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GRAVEL RUNWAY SURFACE STRENGTH MEASUREMENTS

AND AIRCRAFT CERTIFICATION REQUIREMENTS

Roman A. Marushko

Flight Test Engineer

Transport Canada Aircraft Certification

with the assistance of:

Bruce Denyes

Airport Pavement Engineer

Transport Canada Aerodrome Safety

Issue 1

June 30, 1997


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Issue 1

June 30, 1997

GRAVEL RUNWAY SURFACE STRENGTH MEASUREMENTS AND

AIRCRAFT CERTIFICATION REQUIREMENTS

TABLE OF CONTENTS

1.0 INTRODUCTION ................................................................................................................5

2.0 GRAVEL RUNWAY ............................................................................................................6

2.1 Gravel Runway Construction - General ......................................................................6

2.2 Gravel Runway Design Methodologies ......................................................................7

2.3 Frost Effects on Gravel Runways ................................................................................7

2.4 Gravel Runway Strength ..............................................................................................8

2.5 Soil Properties - Effect on Surface Strength ................................................................9

2.6 Indications and Effects of Gravel Runway Failures ....................................................9

2.7 Operational Problems ................................................................................................11

3.0 MEASURING SURFACE STRENGTH ............................................................................12

3.1 California Bearing Ratio (CBR) ................................................................................12

3.2 ASTM D4429 (U.S. Corps of Engineers) CBR Test Method

(Appendix A)..............................................................................................................12

3.3 Boeing High Load Penetrometer (Appendix B) ........................................................13

3.4 Shock Penetrometer (Appendix C) ............................................................................14

3.5 Comparison of CBR Strength Measurement Methods ..............................................15

3.6 Survey of Several Runway CBR Measurements

(Appendix D)..............................................................................................................16

4.0 DESIGN CONSIDERATIONS FOR OPERATION OF AIRCRAFT

ON GRAVEL RUNWAYS ..................................................................................................18

4.1 General........................................................................................................................18

4.2 Rolling Coefficient of Friction ..................................................................................18

4.3 Braking ......................................................................................................................19

4.4 Estimation of Maximum Allowable Tire Pressure ....................................................204.5 Protection of Aircraft..................................................................................................21

5.0 AIRCRAFT CERTIFICATION TEST PROGRAM FOR OPERATIONS

ON GRAVEL RUNWAYS ..................................................................................................24

5.1 General........................................................................................................................24

5.2 Test Surfaces ..............................................................................................................24

5.3 Performance................................................................................................................25

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5.4 Handling ....................................................................................................................26

5.5 Structural Integrity and Systems Operation ..............................................................26

5.6 Aircraft Flight Manual (AFM) ..................................................................................26

5.7 Master Minimum Equipment List (MMEL................................................................28

6.0 SUMMARY 29

7.0 RECOMMENDATIONS ....................................................................................................30

8.0 REFERENCES....................................................................................................................31

FIGURE 1 Cross Section of Flexible Pavement

FIGURE 2 Classification of Soils for Airport Pavement Applications

FIGURE 3 Approximate Relationship of Soil Classification and Bearing Values

FIGURE 4 Gravel Runway Loss of Material

FIGURE 5 Gravel Runway Segregation of Material

FIGURE 6 Gravel Runway Wheel Rutting

FIGURE 7 Gravel Runway Poor Surface Drainage

FIGURE 8 Gravel Runway Poor Sub-surface Drainage

FIGURE 9 Gravel Frost Action Damage

FIGURE 10 Gravel Runway Roughness

FIGURE 11 Gravel Runway Vegetation Growth

FIGURE 12 California Bearing Ration Test Set-Up (ASTM METHOD)

FIGURE 13 Boeing High Load Penetrometer Test Set-up

FIGURE 14 A.M.D. Shock Penetrometer

FIGURE 15 CBR vs Soil Reaction Pressure for THree Penetrometer Test MethodsFIGURE 16 Example of Penetrometer Test Locations

FIGURE 17 Modification of Boeing High Load Penetrometer Test to incorporate a small

flat plate for the determination of Soil Failure Pressure

FIGURE 18 Rolling Coefficient of Friction related to Tire Pressure and Runway CBR

FIGURE 19 Subgrade Spring Reduction Factors Based on Soil Composition

FIGURE 20 Rolling Coefficient of Friction vs Tire Pressure

APPENDIX A ASTM D4429 Standard Test Method for CBR of Soils in Place

APPENDIX B Boeing High Load Penetrometer Soil Strength Tester

APPENDIX C Aerospatiale Method for CBR (Shock Penetrometer)APPENDIX D Summary of Selected Runway CBR Measurements

APPENDIX E Boeing Approvals

APPENDIX F Product Information and Material Safety Data Sheets (MSDS)

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Issue 1

June 30, 1997

GRAVEL RUNWAY SURFACE STRENGTH MEASUREMENTS AND

AIRCRAFT CERTIFICATION REQUIREMENTS

(Issue 1 of this report dated June 30, 1997 has been produced by Transport

Canada Aircraft Certification Flight Test Division to reflect experience gained

and problems encountered during gravel runway certification of transport

category aircraft.)

1.0 INTRODUCTION

The take-off and landing distance requirements of FAR Part 25 require that (in the case of land

planes) take-off and landing distance data be based on a smooth dry hard surfaced runway.

This report will review the characteristics of gravel surfaced runways, which by definition are

not smooth or hard surfaced. Because of these characteristics, the problems involved with the

measurement and definition of these surfaces will be discussed and the effects on aircraft

certification and operations will be examined. This report will limit itself to gravel runway

surfaces composed of coarse gained soils and aggregates rather than unpaved surfaces of all soil

types.

Of particular interest in this report, will be an examination of the adequacy of expressing

runway surface strength in terms of California Bearing Ratio (CBR). The test methods used to

measure CBR will be examined including an indication of their adequacy for this task. The

other problem that will be addressed is the difficulty in providing a suitable description of the

surface condition of the gravel runway for operational use. The effects of surface

characteristics on CBR values will be examined.

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2.0 GRAVEL RUNWAY CONSTRUCTION

2.1 GRAVEL RUNWAY - GENERAL

A runway pavement is defined as a structure consisting of one or more layers of processedmaterials. A flexible pavement consists of layers of material classified as surface course, base

course and subbase course, resting on a prepared subgrade layers. In a flexible pavement arelatively thin surface layer transmits its load to the base layer. Figure 1 is a cross section of atypical flexible runway pavement.

A gravel runway is essentially a flexible pavement with a surface layer of unboundgranular material. Granular materials include coarse grained soils such as sands and gravels.Material availability near the runway site often dictates the specific composition of thepavement layers.

a) Surface Course

Surface courses include hot mix asphaltic-concrete (AC) for flexible pavements. The primarypurpose of the surface course is to prevent the penetration of water to the base course, provide asmooth well bonded surface free from loose particles, to resist the shearing stresses imposed byaircraft loads and transmit bearing loads to the pavement structure. The surface course must

provide texture for skid resistance yet not cause undue wear on the tires.

For a gravel surfaced runway, moisture penetration becomes a significant factor in thepavements strength and resistance to frost action. The relatively lower shear strength of gravelrunway surfaces, especially when wet, may limit aircraft loads imposed on the runway. Aircraftmust also be protected against the hazards of loose particles. Tire wear may increase becauseof the rough texture of the gravel surface and the presence of sharp stones.

b) Base Course

The base course has the major function of distributing the imposed wheel loadings to thepavement foundation, the subbase and/or subgrade. The base course is composed of wellcompacted granular aggregates meeting high standards with respect to stability, durability, andfrost susceptibility.

c) Subbase Course

The function of the subbase is similar to that of the base course. Because this layer is furtherremoved from the surface, the subbase is subject to lower loading intensities, and the material

requirements are not as strict as for the base course. The design thickness of this layer varies asa function of the bearing strength of the subgrade; a lower subgrade bearing strength requiring agreater thickness.

d) Subgrade

The subgrade is that soil beneath the pavement structure which forms the foundation forthe pavement. The subgrade soil ultimately provides support for the pavement static loads,and imposed aircraft and ground vehicle loads. The pavement layers serve to distribute


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the aircraft and vehicle loads over an area on the subgrade greater than the tire contact area.Deformation (or deflection) of the subgrade layer must be controlled so as to remain withinacceptable limits. As in the subbase, the bearing strength of the subgrade influences the

pavement thickness.

The subgrade consists of in-situ soils or imported common material in fill sections. Soils

having the best characteristics in grading and excavation operations are incorporated intothe subgrade. The ability of a particular soil to resist shear and deformation varies with itsdensity and moisture content.

2.2 GRAVEL RUNWAY DESIGN METHODOLOGIES

The methodology applied to the design of flexible pavements at Canadian airports is basedon the use of plate strength test results. The methodology used in the United Statesdetermines the required thickness of the pavement by the use of CBR strength values.

The Canadian design methodology is described in the Public Works Canada, Manual ofPavement Structural Design, ASG-19 (AK-68-12). This methodology determines theminimum thickness of the flexible pavement based on bearing strength values of pavementcomponents. These requirements dictate the pavement thickness to be the greater of thethickness required for structural strength or for frost protection. For granular surfaced

pavements, the total depth of pavement required is divided into base and subbase courses.

Structural thickness requirements consider the subgrade bearing strength, pavement materialsused, planned aircraft design load ratings and tire pressures. Based on the aircraft loading andthe bearing strength of the subgrade soil, an equivalent granular thickness of the pavement isselected so that the subgrade soil will not be overstressed. The equivalent granular thickness isproportioned into surface, base and subbase layers to ensure that layer stability requirementswill be met. Design base course thickness ranges from a minimum of 15 cm (6 in) for tire

pressures less than 0.5 Mpa (75 psi) to 30 cm (12 in) for tire pressures greater than 1.0 Mpa(145 psi).

FAAAdvisory Circular AC 150/5320-6D describes the CBR method of design curves whichprovide the total required thickness of flexible pavement (surface, base and subbase)needed to support a given weight of aircraft over a particular subgrade.

The CBR method of design was developed by the California Division of Highways in1928. The method was subsequently adopted for military airport use by the U.S. ArmyCorps of Engineers, shortly after the outbreak of World War II. The CBR method stillremains in wide spread use today.

2.3 FROST EFFECTS ON GRAVEL RUNWAYS

Gravel pavements are found in regions subject to seasonal frost penetration and are usuallythe preferred surface type in permafrost areas. Gravel pavements often exhibitconsiderable distortion due to frost heave, but are easily regraded. The absence of awaterproof surface is usually not a problem in arctic regions because these areas usuallyhave low precipitation.

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The detrimental effects of frost action may be manifested by non-uniform heave and loss of soilstrength during frost melting. Other effects include loss of compaction, development ofroughness, restriction of drainage and deterioration of the surface. Three conditions must bemet for detrimental frost action to occur; the soil must be frost susceptible, freezingtemperatures must penetrate the frost susceptible soil and there must be sufficient free moistureto form ice.

Soils are categorized into groups for their frost susceptibility. Generally, coarse grainedsoils such as gravels and sands have low frost susceptibility, whereas fine grained soilssuch as silts have high frost susceptibility. The depth of frost penetration is a function ofthe thermal properties of the pavement and soil mass, the severity of air/surfacetemperatures and the temperature of the pavement and soil mass at the start of the freezingseason.

For pavements exposed to seasonal frost, pavement deformations resulting from frostaction are controlled by providing a sufficient combined thickness of non-frost susceptiblematerial to limit frost penetration into the subgrade. Adequate pavement load carryingcapacity may also have to be provided during the critical frost melting period when loadcarrying capacity is reduced.

Permafrost soils occur in arctic regions, where soils are often frozen to considerable depthsyear-around. Seasonal thawing and refreezing of the upper layer can lead to severe loss ofbearing strength and/or differential heave and settlement. In areas with permafrost atshallow depths, satisfactory pavements are assured by containing seasonal thawing withinthe pavement and underlying non-frost susceptible layers. This is intended to prevent thawingof the permafrost layer. Pavement design for permafrost areas must consider thedepth of seasonal thaw penetration.

2.4 GRAVEL RUNWAY STRENGTH

The deflection of the surface of a gravel runway under an applied load depends on the strengthof the surface and the strength of the underlying layers. The strength of the gravel surfacedepends on the interlock of the aggregates, particle friction and cohesion. The surface strengthalso depends on the properties of the surface materials under the influence of moisture. Thisresults in the surfaces of gravel runways being susceptible to shear failures, particularly in wetconditions.

While it is the subgrade strength and overall thickness of the pavement structure that controlsthe amount of surface deflection, the most common cause of operational problems on gravelpavements is the failure of the surface layers due to shear caused by high aircraft tire loading.

Surface shear strength can be estimated by measuring the force required to deflect orpenetrate the surface to a specified depth. This force divided by the area over which it isapplied can be taken as the soil failure pressure. This pressure can be obtained from flatplate or penetrometer type measurement devices and is often correlated to CBR.

Pavements with higher surface strengths have higher soil reaction pressures and higherCBR values.

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2.5 SOIL PROPERTIES - EFFECT ON SURFACE STRENGTH

Granular runway surfaces are typically non-homogeneous in composition, and may containvarious types of soils. The standard method of classifying soils for engineering purposes isASTM D 2487 commonly called the Unified System. Figure 2 is an example of soilclassification based on the Unified System. One of the purposes of soil classification is topredict the probable behavior of soils under the influence of frost and moisture. A soilclassification system could also be useful for the identification and definition of the gravelrunway surface for certification and operational use.

The Unified System classifies soils first on the basis of grain size (coarse grained and finegrained soils), then further subgroups soils based on the plasticity constants. Soils areexpressed by soil group symbols (e.g. GW; which is described as Well graded gravel andgravel sand mixtures, little or no fines). In Figure 3, the bearing strengths of soils identifiedby Unified System group symbols may be estimated in terms of CBR values.

AC 150/5320-6D Airport Pavement Design and Evaluation states that field CBR valuesrange from 60-80 for well graded gravel soils and 20-40 for well graded sands. The

potential for frost action of these soils is minimal, with almost no compressibility andexpansion and the drainage characteristics are generally excellent. The presence of claysoils can result in a marked reduction of the strength values and frost properties of thesesoils.

The use of a soil classification system may be useful for the estimation of the surfacestrength of a gravel runway or as a check on the validity of specific CBR measurements.

2.6 INDICATIONS AND EFFECTS OF GRAVEL RUNWAY FAILURES

The definition of runway failure is somewhat arbitrary but in the case of gravel runways itis typically identified by the formation of ruts, increasing roughness in the surface or

permanent deformation. Aircraft operations that overload the gravel runway pavementstructure can result in surface deformations which may adversely affect aircraftperformance and could cause structural damage. Some of the indications of gravel runwayfailures are discussed below.

a) Loss of Material (Figure 4)

Indications of surface material loss are bare spots, base, subbase or subgrade material appearingon the surface, and a buildup of granular material at the edge of the runway. The primarycauses are loss of material during snow removal, tire action or infiltration of lower layermaterial into the surface layer.

This condition results in a reduction of the surface strength of the runway and reduced brakingaction as a result of the loss of coarse material.

This condition can be corrected by adding new material and compacting.

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b) Segregation (Figure 5)

Segregation is the accumulation of loose non-cohesive aggregates on the surface. Causes arethe loss of finer materials due to jet or propeller blast, tire action and weathering. Adverseperformance effects and increased landing gear loads from the accumulation of loose materialsmay occur.

The potential for damage to the aircraft from debris is increased.

This condition is corrected by regrading and adding new material.

c) Rutting (Figure 6)

Rutting is defined as longitudinal deformation in the wheel path. Rutting without shoving ofadjacent material is an indication of failure in deeper layers of the pavement due to inadequatefoundation strength. Rutting with shoving indicates a shear failure in the surface layer due topoor cohesion (low surface shear strength). Poor cohesion may be the result of high moisturecontent, poor gradation, segregation or poor compaction.

Rutting may cause reduced acceleration and braking performance, directional control

problems and higher landing gear loads.

Rutting is corrected by regrading of the runway surface.

d) Drainage (Figures 7 & 8)

Poor surface drainage is indicated by damp surface areas persisting after rainfall orsnowmelt, without rutting. Soft areas with rutting and shoving during spring thaw or wetconditions and frost heaving during winter are indications of poor subsurface drainage.

Areas of wetness may result in decreased acceleration, especially when rutting is occurring.Damp areas may have an adverse effect on braking performance.

Dampness can usually be corrected by improving drainage.

e) Frost Action, Roughness (Figure 9 & 10)

Frost action is indicated by differential heaving of the surface or depressions which appearin the same place yearly during the frozen season. Inadequate drainage of the subgradeand inadequate granular thickness over frost susceptible material may be the cause of frostheave. Frost action may raise boulders in the subgrade.

Roughness may be caused by loss of material, frost action or settlement.

Runway roughness may result in decreased acceleration, increased landing gear loads as

well as the possibility of damage from larger debris.

Repair of the damaging effects of frost action may require more substantial work,including runway rehabilitation and the improvement of drainage. Roughness can becorrected by regrading.

f) Vegetation (Figure 11)

Uncontrolled vegetation growth may occur on the gravelled operational surface itselfand/or in the graded area at the runway edge. Vegetation growth may be caused by poor

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drainage or accumulation of organic soils (earth) in the surface. Low traffic volume mayalso encourage the growth of vegetation.

Vegetation will result in uneven surface characteristics, which may adversely affect theacceleration and braking performance of the aircraft.

Vegetation may be reduced by regrading, improving drainage and introducing chemicalgrowth inhibitors.

2.7 OPERATIONAL PROBLEMS

Report AK-67-09-280 Gravel Runways Condition Reporting Procedures and SurfaceStability Test Methods, lists anecdotal evidence of the types of operational problems whichmay be encountered during gravel runway operations.

An operator of medium sized propeller and jet aircraft reported that the aircraft wouldoccasionally get stuck during the spring or after rainy periods because of inadequatesurface hardness. Poor drainage was noted as a cause of rutting during wet conditions andsevere roughness occurred after the surface dried. There were also periods when runwayswere closed during the spring thaw.

One operator noted that gravel runway conditions change almost daily with the weatherand season.

All of the operators reported problems with fine materials damaging propellers and engines.Procedures to minimize damage included revising power application techniques. One operatorpainted the propellers of their aircraft each day to identify new nicks at the end of the day.Another operator suggested the use of chemical additives on the runway surface to prevent theraising of dust and the loss of fine soils.

Vegetation was noted by a small aircraft operator as being difficult to brake on with sandbeing the best surface for braking.

One operator would vary tire pressure on their Boeing 737 seasonally, or use low pressuretires on specific runways. Prohibition of Boeing 737 operations during the spring thawwas cited.

For Boeing 737 operation, Boeing commented that surfaces must be hard enough over theentire runway length and the use of an average strength value was not permissible. Bumpsthat are visible on a runway are often too severe for aircraft operation. The ride quality in a

pickup truck driven at automobile highway speeds can be used to check the smoothness ofthe runway for the suitability of aircraft operations. It was also noted that jet exhaust fromone typical Boeing 737 takeoff can result in the loss of up to two cubic yards of material.The material may blow off unevenly causing the formation of bare spots and hollows. Thiscan become the mechanism for the start of longer term damage to the runway. This

problem can be corrected by regular grading, the addition of new material, and compacting.

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3.0 MEASURING SURFACE STRENGTH

3.1 CALIFORNIA BEARING RATION (CBR)

The CBR is the ratio of the load bearing capability of a given sample of soil to that ofcrushed limestone. The bearing strength of crushed limestone has been adopted as one of

the criteria to which other types of soils are compared. Limestone has a CBR value of 100,which is expressed as CBR 100. A soil with CBR 10 has 10% of the bearing strength ofcrushed limestone.

The CBR test is basically a penetration test conducted at a uniform rate of strain. The CBRtest is considered to be a measure of the confined strength of a soil. CBR tests may beconducted in the laboratory or the field. ASTM D 1883 Bearing Ratio of LaboratoryCompacted Soils is the laboratory CBR test method. ASTM D 4429, Standard TestMethod for the Bearing Ratio of Soils in Place is the field CBR test method. (Appendix A)

The laboratory method is useful at the design stage of a pavement, but is of limited use foroperational purposes. The ASTM D 4429 method is of greater interest for themeasurement of gravel runway surface strength and may be considered to be the definitiveCBR test method.

Field CBR tests are intended for the measurement of pavement foundations that have beenin place for several years, where moisture has been allowed to reach an equilibriumcondition. Field CBR tests can also used to measure the strength of the surface layer of thegravel pavement.

3.2 ASTM D4429 (U.S. CORPS OF ENGINEERS) CBR TEST METHOD (APPENDIX A)

This method is the standard test method used to determine the CBR of soils in place. This

method shall be referred to as the ASTM method in the context of this report. Figure 12 isal illustration of the test setup.

The ASTM method is essentially the determination of the load required to cause theuniform rate of penetration of a piston into the soil. This method is applicable to sub-baseand base course materials. A test pit may have to be opened for the measurement of theselayers. This method is also applicable to the measurement of a granular surface layer forwhich CBR is the desired parameter.

The ASTM method requires that consideration be given to moisture content. If the CBRresults are to be used without any correction for moisture, then the test must be conducted

under the following conditions:

1. The degree of soil saturation (percentage of voids filled with water) must be 80% orgreater;

2. Materials must not be significantly affected by moisture. Such materials include coarsegrained or cohesionless soils;

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3. The soil must not have been modified by construction activity in the two years prior tothe test. Construction activities such as compacting or grading subsequent to thebearing test will invalidate the test.

The test apparatus consists of a mechanical screw jack, proving rings, penetration piston,dial gauges, surcharge weights, jacks and a large reaction load (Figure 12). Reaction loads

used are typically large vehicles or construction equipment.

The ASTM method measures the soil reaction pressure when a 2 inch diameter (nominally3 sq in) piston is driven up to 0.5 inches deep into a confined soil sample. The test

procedure involves preparation of a test area, placing the apparatus under the reaction load,and applying a load to the penetration piston to achieve a 0.05 inch per minute rate ofpenetration. Penetration is recorded at .025 inch increments to a depth of 0.50 inches.Penetration stress is computed at each increment of penetration. This value is the appliedforce (as measured on load cell) divided by the piston area.

A curve is derived of penetration stress versus depth of penetration. The CBR is calculatedby dividing the penetration stress at a 0.1 in and 0.2 in depth to the same penetration stress

for the standard material (crushed limestone). This test method allows for tolerances inCBR value. Tolerances increase with increasing CBR values. Normally the CBR is in thevalue derived from 0.1 penetration but 0.2 in may be used if the resulting CBR value ishigher and results are more consistent.

The test points for the ASTM method should be spaced at a minimum of 7 in apart forcohesive soils and 15 in for non-cohesive soils. Test results may be invalidated by the

presence of a rock or voids beneath the penetration piston. At the end of the test, a soilsample is obtained to determine the water content.

3.3 BOEING HIGH LOAD PENETROMETER(APPENDIX B)

The Boeing High Load Penetrometer test is used for the measurement of surface strengthand may also be used to measure the subgrade strength of paved and unpaved runways.This measuring device consists of a hydraulic cylinder to provide a large penetration forceat a test probe (Figure 13). The test probe is a conical projection at the end of a cylinder.This method is not a confined test as is the case with the ASTM method.

The test apparatus is similar to that of the ASTM method. A hand pump is used instead ofa mechanical screw jack. Pressure is read off of the hand pump to calculate the loadapplied on the penetrometer. Unlike the ASTM method, the Boeing High LoadPenetrometer test method does not provide any guidance on the correction of soil strengthdue to moisture, soil disturbance or construction activity.

In this test procedure, the cone reference point is driven at a steady rate to a four inch depthinto the surface, by the application of pressure through the hand pump. Penetrometerpressure is taken when a condition of equilibrium is reached between the hydraulicpressure and the surface reaction pressure. Generally a pressure reading is taken 30seconds after movement of the penetrometer has stopped at the 4 inch depth.

Soil failure pressure is derived from the penetrometer force divided by the projected areaof the 2 inch diameter penetrometer cone point. Friction and shear forces at the side of the

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cone point are included in the total force. The CBR value is determined by a formularelating the CBR to the soil failure pressure.

For the Boeing test method the surface material must be homogeneous to a depth beyondthe cone point tip. the presence of large stones may introduce errors, but Boeing statesthat the presence of a large stone will be clearly evident to the operator during

measurement. This is indicated by a sudden large increase in penetrometer pressurereadings during pressure application. Relocation to another test position several feet awayis recommended. Inaccuracies may also result from friction in the jack and sides of the

penetrometer if any tilting occurs. This method does not specify the spacing of test points.

Boeing has provided a curve of the CBR versus Soil Failure Pressure (Boeing High LoadPenetrometer) which indicates that the CBR value derived from the Boeing Penetrometergenerally corresponds to that of the ASTM method at a penetration of 0.50 inches (Figure15). However it is noted that the ASTM method normally requires the CBR to becalculated at a depth of 0.1 or 0.2 in. The CBR values from the Boeing method are on theaverage 10% less than that of the ASTM method for the same soil reaction pressure.

Boeing claims that this method has the advantages of accuracy, versatility, speed of useand may be performed by relatively untrained personnel.

3.4 SHOCKPENETROMETER (APPENDIX C)

The shock penetrometer has been employed by Avions Marcel Dassault - Breguet Aviation(AMD-BA) and Aerospatiale for the determination of runway CBR. Both methods aresimilar, but because of available information, the Aerospatiale method is described here ingreater detail. (Figure 14)

The manufacturer states that this test method is applicable to laterite, grassed or gravel

runways. The range of CBR values derived by this method are from a CBR of 2.5 to 15.The assumption behind this method is that the minimum depth of soil for aircraft operationis 10 cm (4 in). This test method does not provide any correction to CBR values due tomoisture or soil disturbance.

The shock penetrometer consists of a long rod with a cone in contact with the soil and asliding weight. The penetrometer is driven into the soil to a depth of 10 cm by the releaseof a sliding drop weight along a handle. The soil strength is indicated by the number ofdrops required to achieve the 10 cm penetration. CBR is read off of a calibration chart as afunction of the number of drops.

This test method determines a soil reaction pressure derived from the number of drops ofthe falling weight divided by the projected surface area of the cone. The CBR is read froma chart of CBR versus Soil Reaction Pressure. When this curve is compared against asimilar curve produced by the Boeing method, the CBR values from this method areapproximately 70% of those of the Boeing method for the same soil reaction pressure(Figure 15). As an example, a CBR value of 10 by this method corresponds to a CBR of14 by the Boeing method. The soil reaction pressure to CBR conversion appears to be onlyapplicable to soils having CBR values below 20.

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3.5 COMPARISON OF CBR STRENGTH MEASUREMENT METHODS

For the purposes of this discussion, the CBR value derived from the ASTM method will beconsidered as the standard to which the other methods will be compared.

The ASTM method using a 3 sq in piston directly measures the bearing strength of the soil.

Shear reaction to this test method is minimal because of the confinement of the soil and thesmall depth of penetration. The Boeing penetrometer and Shock penetrometer have conesof similar projected areas (3.14 sq in vs. 3.25 sq in) being driven into the soil to anapproximate depth of 4 in. The geometry of these cone penetrometers are different fromeach other, however, the principle forces reacting against these cone penetrometers are theshear strength and bearing reaction of the soil. The ASTM method and Boeingpenetrometer also work on the principle of a steady application of pressure to drive thepiston/cone into the soil. The shock penetrometer is driven into the soil by a series ofimpacts.

Figure 15 is a plot of CBR versus oil reaction pressure provided by Boeing to show thecorrelation of their penetrometer to the ASTM method. Aerospatiale provides a similarchart for their Shock penetrometer, which is overlaid on this chart. Here, the soil reactionpressure is the penetration force divided by the contact area of the penetrometer device.This chart indicates that for soils with CBRs less than 40, the Boeing High LoadPenetrometer will yield CBR estimates that agree very closely with results of the ASTmmethod (0.5 inch penetration). CBR values estimated from Shock penetrometer do notcorrelate closely to either the ASTM or Boeing methods.

The Shock penetrometer may not correlate well to CBR because it is an impact test, ratherthan a constant rate of loading, like the other two penetrometer methods. A comparison ofsurface strength measurements using the Shock penetrometer has identified significantdiscrepancies in CBR values as compared to the Boeing penetrometer when measured on

the same runways. (See paragraph 3.6 and Appendix D)

Figure 15 suggests that the application of CBR values for operational use without statingthe test method may be misleading. For example, if the soil reaction pressure is 500 psi,the ASTM method and Boeing penetrometer test methods would yield a CBR value ofapproximately 14, whereas the Shock penetrometer derivation of soil failure pressurewould give a CBR value in the range of 10. Because of the errors associated with using theresults of various test equipment and methods to estimate the standard CBR value of thesoil, it is important that the test method be identified.

Current practice is to state a minimum CBR value in the Airplane Flight Manual (AFM),

but the method of measurement is usually not indicated. Without knowing the method,aircraft operations may take place on a surface that is weaker than expected. For thisreason the AFM should state the measuring technique associated with CBR value. Forexample, CBR would be expressed as CBR 14 as determined by the BoeingPenetrometer. Similarly, if any other units or methods are used to express the strength ofa runway, they should be clearly stated along with the strength value and the layer of therunway to which they apply.

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The Boeing and Shock penetrometer methods do not provide any CBR correction factors toaccount for soil moisture content. As an unpaved surface may be significantly weakerfollowing spring thaw or heavy precipitation, the condition under which themeasurements were made should be recorded and provided in the AFM. These should besimilar to those specified for the ASTM method and include, the degree of soil saturation,the materials in the surface and the date of measurement.

The above test methods do not specify where the tests should be performed on the runway.The locations specified for the Transport Canada plate test, AK-68-31-000 AirportPavement Evaluation - Bearing Strength, where measurements are taken in the aircraftwheel tracks at specified intervals, appears to be reasonable. Figure 16 is an example of astrength survey conducted at Baker Lake by Boeing using their penetrometer.

The Boeing and Shock penetrometer test methods also do not provide any information onthe precision and bias of the CBR results. The Boeing penetrometer has been calibrated bymaking tests on various soil samples for which the CBR has been determined usingconventional CBR test methods. CBR correlation data should be obtained from otherequipment manufacturers to qualify their equipment, before performing any tests.

Report AK-67-09-280 discusses the modification of a Boeing penetrometer by replacingthe cone point with a small flat plate. Figure 17 is an illustration of the test set-up. The

plate diameter proposed is 150mm (6 in), and is sized to approximate the wheel loadsimposed by a single aircraft tire. The plate diameter may be varied to account for thefailure criteria of the soil and the available size of the reactive load. The contact areashould equate as closely as possible to the tire contact area of a typical aircraft operating onthe gravel surface. Soil failure would be expressed as the pressure required to indent thesoil to a specified depth. This proposal would require further investigation and testing, asthe definition of soil failure is a critical consideration. Soils in general, do not have welldefined failure points.

3.6 SURVEY OF SEVERAL RUNWAY CBR MEASUREMENTS (APPENDIX D)

Appendix D is a summary of CBR measurements of various runways in support ofCanadian runway measurement and aircraft certification programs. CBR values arepresented as average for the runway and the range from minimum to maximum. The rangeindicates the variability of CBR values for a particular runway. Details are also provided(if available) on the date of the measurement, dry or wet conditions and lateral locationwith respect to the centerline. More detailed information may be obtained by review of thereferences used.

The composition of the various runways tested was found to vary considerably. Only afew runways such as Rankin Inlet were composed entirely of gravel. Many runways werecomposed of sand and fine gravel such as Churchill (Manitoba) or sand, clay and finegravel such as Hall Beach (NWT).

The CBR measurements taken by the Canadian DOT Gravel Runway Survey did notspecify the measurement method used although it is believed it was the Boeing High LoadPenetrometer. The DOT surveys provided an average CBR measurement and did not

provide a range of measurements.

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Test results for Churchill (Manitoba), Kuujuaq (Quebec) and Nanivisik (NWT) providedCBR values for wet and dry runways. At Churchill, wet and dry CBR values varied by afactor 2. Kuujuaq and Nanisivik showed no apparent reduction in CBR for wet runwaysas compared to dry. These particular runways are all composed of sand and gravel. Therewere no wet runway CBR values obtained at Hall Beach, which would have beeninteresting considering the presence of clay in this runway.

CBR measurements taken were consistent in value with the exception of those taken byAMD-BA for the Falcon 900 program using the Shock penetrometer. CBR measurementsrecorded by AMD-BA were approximately one third of the magnitude of those recorded byother agencies. The Shock penetrometer CBR measurements for the Kuujuarapik runwaygave an average CBR of 10. AMD-BA explains that the low CBR values were attained atKuujuarapik because the runway was composed of sandy soil with a lot of free gravels.This same test method gave an average CBR of 15 for the Kuujuaq runway which appearedto be more highly compacted.

Based on approximate CBR strengths of materials present in these runways, the AMDderived values for Kuujuaq would still be low for either poorly graded sand or gravelrunways (See figure 3). As a comparison, the runway at Lake Havasu, Arizona wasdescribed by one manufacturer as being composed of soft sandy areas on a runwayconstructed of deep loose gravel with large stones. A low CBR would be expected herebut measurements taken by the Boeing penetrometer on different occasions had indicated aminimum single CBR value of 34 and minimum average CBR of 50.

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4.0 DESIGN CONSIDERATIONS FOR OPERATIONS OFAIRCRAFTON GRAVEL RUNWAYS

4.1 GENERAL

Operation of aircraft from gravel runways should be as safe as that from paved runways.

This includes consideration of aircraft performance, handling, structure, systems andpowerplant operation, as well as the effect of damage from debris.

4.2 ROLLING COEFFICIENT OF FRICTION

The relatively weaker surface of gravel runways have an adverse effect on aircraft rollingresistance. Excessive tire pressures may cause shear failures of the surface and deflectionsin the form of rutting. This action extracts energy from the wheel motion and causes anincreased rolling coefficient of friction. The result is reduced aircraft acceleration on take-off from a gravel surface resulting in increased take-off and accelerate-stop distances.

For a given aircraft tire pressure, the lower the strength of the surface, then the higher therolling coefficient of friction. A useful parameter against which to correlate rolling frictionis the ratio of aircraft tire pressure divided by the runway surface CBR value (Figure 18).A reduction in tire pressure reduces this ratio and in turn the rolling coefficient of friction.The minimum permissible tire pressure is constrained by limits on tire size and tiredeflection for a given wheel load. Reducing wheel load (aircraft weight) is a method oflimiting tire pressure without exceeding tire deflection limits. Another method of reducingthis ratio is by the selection of oversize tires inflated to a lower pressure. This results inthe wheel load being distributed over a larger surface area, thereby reducing shear stressesin the runway surface.

Tire rigidity should also be considered in its effect on the rolling coefficient of friction. A

less rigid tire tends to flatten when under load allowing it o create shallower rolling trackswhich results in reduced rolling friction. Ideally, it is desirable to have a combination oflow tire pressure and reduced tire rigidity on surfaces having a low CBR value.

On weak runway surfaces, the rolling coefficient of friction varies with moisture dependingon the specific soil. Loose sandy soils exhibit a decrease in rolling coefficient of frictionwith increasing moisture content, because when dry, more effort is required to push sandysoils away by the tires. This characteristic is independent of tire pressure. The rollingcoefficient of friction is increased considerably with moisture for clay and silt soils becausethese soils tend to stick to tires and increase their rolling friction. Reducing tire pressurewhen on these soils has little effect.

The rolling coefficient of friction may also be a function of tire rotation speed for softsoils. On a soft soil, the static equilibrium is disturbed on initial motion resulting in thetotal load being distributed over a smaller area. This causes an increase in tire contact

pressure causing the tire to sink deeper into the soil thereby increasing the rollingcoefficient of friction. Once in motion, the soil beneath a wheel has little time to movewhich prohibits the tire from sinking in, and results in a reduction in the rolling coefficientof friction with tire speed increase.


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Wheel arrangement has an effect on the rolling coefficient of friction. Tandem wheelshave a lower rolling coefficient of friction when compared to side by side wheels. Intandem arrangements, the front wheel does the main work in forming a track. The rearwheel runs on a smoothed level track and experiences less rolling resistance.

The wheel configuration of the aircraft will be fixed prior to certification for unpaved

runway operations. Initial design configurations may not anticipate future gravel runwayoperations.

Figure 20 is a graph of tire pressure versus the rolling coefficient of friction from test resultsconducted by Boeing and the U.S. Corps of Engineers. For surfaces having CBR valuesranging from 10 to 20, the rolling coefficient of friction increases as the tire pressure increasesto a value of about 0.075, at a tire pressure of 150 psi. On surfaces having a CBR of 30 andabove, the rolling coefficient of friction remains fairly constant as the tire pressure increasesand may actually trend slightly downwards.

4.3 BRAKING

Test results indicate that the braking coefficient is a function of the surface characteristicsand is independent of runway CBR. Soil properties and moisture are the primary factorsaffecting the adhesion between the surface and the tire, which directly affects the brakingcoefficient of friction. Significant degradation in braking was reported by onemanufacturer for tests conducted on a wet gravel runway which had experiencedconsiderable loss of gravel due to weathering. Similarly, accumulations of loose andsmooth gravel particles may reduce tire adhesion. Tire adhesion may improve under someconditions such as frozen well graded gravel surfaced runways, when the surface layer isbonded by frozen moisture.

Accumulations of fine sandy soils may also cause a reduction in the braking

coefficient. Fine soils tend to be more slippery than coarse grained soils and may plug tiretreads, resulting in reduced adhesion to the surface. Generally, low pressure tires havebetter adhesion on wet soil surfaces than high pressure tires and also adhere better tosurfaces containing any vegetation. Surface slipperiness is also significant when the upperlayers of a frozen runway thaw, while the lower layers remain frozen. In all cases, reducedadhesion increases the possibility of skidding during braking, especially for aircraft withouteffective anti-skid systems.

Anti-skid systems are desirable on gravel runways since they compensate for changes intire adhesion to the runway surface and result in shorter deceleration distances. Unevensurfaces however will degrade anti-skid response. One manufacturer has concluded that

the response of aircraft anti-skid performance is a function of runway surface conditionrather than the CBR strength.

The dependence of braking performance on surface characteristics requires that the surfacebe defined to ensure that predicted braking response will be achieved. The Unified soilclassification system may represent a method of identifying runway surfaces for aircraftperformance charts. Because of the variability of gravel runway surfaces, a worst casesurface should be used during performance testing to obtain conservative flight manualdata.


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4.4 ESTIMATION OF MAXIMUM ALLOWABLE TIRE PRESSURE

Report AK-67-09-280 provides some general guidelines for estimating the maximumallowable tire pressure for a gravel surfaced pavement. An essential step is to define anddetermine the soil failure pressure which provides an indication of the shear strength of thesurface. This may be measured in place by a small 150 mm (6 in) diameter static plate

load test or by the Boeing penetrometer test. In the case of the plate load test, it would benecessary to standardize the test deflection which would constitute the failure for the sizeof the plate used. The failure load could then be determined and the failure pressurecalculated as the failure load divided by the contact area of the test plate. The plate contactarea should approximate closely as possible to the aircraft tire contact area.

The allowable tire pressure may be estimated by applying a factor of two to the soil failurepressure. The factor of safety is to account for the effects of tire motion which are notpresent in the static test. Tire motion effects such as tire scrub, braking, localized tirecontact pressure, uneven wheel loads and surface degradation can produce shear stresses30% greater than those of the average tire contact pressure.

Regardless of whether a plate load or CBR procedure is employed, a series of tests shouldbe made along the length of the runway, generally in the aircraft wheel paths; the resultsshould be averaged and a statistical measure such as lower quartile point or standarddeviation applied to account for strength variations along the runway length. Dependingon the surface and/or subgrade soil type, it may also be necessary to apply a furtherstrength reduction factor to account for weakening of the pavement during spring thaw.Suggested spring reduction factors based on soil type are given in Figure 19.

An example calculation for the estimation of allowable tire pressure on a gravel surfacedpavement is given below:

Parameter Value Note

Soil Failure Pressure 400 psi as measured from a plateload test or converted froma CBR value

Allowable Tire Pressure 400/2 = 200 psi factor of safety of 2 applied

Allowable Tire Pressure 200*(1.0-0.25)=150 psi Spring Reduction Factor of(Spring) 25% applied for a soil of

type GC -Gravel with Clayfines

The estimated maximum allowable tire pressure for the gravel surface would therefore be150 psi. As this value is only a static approximation, aircraft testing is still necessary toensure that this value is adequate.

In another approach, the U.S. Army Corps of Engineers have applied a method ofestimating allowable tire pressure based on the rolling coefficient of friction on a low

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strength surface. A criteria was established for determining a tire pressure to give a rollingcoefficient of friction of .075. The .075 value was based on the typical thrust to weightratio of .3 of a transport category aircraft and 25% of maximum thrust required to start theaircraft rolling from a stopped condition on a weak surface. (.075 is calculated as the

product of .3 and 25%).

In the tests conducted by the U.S. Corps of Engineers, a range of tire pressures wereevaluated to establish this rolling coefficient of friction on surfaces with varying CBR. Fora given tire pressure, the thrust required to start the aircraft moving was measured. Testresults are shown in Figure 20. For example assuming a rolling coefficient of friction of.075 and a CBR of 20, the allowable tire pressure would be 1.03 Mpa (150 psi). Notehowever that the slope of the variation of the rolling coefficient of friction with tire

pressure is relatively flat at higher CBR values and hence small differences in CBR mayresult in an optimistic value of the maximum allowable tire pressure.

Tire deflection depends on the ratio of aircraft weight to tire pressure. Aircraft weight mayhave to be reduced in proportion to tire pressure to satisfy tire deflection limitations. If testsurfaces have a different strength than the minimum proposed, then individual tirepressures for each surface strength tested will have to be established. Maximum allowableaircraft weight will correspond to the tire pressure tested.

4.5 PROTECTION OF AIRCRAFT

Operations on gravel runway surfaces require protecting the aircraft from the effects of flyingdust, debris and stones. Protection systems are typically characterized as Gravel RunwayKits and are comprehensive modifications to minimize the adverse effects of gravel runwayoperations.

a) Reduction of Gravel Spray from Landing Gear

The debris spray in the wake of the rolling wheels may be reduced by the installation of graveldeflectors, which are typically flat plate shields attached to the landing gear between, around orbehind the wheels. On retractable landing gear, mechanisms may be necessary for the fairingof these deflectors into the profile of the aircraft to reduce aerodynamic drag. Other methodsinclude the installation of tires with chines facing inboard to prevent the debris spray fromimpinging on the gear struts. One manufacturer has employed mud guard type nose wheeldeflectors on their aircraft.

b) Protection of Aircraft Surfaces

The gravel kit may require the installation of protective panels or the bonding of materialsto the skin of the aircraft to protect surfaces from the effects of flying debris. Areas to beprotected include belly surfaces, inboard flap panels, cables and pipes routed on thelanding gear, external lights and antennas. External light protection may include therequirement that belly lights be retractable or covered in a protective wire mesh. Flapextension angles may have to be decreased for take-off and landing to minimize damagecaused by debris spray from the wheels.

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c) Protection of Engines

Jet and propeller driven aircraft have unique protection requirements for operation ongravel surfaces. Both types of propulsion must avoid the ingestion of debris, especially gasturbine engines because of the possibility of severe engine damage occurring.

Propeller driven aircraft are susceptible to impact damage caused by the close proximity ofthe blade tip to the runway surface, especially for engines mounted close to the ground.The most critical condition for blade damage generally occurs at high propeller RMP and alow aircraft ground speed, but may also occur during static operation at ground idle. Inthese conditions, the suction effect from the propeller has the greatest possibility of raisingdebris. Damage is minimized by gradual application of power until the aircraft achieves aminimal ground speed. Power levers may also have to be set to fine pitch during taxi.

Jet engines are primarily susceptible to the ingestion of debris from wheel spray. Graveldeflectors may be required to prevent this. Jet engines mounted close to the ground havesuction effects similar to those found on propeller driven aircraft. The Boeing 737 gravelkit incorporates engine vortex dissipators, which are probes at the front of the engine

designed to destroy a vortex created by low pressure at the engine intake. The vortex tendsto raise debris especially at low ground speeds. The probes extract engine bleed air anddirect it at the vortex to destroy it.

The ability to apply maximum takeoff thrust while the aircraft is stationary is usually notpossible on an unpaved runway. Take-off distances will increase when the application oftake-off power or thrust is scheduled by a gradual rate of application or a rolling take-off isperformed. Devices such as vortex dissipators which extract bleed air may also reduceavailable take-off thrust.

d) Tires

Increased tire wear is common on unpaved surfaces because of the rough texture ofunpaved surfaces. Tires are also vulnerable to cutting and penetration from sharp stonesthat may be found on gravel surfaced runways. Braking application may have to bereduced to minimize tire damage from inadvertent skidding which in turn will have anadverse effect on braking performance. This may be more critical at lower speeds.

e) Effects of Dust and Debris on Systems

Flying debris and dust may result in the gradual blockage of intakes, ducts, drains and airdata sources, and may eventually increase the possibility of flight controls jamming. Theabrasive qualities of dust may promote the erosion of paint surfaces and the crazing ofwindows if correct cleaning procedures are not applied. The infiltration of dust into

mechanical linkages may promote increased wear. Limitation against the operation of airconditioning systems or the prohibition of specific bleed air configurations while on theground may be required.

f) Increased Structural Loading

Aircraft landing gear and tire structures will likely be exposed to greater static and dynamicloadings on gravel runways as compared to hard surfaced runways. The relatively roughersurfaces of gravel runways may allow large forces to be transmitted to landing gear.

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Lateral loads are considerably higher on low CBR runways, especially during turningmanoeuvres and during the passage of the nose gear through deposits of softer materials.

Since tire deflection will be increased under the influence of a reduced tire pressure for agiven weight, it may be necessary to limit aircraft weight. Along with a weight restriction,loads on the nose landing gear may have to be reduced by applying a limitation to the

forward centre of gravity range.

g) Inspection and Maintenance

Increased inspection requirements are necessary for gravel runway operations. Propellers,engine intakes, compressor blades, aircraft surfaces, landing gear systems, tires, filters andproturbances should be checked more frequently for dust and debris damage. It may benecessary to perform an inspection before each flight for any damage from the previousflight. Minor paint touch-ups and repairs in the field may be necessary before morepermanent repairs can be made at a maintenance base. The frequency of cleaning of theaircraft and the lubrication of linkages should also be increased because of the increaseddust in the gravel runway environment.

Operations on gravel runways require maintenance programs to be modified and approvedfor such operations. An approved Maintenance Manual Supplement or specificmaintenance instructions may be necessary, depending on the aircraft basis of certification.

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5.0 AIRCRAFT CERTIFICATION TEST PROGRAM FOROPERATIONS ON GRAVEL RUNWAYS

5.1 GENERAL

The objective of a certification test program is to produce a set of limitations, procedures

and performance appropriate to operation on gravel runways. If it is intended thatperformance data cover all likely operating conditions, then the tests must cover worstcase conditions. For conservative aircraft performance data, the strength of the surfaceshould be measured prior to conducting any aircraft gravel runway performance testing,since the passage of the aircraft during the test program may further weaken the surface.

For the purpose of certifying aircraft for operation on gravel surfaces, restrictingmeasurement to the shear strength to the surface, rather than the bearing strength of theentire foundation, is generally sufficient. It will however still be necessary to ensure thatthe pavement has sufficient overall strength to support the load of the aircraft, withoutcausing subgrade failure. Construction records or other sources giving the maximumpavement strength load ratings (such as the ICAO ACN/PCN system) should be referred toat this stage.

Strength of the gravel runway surface measured in the summer and fall may also be higherthan the spring because of moisture from the spring thaw. It is therefore important to notethe time and conditions under which the strength measurements were obtained.

Flight test demonstrations should be conducted on both wet and dry runway surfacesbecause of the effects of moisture on rolling resistance and braking performance. Criteriamay have to be established on the exact definition of a wet surface; whether it is wetnessfollowing spring thaw conditions or as a result of long lasting precipitation.

The tire pressure will be constrained by the strength of the surface proposed for operation.The first step is to establish a minimum surface strength and determine the maximumallowable tire pressure for this strength level. Section 4.4 discusses the determination ofmaximum allowable tire pressure.

5.2 TEST SURFACES

a) Measurement of Runway Strength

Surface strength for each test surface should be measured at the start of the test program.The method used to measure the surface strength should be recorded as well as the

locations of the test measurements. The date of the measurement, the moisture conditions,runway conditions, presence of frost and other effects should also be recorded.

b) Soil Analysis

A soil analysis of each test surface should be conducted using a standard method such asthe Unified system. This data would be used to define the composition of the runwaysurface tested for the AFM and for the determination of seasonal reduction factors for tire

pressure. Braking performance may also be affected by the soil composition of the surface.

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5.3 PERFORMANCE

a) Rolling Coefficient of Friction

Rolling coefficient of friction is a function of tire pressure and runway surface strength.Data for testing should be obtained for the worst case of tire pressure divided by runway

strength (CBR) ratio. As mentioned before, maximum aircraft weight may be constrainedby deflection limits of the tire. Therefore a limiting configuration should be defined by theaircraft weight over tire pressure ratio.

It may not be possible to find a gravel runway with the minimum strength proposed for theoperation. Tests should then be conducted on two runways of differing strength usingcorresponding tire pressures to obtain the rolling coefficient of friction for each surfacestrength. The higher of the two rolling friction values obtained should be applied to the

proposed runway strength for conservative results.

Tests may be conducted by accelerate-coasts or landing on the unpaved runway using take-off flap setting. The aircraft should be allowed to decelerate to the lowest practical speed

with the engines at an idle setting and without using brakes, spoilers or reverse thrust.

b) Braking Coefficient of Friction

Braking performance is primarily a function of the characteristics of the braking surfacerather than the surface strength. Braking tests should therefore be conducted on surfaceshaving the characteristics that will result in the worst braking performance. Surfacecharacteristics of the test runway should be defined in terms of a soil classification system,such as the soil group descriptions found in the Unified soil classification system (Figure3). Testing under wet conditions may result in the worst possible braking performance.Factors may have to be applied to braking distance to account for reduced brakingperformance under wet conditions. In general the braking performance of at least threedifferent runways typical of expected operation, should be assessed.

Braking coefficient data should be collected for the take-off and landing configurations.Braking data may be obtained during accelerate-stop and/or landing tests with spoilersdeployed and full braking.

c) Performance Take-offs and Landings

Performance take-offs and landings should be conducted to verify the performance data.Weights should be varied from minimum to the maximum for the gravel runway. Testsshould also be conducted to establish procedures for scheduling thrust or power and

establish limits for thrust reverse. Landing touchdown characteristics should be evaluatedduring the landing performance tests.

d) Drag from Gravel Kit Installation

Tests to determine drag polars and assessment of handling qualities due to the addition ofgravel kits should be conducted. Performance data for takeoff distance and climbperformance may require adjustment. Indicated airspeed and altitude may also requirecalibration, depending on the external configuration.

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5.4 HANDLING

a) Ground Handling Characteristics

Taxi tests should be conducted to determine lowest runway strength and minimum

allowable tire pressures necessary for acceptable handling characteristics. Any limits on

taxi operation should be determined.

The ground handling characteristics should be checked on the various types of surfaces on

which they are intended to operate. Particular attention should be made to ground handling

with low pressure or oversized tires for controllability during the take-off and landing rolls,

and turns while taxiing.

5.5 STRUCTURAL INTEGRITY AND SYSTEMS OPERATION

a) Protection

Functional performance of protection equipment must be tested including the effectivenessin minimizing debris spray and satisfactory operation during gear retraction and extension.

Water trough testing may be employed in some cases, by using water to simulate the debris

spray pattern. Flight testing of deflectors includes determining any increase in the drag

coefficient caused by the installation of these devices, and checking that there are no

adverse effects on the handling qualities of the aircraft. Increased gear retraction time as a

result of the installation of gravel deflectors may require adjustments to take-off

performance data.

5.6 AIRCRAFT FLIGHT MANUAL (AFM)

Aircraft Flight Manual information will be required for aircraft gravel runwayairworthiness approval. Transport Canada Airworthiness Manual Advisory (AMA) 525/4,

Operations from Unpaved Surfaces provides guidance for information to be included in

the AFM. Normally, the information is included in an AFM supplement. The AFM

should include the following information as applicable:

a) Limitations Section

1. Weight and centre of gravity limitations

2. Approved take-off and landing configurations

3. Minimum and maximum tire pressures and tire types

4. Specified gravel protection system installed and operative5. Minimum strength of runway surface and approved types of unpaved runways

6. Prohibition of reduced thrust take-offs

7. Wheel brakes, spoilers/lift dumpers and anti-skid to be operative for take-off and landing

8. Continuous ignition on for take-off and landing

9. Nose wheel steering operative

10. Any other applicable limitation

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b) Normal Procedures Section

1. Procedures for taxiing, take-off, rejected take-off and landing on gravel runways2. Setting of thrust or power on take-off3. Selection of reverse thrust on landing and minimum speeds for operation4. Procedures for operation of braking system

5. Note on minimum turn radius available6. Note that engine runups should be avoided over surfaces composed of loose material

c) Abnormal Procedures and Emergency Procedures Section

There are typically no changes to the emergency procedures section with the incorporationof gravel runway kits or for gravel runway operations. Abnormal procedures may howeverrequire a review. For example, the failure of the bleed source to an engine vortex dissipatormay require diversion to another airport having a paved runway.

d) Performance Section

This section includes all applicable charts pertaining to aircraft field performance and otherparameters affected by the gravel kit installation. The application of distance factors maybe necessary for wet gravel runways especially those with standing water. No credit forclearway and/or stopway should be allowed.

The surface definition of gravel runways has been typically specified in the performancesection of the AFM supplement instead of the limitations section. The following areexamples of surface definitions applied in some AFM supplements:

1. Take-off and landing field lengths shown in this section were determined on a wetgravel runway

2. The runway surface should have a uniform covering of gravel that is graded smoothand kept free from ruts to avoid collection of undrained water during periods ofprecipitation;

3. Surface material at least 6 inches thick, well compacted and with a CaliforniaBearing Ratio of at least 30, and no areas of deep loose gravel deficient in fines;

4. The subbase strength at a depth of 8 inches below the runway surface capable ofsupporting 260 psi at 0.1 inch penetration when tested by the in place CBR methodor by equivalent method;

5. Runway subsurfaces constructed of materials impervious to water should be gradedto facilitate water drainage;

6. Gravel runway should be inspected at a frequency dictated by local conditions toassure that it is in a satisfactory condition.

It is pertinent to consider the preceding examples on their degree of usefulness foroperational use. The use of soil classification system group symbols to describe gravel

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runway surfaces in AFM performance charts may be clearer in providing a definition of thetype of surface the performance chart is based on. Acceptable soil types as defined by theUnified group symbols would be listed and described here. The AFM supplement mayinclude accompanying information on the corresponding properties of gravel runways.This would provide flight crews and operators with general information of the probablebehavior of gravel runway surfaces under varying conditions affecting aircraft

performance. This would also provide a link between the identification of gravel runwaysurfaces used during certification testing to those used operationally.

5.7 MASTERMINIMUM EQUIPMENT LIST (MMEL)

Specific gravel protection equipment will usually lack redundancy for dispatch withfailures. Unless specifically evaluated there should be no dispatch with inoperativeequipment. It is recognized that a certain amount of damage from gravel runwayoperations may occur as long as this does not affect flight safety.

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6.0 SUMMARY

A gravel runway is essentially a flexible pavement with a surface course of unboundgranular material. Performance data based on a hard surfaced smooth dry runway isusually not valid when applied to a gravel runway. The lower strength and less uniformproperties of gravel surfaces results in significant differences in rolling friction and braking

performance. Braking performance is usually most affected by surface soil propertiesrather than surface strength.

Although there is a minimum surface strength requirement for a given aircraft weight andtire loading, the most common operational problems result from the shear failure of thesurface (rutting) caused by excessive aircraft tire loading. Gravel runway surface strengthdepends on the surface composition, moisture content, gradation, compaction andaggregate interlock. Gravel surfaces are susceptible to weakening from moisturepenetration and frost action. Loose material associated with gravel runway surfaces alsoresults in the requirement to protect the aircraft from debris. The rough texture of gravelsurfaces contribute to increased tire wear.

California Bearing Ratio (CBR) is a common parameter used to express soil bearingstrength. CBR also indicates runway surface shear strength if a penetrometer method isused. On a given runway, the CBR value is dependent on the test method used. Largediscrepancies in measured CBR values have been identified dependent on measurementtechnique. The specific CBR method used for gravel runway certification testing shouldbe identified in the AFM. The same CBR method should be applied for operationalmeasurements.

Airworthiness approval requires consideration of aircraft performance, handling, structuraland systems aspects, and the provision of AFM limitations, procedures, and performanceinformation.

Test programs are necessary to determine aircraft performance, handling characteristicsand systems operation on gravel runway surfaces. For conservative performance data, testsshould be conducted on surfaces having the most adverse effects on rolling resistance andbraking performance. Measurement of surface shear strength, rather than the bearingstrength of the entire foundation is generally sufficient for certification testing.

Because of the variability of gravel runways, a description of the surface composition is animportant parameter to relay to the operator/crew to ensure that the performance dataobtained during testing is appropriate for aircraft operations. The AFM should describe thecharacteristics of the gravel runway surface for which the performance testing was

conducted. This information must be presented in a manner, that will provide a practicalaid in the identification of the runway surface for operational use.

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7.0 RECOMMENDATIONS

1. Safe gravel runway operation requires an airworthiness approval because of significanteffects on performance, handling, structural integrity and systems operation.Airworthiness approval should be a prerequisite for any operational approval.

2. The AFM should express the strength of the runway using a strength classificationsystem that can be easily interpreted by the flight crew and aircraft operator. Themethod of strength measurement used for certification testing should be stated. Thisshould include the conditions under which the testing was conducted and the specificrunway layers on which measurements were taken.

3. The ASTM method and Boeing High Load Penetrometer should be the only approvedmethods for determining gravel runway surface CBR for the time being. The Shock

penetrometer results may be unreliable in predicting CBR and should not be approvedfor use until differences can be resolved

4. The AFM performance section should provide a description of the gravel runwaysurface on which performance testing was conducted. This information must bepresented in a manner, that will aid in the identification of the runway for operationaluse.

5. The AFM should include a description of the soil types tested. This would serve as anaid in identifying gravel surfaces for the operator when performing their ownassessment. The terminology used to identify soil types should be such that it isreadily understandable. Information regarding the influence of moisture, frost andweathering of specific soil types on the performance and handling of the aircraft shouldbe provided.

6. The establishment of an approved Maintenance Manual Supplement or specific aircraftmaintenance instructions should be considered for gravel runway airworthinessapproval.

7. Operational approval should consider the need for gravel runways to undergocontinued inspection and maintenance to ensure that runway thickness, material andstrength specifications continue to be met for operational use.

8. Airworthiness Manual Advisory, AMA 525/4, Operations on Unpaved Runwaysshould be reviewed and updated to incorporate the above recommendations, and thematerial discussed and referenced in this report. Specific tests, aircraft configurations

and acceptable criteria should be provided in this document.

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8.0 REFERENCES

1. AGARDograph 45; Operations from Unprepared and Semi-prepared Airfields;September 1960.

2. Aircraft Landing Gear Design; Principles and Practices; Norman S. Currey; Lockheed

Aeronautical Systems Company Marietta, Georgia.

3. ASTM D 4429-93; Standard Test Method for CBR (California Bearing Ratio) of Soilsin Place.

4. Avions Marcel Dassault-Breguet Aviation report; Mystere-Falcon 900; Operation onUnpaved Runways, September 15, 1988.

5. Boeing Model 737 Advanced Low Pressure Tire Gravel Runway CertificationProgram; Forest W. Worthington, The Boeing Company, Seattle, Washington.

6. Boeing Document No. D6-24555, High Load Penetrometer Soil Strength Tester, Rev.E, dated 4/5//84.

7. Boeing Model 737-200 Document No. D6-32021, Section or Addendum No. 15-1,Gravel Runway Certification Results, dated 11-5-68.

8. Canadair Challenger, MAA 601-138: Performance and Handling of the CL-601Equipped to Operate on Gravel Runways, dated 15 February 1985.

9. F.A.A. Advisory Circular AC No. 150//5335-5; Standardized Method of ReportingAirport Pavement Strength-PCN.

10. F.A.A. Advisory Circular, AC No: 150//5320-6D; Airport Pavement Design andEvaluation; Jan. 30, 1996.

11. F.A.A. Advisory Circular AC 25-7-X, Flight Test Guide for the Certification ofTransport Category Airplanes.

12. MIL-STD-612A Method 101, California Bearing Ratio of Soils.

13. Public Works Canada, Manual of Pavement Structural Design; ASG-a9 (AK-69-12);Public Works Canada; Architectural and Engineering Services; Air Transportation;July 1992.

14. Transport Canada Air, Airports and Construction; AK-67-09-280, Gravel RunwaysCondition Reporting Procedures and Surface Stability Test Methods, dated June 1984.

15. Transport Canada Airports; Safety and Technical Services; Airport PavementEvaluation - Bearing Strength; AK-68-31-000; September 1987.

16. Transport Canada Airworthiness Manual Advisory, AMA 525/4, Operations onUnpaved.

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Documents

gravel strips details.pdf