualified engineer should be consulted to

characterize the existing pavement layers. 

 (1) Hot Mix Overlay of an Existing Flexible Pavement. A trial thickness of overlay is selected, 

and the program iterates until a CDF of 1.0 is reached. The overlay thickness required to achieve a CDF of 1.0 is the 

design thickness. 

 (2) Concrete Overlay of an Existing Flexible Pavement. The design of a concrete overlay on an

existing flexible pavement is essentially the same as designing a new rigid pavement. The existing flexible pavement is

characterized by assigning the appropriate thicknesses and moduli of the existing layers. A trial thickness of overlay is 

selected, and the program iterates until a CDF of 1.0 is reached. The overlay thickness required to achieve a CDF of 1.0

is the design thickness. The design process is relatively simple; however, the characterization of the existing pavement

layers requires judgment by a qualified engineering. The program assumes the interface between the concrete overlay 

and the existing flexible surface is frictionless. 

b. Overlays of Existing Rigid Pavements. The design of overlays for an existing rigid pavement is

complex because deterioration of the underlying pavement as well as deterioration of the overlay must be considered.

The condition of the existing rigid pavement prior to overlay is important and is expressed in terms of the structural 

condition index (SCI). The SCI is derived from the pavement condition index (PCI). (Additional guidance on deriving

an SCI is provided in the LEDFAA user’s manual.) The PCI is a numerical rating indicating the operational condition 

of an airport pavement based on visual survey. The scale ranges from a high of 100 to a low of 0, with 100 representing 

a pavement in excellent condition. The PCI is measured using ASTM standard test method D 5340, Standard Test

Method for Airport Pavement Condition Index Survey. For rigid pavements, 15 different types of distress are 

considered in measuring the PCI. These distress types all reduce the PCI of a pavement, depending on their severity

and relative effect on performance. Not all distress types are indicative of structural distress. Rollings has identified 6

distress types that are indicative of the structural condition of the pavement. Table 7-1 lists these 6 distress types. 

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TABLE 7-1. RIGID PAVEMENT DISTRESS TYPES USED TO CALCULATE THE 

STRUCTURAL CONDITION INDEX, SCI 

Corner Break

Longitudinal/Transverse/Diagonal Cracking 

Shattered Slab

Shrinkage Cracks

a

 (cracking partial width of slab) 

Spalling–Joint

Spalling–Corner 

a Used only to describe a load-induced crack that extends only part of the way across a slab. 

The SCI does not include conventional shrinkage cracks due to curing problems. 

An SCI of 80 is consistent with the current FAA definition of initial failure of a rigid pavement, i.e., 50 percent of the 

slabs in the traffic area exhibit initial structural cracking. The SCI allows a more precise and reproducible rating of a 

pavement’s condition than previous FAA condition factor ratings, Cb and Cr. 

 (1) Hot Mix Asphalt Overlays of Existing Rigid Pavements. The design process for hot mix 

overlays of rigid pavements considers two conditions for the existing rigid pavement to be overlaid: a SCI of the 

existing pavement that is equal to or less than 100. 

 (i) Structural Condition Index Less Than 100. The most likely situation is one in

which the existing pavement is exhibiting some structural distress, i.e., the SCI is less than 100. If the SCI is less than 

100, the overlay and base pavement deteriorate at a given rate until failure is reached. LEDFAA assumes an overlay 

thickness and iterates on the thickness of overlay until a 20-year life is predicted. A 20-year predicted life satisfies the 

design requirements. 

 (ii) Structural Condition Index Equal to 100. An existing pavement with an SCI of

100 might require an overlay to strengthen the pavement in order to accept heavier aircraft. If the SCI of the base 

pavement is equal to 100, an additional input is required—the CDFU, cumulative damage factor used, which estimates 

the amount of pavement life used up prior to overlay. LEDFAA assumes the base pavement will deteriorate at one rate 

while the SCI is equal to 100 and at a different rate after the SCI drops below 100. As with an SCI less than 100, a trial 

overlay thickness is input, and the program iterates on that thickness until a 20-year life is predicted. The design

thickness is the thickness that provides a 20-year predicted life. 

 (2) Concrete Overlays of Existing Concrete Pavements. The design of a concrete overlay of

an existing rigid pavement is the most complex type of overlay to be designed. Deterioration of the concrete overlay

and existing rigid pavement must be considered as well as the degree of bond between the overlay and existing 

pavement. LEDFAA considers two degrees of bond and addressed each separately for thickness design. 

 (i) Fully Unbonded Concrete Overlay. An unbonded concrete overlay of an existing 

rigid pavement is one in which steps are taken to intentionally eliminate bonding between the overlay and existing

pavement. Commonly, the bond is broken by applying a thin hot mix layer to the existing rigid pavement. The

interface friction coefficient between the overlay and existing pavement is set to reflect an unbonded condition. The 

interface coefficient is fixed and cannot be changed by the user. As with hot mix asphalt overlays, an SCI is required to

describe the condition of the existing pavement. A trial overlay thickness is input, and LEDFAA iterates until a 20-year 

service life is predicted. The thickness that yields a 20-year service life is the design thickness. 

 (ii) Partially Bonded Concrete Overlay. A partially bonded overlay is one in which 

no particular effort is made to either eliminate or achieve bond between the concrete overlay and the existing rigid 

pavement. Such overlays are normally appropriate for existing rigid pavements when the SCI is 77 or greater. The

interface coefficient is set to reflect a small degree of friction between the overlay and base pavement. This coefficient

is fixed and cannot be changed by the user. An SCI for the existing pavement is required. A trial overlay thickness is

input, and LEDFAA iterates until a 20-year service life is predicted. The thickness that yields a 20-year service life is 

the design thickness. 

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APPENDIX 4. RELATED READING MATERIAL 

1. Electronic copies of the latest versions of the following FAA publications are available on the FAA website at 

http://www.faa.gov/. Printed copies can be requested from the Department of Transportation, Subsequent Distribution

Office, Ardmore East Business Center, 3341 Q 75th Ave, Landover, MD 20785. The Department of Transportation,

however, will charge a fee for some of these documents. Advisory Circular 00-2, Advisory Circular Checklist, provides 

a list of all current ACs. 

a. AC 00-2, Advisory Circular Checklist and Status of Other FAA Publications. 

b. AC 150/5300-13, Airport Design. 

c. AC 150/5320-5, Airport Drainage. 

d. AC 150/5320-12, Measurement, Construction, and Maintenance of Skid Resistant Airport Pavement

Surfaces. 

e. AC 150/5300-9, Predesign, Prebid, and Preconstruction Conferences for Airport Grant Projects. 

 

f. AC 150/5335-5, Standardized Method of Reporting Airport Pavement Strength–PCN. 

g. AC 150/5370-10, Standards for Specifying Construction of Airports. 

h. AC 150/5370-11, Use of Nondestructive Testing Devices in the Evaluation of Airport Pavements. 

i. AC 150/5380-6, Guidelines and Procedures for Maintenance of Airport Pavements. 

2. Copies of the following reports can be obtained from the National Technical Information Service, 5285 Port

Royal Road, Springfield, VA 22161 or at http://www.ntis.gov/. 

a. FAA-RD-73-169, Review of Soil Classification Systems Applicable to Airport Pavement Design, 

May 1974, by Yoder; AD-783-190. 

b. FAA-RD-74-30, Design of Civil Airfield Pavement for Seasonal Frost and Permafrost Conditions, 

October 1974, by Berg; ADA-006-284.

c. FAA-RD-74-36, Field Survey and Analysis of Aircraft Distribution on Airport Pavements, February

1975, by Ho Sang; ADA-011-488. 

d. FAA-RD-76-66, Design and Construction of Airport Pavements on Expansive Soils, June 1976, by

McKeen; ADA-28-094.

e. FAA-RD-73-198-1, Design and Construction and Behavior Under Traffic of Pavement Test Sections, 

June 1974, by Burns, Rone, Brabston, Ulery; AD-785-024. 

f. FAA-RD-74-33, III, Design Manual for Continuously Reinforced Concrete Pavements, May 1974, by

Treybig, McCullough, Hudson; AD-780-512. 

g. FAA-RD-75-110-II, Methodology for Determining, Isolating and Correcting Runway Roughness, 

June 1977, by Seeman and Nielsen; ADA-044-378. 

h. FAA-RD-73-198-111, Design and Construction of MESL, December 1974, by Hammitt; AD-005-

893. 

i. FAA-RD-76-179, Structural Design of Pavements for Light Aircraft, December 1976, by Ladd, 

Parker, Pereira; ADA-04 l-300. 

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AC 150/5320-6D Change 3 4/30/2004 

j. FAA-RD-74-39, Pavement Response to Aircraft Dynamic Loads, Volume II–Presentation and 

Analysis of Data, 1974, by Ledbetter; ADA-022-806. 

k. DOT/FAA/RD-74/199, Development of a Structural Design Procedure for Flexible Airport 

Pavements, November 1974, by Barker and Brabston, ADA-019-205. 

l. DOT/FAA/RD-77/81, Development of a Structural Design Procedure for Rigid Airport Pavements, 

April 1979, by Parker, Barker, Gunkel, and Odom, ADA-069-548. 

m. FAA-RD-81-78, Economic Analysis of Airport Pavement Rehabilitation Alternatives, October 1981, 

by Epps and Wootan, ADA-112-550 

n. DOT/FAA/PM-87/19, Design of Overlays for Rigid Airport Pavements, April 1988, by R. S.

Rollings, ADA-194-331.

3. Copies of ASTM standards can be obtained from the ASTM International, 100 Barr Harbor Drive, P.O. Box

C700, West Conshohocken, PA 19428 or at http://www.astm.org/. 

4. Copies of AASHTO standards can be obtained from the American Association of State Highway and 

Transportation Officials, 444 North Capitol Street NW, Suite 249, Washington, DC 20001 or at 

http://www.transportation.org/aashto/home.nsf/FrontPage/. 

5. Copies of UFC 3-260-02, Pavement Design for Airfields, Department of the Army, Unified Facility Criteria 

(UFC), June, 2001 can be obtained from http://65.204.17.188//report/doc_ufc.html 

The Unified Facility Criteria supersedes the following technical manuals previously noted in this AC: 

TM5-824-2, Flexible Airfield Pavements, Department of the Army Technical Manual

TM5-824-3, Rigid Pavements for Airfields Other than Army, Departments of the Army and the Air Force, 

TM5-818-2, Pavement Design for Frost Conditions, Department of the Army

6. Copies of MS-11, Full Depth Asphalt Pavements for Air Carrier Airports, January 1973; IS-154, Full Depth 

Asphalt Pavements for General Aviation, January 1973; and MS-10, Soils Manual, Fourth Edition (1986), can be

obtained from the Asphalt Institute, Research Park Drive, PO Box 14052, Lexington, KY 40512 or at 

http://www.asphaltinstitute.org/. 

7. Copies of Special Technical Publication M-5, The Estimation of Concrete Flexural Strength from Other Types 

of Strength Tests, DATE, by W. Charles Greer, can be obtained from MACTEC Inc., Director of Publications, 1105

Sanctuary Parkway, Suite 300 Alpharetta, Georgia. 30004. 

8. Copies of FHWA-HI-90-001, Geotextile Design and Construction Guidelines, October 1989, can be obtained

from the Department of Transportation, Federal Highway Administration, Turner Fairbanks Highway Research Center, 

6300 Georgetown Pike, McLean, VA 22101. 

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4/30/2004 AC 150/5320-6D Change 3 

APPENDIX 5. AIRFIELD PAVEMENT DESIGN SOFTWARE 

1. BACKGROUND. This appendix announces software to aid with the design of airfield pavements in

accordance with the methods presented in Chapters 3 and 4 of this AC. The software presented in this appendix uses

Microsoft Excel® as a platform with Visual Basic® for Applications (VBA) Macros to facilitate the design process. 

2. AVAILABLE SOFTWARE AND SUPPORT MATERIAL. Two programs (spreadsheets) are available to 

determine pavement thickness requirements in accordance with this AC. Program F805FAA.XLS determines 

pavement thickness requirements for flexible pavement sections and bituminous overlays on existing flexible pavement

sections. Program R805FAA.XLS determines pavement thickness requirements for rigid pavement sections and 

bituminous or Portland cement concrete overlays on existing rigid or flexible pavement sections. 

Reference manuals, which guide users through each step, are available for both programs. The manuals assume users 

are familiar with the design requirements of the AC. 

Pavement designs developed using the Frost Design feature of the spreadsheets are consistent with the Reduced

Subgrade Strength method described in Chapter 3.

The spreadsheets will produce thickness designs consistent with the nomographs provided in this AC. Small variations 

should be expected due to difficulties with visual interpretation of the nomographs. 

3. ACCESS TO SOFTWARE. Design software and user manuals may be downloaded directly from the FAA 

Office of Airport Safety and Standards website (http://www.faa.gov/arp/). Software links are located on the 

"resources" and "engineering" pages of this site. Updates or additions to the design software and manuals will be posted

online, as well. 

4. USE OF SOFTWARE. Numerical results from the programs may be used to complete FAA Form 5100-1, 

Airport Pavement Design. When used to develop the pavement design, the printed results of the software should be

attached to Form 5100-1. Results from the program design summary and the aircraft mixture data provide sufficient

information to reproduce and review the pavement thickness design. Additional design information is required to

complete Form 5100-1. 

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