GRAVEL RUNWAY SURFACE STRENGTH MEASUREMENTS AND AIRCRAFT CERTIFICATION REQUIREMENTSRoman 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 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 ....................................................20 4.5 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 3 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 FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Cross Section of Flexible Pavement Classification of Soils for Airport Pavement Applications Approximate Relationship of Soil Classification and Bearing Values Gravel Runway Loss of Material Gravel Runway Segregation of Material Gravel Runway Wheel Rutting Gravel Runway Poor Surface Drainage Gravel Runway Poor Sub-surface Drainage Gravel Frost Action Damage Gravel Runway Roughness Gravel Runway Vegetation Growth California Bearing Ration Test Set-Up (ASTM METHOD) Boeing High Load Penetrometer Test Set-up A.M.D. Shock Penetrometer CBR vs Soil Reaction Pressure for THree Penetrometer Test Methods Example of Penetrometer Test Locations Modification of Boeing High Load Penetrometer Test to incorporate a small flat plate for the determination of Soil Failure Pressure Rolling Coefficient of Friction related to Tire Pressure and Runway CBR Subgrade Spring Reduction Factors Based on Soil Composition Rolling Coefficient of Friction vs Tire Pressure ASTM D4429 Standard Test Method for CBR of Soils in Place Boeing High Load Penetrometer Soil Strength Tester Aerospatiale Method for CBR (Shock Penetrometer) Summary of Selected Runway CBR Measurements Boeing Approvals Product Information and Material Safety Data Sheets (MSDS) FIGURE 18 FIGURE 19 FIGURE 20 APPENDIX A APPENDIX B APPENDIX C APPENDIX D APPENDIX E APPENDIX F 4 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.) 1. Of particular interest in this report. This report will limit itself to gravel runway surfaces composed of coarse gained soils and aggregates rather than unpaved surfaces of all soil types. 5 . The test methods used to measure CBR will be examined including an indication of their adequacy for this task. The effects of surface characteristics on CBR values will be examined. 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.Issue 1 June 30. This report will review the characteristics of gravel surfaced runways. will be an examination of the adequacy of expressing runway surface strength in terms of California Bearing Ratio (CBR). Because of these characteristics.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. 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. which by definition are not smooth or hard surfaced. 1 GRAVEL RUNWAY CONSTRUCTION GRAVEL RUNWAY . c) Subbase Course The function of the subbase is similar to that of the base course. The primary purpose of the surface course is to prevent the penetration of water to the base course. Figure 1 is a cross section of a typical flexible runway pavement. The relatively lower shear strength of gravel runway surfaces. a lower subgrade bearing strength requiring a greater thickness. b) Base Course The base course has the major function of distributing the imposed wheel loadings to the pavement foundation. Because this layer is further removed from the surface. especially when wet. A gravel runway is essentially a flexible pavement with a surface layer of unbound granular material. the subbase is subject to lower loading intensities. resting on a prepared subgrade layers. The pavement layers serve to distribute 6 . Tire wear may increase because of the rough texture of the gravel surface and the presence of sharp stones. A flexible pavement consists of layers of material classified as surface course. provide a smooth well bonded surface free from loose particles. a) Surface Course Surface courses include hot mix asphaltic-concrete (AC) for flexible pavements. Granular materials include coarse grained soils such as sands and gravels. For a gravel surfaced runway. The design thickness of this layer varies as a function of the bearing strength of the subgrade. In a flexible pavement a relatively thin surface layer transmits its load to the base layer. The subgrade soil ultimately provides support for the pavement static loads. to resist the shearing stresses imposed by aircraft loads and transmit bearing loads to the pavement structure. may limit aircraft loads imposed on the runway. and frost susceptibility. the subbase and/or subgrade. The surface course must provide texture for skid resistance yet not cause undue wear on the tires. and imposed aircraft and ground vehicle loads. durability. The base course is composed of well compacted granular aggregates meeting high standards with respect to stability. moisture penetration becomes a significant factor in the pavement’s strength and resistance to frost action.2. base course and subbase course.0 2. Aircraft must also be protected against the hazards of loose particles. and the material requirements are not as strict as for the base course. d) Subgrade The subgrade is that soil beneath the pavement structure which forms the foundation for the pavement.GENERAL A runway pavement is defined as a structure consisting of one or more layers of processed materials. Material availability near the runway site often dictates the specific composition of the pavement layers. Army Corps of Engineers. Soils having the best characteristics in grading and excavation operations are incorporated into the subgrade. 2. base and subbase layers to ensure that layer stability requirements will be met. The ability of a particular soil to resist shear and deformation varies with its density and moisture content.2 GRAVEL RUNWAY DESIGN METHODOLOGIES The methodology applied to the design of flexible pavements at Canadian airports is based on the use of plate strength test results. The CBR method of design was developed by the California Division of Highways in 1928. The equivalent granular thickness is proportioned into surface. FAAAdvisory Circular AC 150/5320-6D describes the CBR method of design curves which provide the total required thickness of flexible pavement (surface. The absence of a waterproof surface is usually not a problem in arctic regions because these areas usually have low precipitation.the aircraft and vehicle loads over an area on the subgrade greater than the tire contact area. ASG-19 (AK-68-12). The subgrade consists of in-situ soils or imported common material in fill sections. Gravel pavements often exhibit considerable distortion due to frost heave. 7 .3 FROST EFFECTS ON GRAVEL RUNWAYS Gravel pavements are found in regions subject to seasonal frost penetration and are usually the preferred surface type in permafrost areas. the total depth of pavement required is divided into base and subbase courses. Based on the aircraft loading and the bearing strength of the subgrade soil.5 Mpa (75 psi) to 30 cm (12 in) for tire pressures greater than 1. For granular surfaced pavements. This methodology determines the minimum thickness of the flexible pavement based on bearing strength values of pavement components. The Canadian design methodology is described in the Public Works Canada.S. The methodology used in the United States determines the required thickness of the pavement by the use of CBR strength values. The CBR method still remains in wide spread use today. pavement materials used. The method was subsequently adopted for military airport use by the U. shortly after the outbreak of World War II. the bearing strength of the subgrade influences the pavement thickness. but are easily regraded. Design base course thickness ranges from a minimum of 15 cm (6 in) for tire pressures less than 0. an equivalent granular thickness of the pavement is selected so that the subgrade soil will not be overstressed. Deformation (or deflection) of the subgrade layer must be controlled so as to remain within acceptable limits. Manual of Pavement Structural Design. base and subbase) needed to support a given weight of aircraft over a particular subgrade. planned aircraft design load ratings and tire pressures. These requirements dictate the pavement thickness to be the greater of the thickness required for structural strength or for frost protection.0 Mpa (145 psi). As in the subbase. Structural thickness requirements consider the subgrade bearing strength. 2. Adequate pavement load carrying capacity may also have to be provided during the critical frost melting period when load carrying capacity is reduced. In areas with permafrost at shallow depths. While it is the subgrade strength and overall thickness of the pavement structure that controls the amount of surface deflection. the most common cause of operational problems on gravel pavements is the failure of the surface layers due to shear caused by high aircraft tire loading. particle friction and cohesion. This force divided by the area over which it is applied can be taken as the soil failure pressure. coarse grained soils such as gravels and sands have low frost susceptibility. restriction of drainage and deterioration of the surface. Surface shear strength can be estimated by measuring the force required to deflect or penetrate the surface to a specified depth. For pavements exposed to seasonal frost. freezing temperatures must penetrate the frost susceptible soil and there must be sufficient free moisture to form ice. Other effects include loss of compaction. the soil must be frost susceptible. whereas fine grained soils such as silts have high frost susceptibility. Permafrost soils occur in arctic regions.The detrimental effects of frost action may be manifested by non-uniform heave and loss of soil strength during frost melting. Seasonal thawing and refreezing of the upper layer can lead to severe loss of bearing strength and/or differential heave and settlement. where soils are often frozen to considerable depths year-around. The strength of the gravel surface depends on the interlock of the aggregates. 2. This results in the surfaces of gravel runways being susceptible to shear failures. particularly in wet conditions. development of roughness. This is intended to prevent thawing of the permafrost layer. The depth of frost penetration is a function of the thermal properties of the pavement and soil mass. Pavements with higher surface strengths have higher soil reaction pressures and higher CBR values. Pavement design for permafrost areas must consider the depth of seasonal thaw penetration. the severity of air/surface temperatures and the temperature of the pavement and soil mass at the start of the freezing season. This pressure can be obtained from flat plate or penetrometer type measurement devices and is often correlated to CBR.4 GRAVEL RUNWAY STRENGTH The deflection of the surface of a gravel runway under an applied load depends on the strength of the surface and the strength of the underlying layers. Three conditions must be met for detrimental frost action to occur. pavement deformations resulting from frost action are controlled by providing a sufficient combined thickness of non-frost susceptible material to limit frost penetration into the subgrade. 8 . Soils are categorized into groups for their frost susceptibility. satisfactory pavements are assured by containing seasonal thawing within the pavement and underlying non-frost susceptible layers. Generally. The surface strength also depends on the properties of the surface materials under the influence of moisture. with almost no compressibility and expansion and the drainage characteristics are generally excellent. The potential for frost action of these soils is minimal. 9 . One of the purposes of soil classification is to predict the probable behavior of soils under the influence of frost and moisture. This condition can be corrected by adding new material and compacting. subbase or subgrade material appearing on the surface. This condition results in a reduction of the surface strength of the runway and reduced braking action as a result of the loss of coarse material. The presence of clay soils can result in a marked reduction of the strength values and frost properties of these soils. GW. tire action or infiltration of lower layer material into the surface layer. Soils are expressed by soil group symbols (e. Aircraft operations that overload the gravel runway pavement structure can result in surface deformations which may adversely affect aircraft performance and could cause structural damage. AC 150/5320-6D Airport Pavement Design and Evaluation states that field CBR values range from 60-80 for well graded gravel soils and 20-40 for well graded sands. The use of a soil classification system may be useful for the estimation of the surface strength of a gravel runway or as a check on the validity of specific CBR measurements. The standard method of classifying soils for engineering purposes is ASTM D 2487 commonly called the Unified System. which is described as “Well graded gravel and gravel sand mixtures. base. Figure 2 is an example of soil classification based on the Unified System. the bearing strengths of soils identified by Unified System group symbols may be estimated in terms of CBR values. A soil classification system could also be useful for the identification and definition of the gravel runway surface for certification and operational use. Some of the indications of gravel runway failures are discussed below. increasing roughness in the surface or permanent deformation. and a buildup of granular material at the edge of the runway. The primary causes are loss of material during snow removal. The Unified System classifies soils first on the basis of grain size (coarse grained and fine grained soils).g. then further subgroups soils based on the plasticity constants. 2.6 INDICATIONS AND EFFECTS OF GRAVEL RUNWAY FAILURES The definition of runway failure is somewhat arbitrary but in the case of gravel runways it is typically identified by the formation of ruts. and may contain various types of soils.5 SOIL PROPERTIES .2. In Figure 3. little or no fines”).EFFECT ON SURFACE STRENGTH Granular runway surfaces are typically non-homogeneous in composition. a) Loss of Material (Figure 4) Indications of surface material loss are bare spots. directional control problems and higher landing gear loads. Adverse performance effects and increased landing gear loads from the accumulation of loose materials may occur. Causes are the loss of finer materials due to jet or propeller blast. Rutting may cause reduced acceleration and braking performance. Poor cohesion may be the result of high moisture content. Vegetation growth may be caused by poor 10 . especially when rutting is occurring. Rutting is corrected by regrading of the runway surface. increased landing gear loads as well as the possibility of damage from larger debris. Dampness can usually be corrected by improving drainage. Roughness may be caused by loss of material. Repair of the damaging effects of frost action may require more substantial work. poor gradation. This condition is corrected by regrading and adding new material. Damp areas may have an adverse effect on braking performance. Roughness can be corrected by regrading. frost action or settlement. segregation or poor compaction. The potential for damage to the aircraft from debris is increased.b) Segregation (Figure 5) Segregation is the accumulation of loose non-cohesive aggregates on the surface. including runway rehabilitation and the improvement of drainage. Runway roughness may result in decreased acceleration. tire action and weathering. f) Vegetation (Figure 11) Uncontrolled vegetation growth may occur on the gravelled operational surface itself and/or in the graded area at the runway edge. e) Frost Action. Areas of wetness may result in decreased acceleration. Soft areas with rutting and shoving during spring thaw or wet conditions and frost heaving during winter are indications of poor subsurface drainage. d) Drainage (Figures 7 & 8) Poor surface drainage is indicated by damp surface areas persisting after rainfall or snowmelt. Roughness (Figure 9 & 10) Frost action is indicated by differential heaving of the surface or depressions which appear in the same place yearly during the frozen season. Rutting with shoving indicates a shear failure in the surface layer due to poor cohesion (low surface shear strength). without rutting. Frost action may raise boulders in the subgrade. c) Rutting (Figure 6) Rutting is defined as longitudinal deformation in the wheel path. Rutting without shoving of adjacent material is an indication of failure in deeper layers of the pavement due to inadequate foundation strength. Inadequate drainage of the subgrade and inadequate granular thickness over frost susceptible material may be the cause of frost heave. There were also periods when runways were closed during the spring thaw.drainage or accumulation of organic soils (earth) in the surface. the addition of new material. The material may blow off unevenly causing the formation of bare spots and hollows. Prohibition of Boeing 737 operations during the spring thaw was cited. 11 . Vegetation may be reduced by regrading. 2. One operator would vary tire pressure on their Boeing 737 seasonally. Poor drainage was noted as a cause of rutting during wet conditions and severe roughness occurred after the surface dried. For Boeing 737 operation. This can become the mechanism for the start of longer term damage to the runway. This problem can be corrected by regular grading. All of the operators reported problems with fine materials damaging propellers and engines. Vegetation will result in uneven surface characteristics. improving drainage and introducing chemical growth inhibitors. which may adversely affect the acceleration and braking performance of the aircraft. Procedures to minimize damage included revising power application techniques. The ride quality in a pickup truck driven at automobile highway speeds can be used to check the smoothness of the runway for the suitability of aircraft operations. and compacting. An operator of medium sized propeller and jet aircraft reported that the aircraft would occasionally get stuck during the spring or after rainy periods because of inadequate surface hardness. It was also noted that jet exhaust from one typical Boeing 737 takeoff can result in the loss of up to two cubic yards of material.7 OPERATIONAL PROBLEMS Report AK-67-09-280 Gravel Runways Condition Reporting Procedures and Surface Stability Test Methods. One operator noted that gravel runway conditions change almost daily with the weather and season. Bumps that are visible on a runway are often too severe for aircraft operation. Another operator suggested the use of chemical additives on the runway surface to prevent the raising of dust and the loss of fine soils. lists anecdotal evidence of the types of operational problems which may be encountered during gravel runway operations. Low traffic volume may also encourage the growth of vegetation. Vegetation was noted by a small aircraft operator as being difficult to brake on with sand being the best surface for braking. or use low pressure tires on specific runways. One operator painted the propellers of their aircraft each day to identify new nicks at the end of the day. Boeing commented that surfaces must be hard enough over the entire runway length and the use of an average strength value was not permissible. The ASTM method is essentially the determination of the load required to cause the uniform rate of penetration of a piston into the soil.2 ASTM D4429 (U. The bearing strength of crushed limestone has been adopted as one of the criteria to which other types of soils are compared. The ASTM method requires that consideration be given to moisture content. Figure 12 is al illustration of the test setup. If the CBR results are to be used without any correction for moisture. This method shall be referred to as the ASTM method in the context of this report. The CBR test is basically a penetration test conducted at a uniform rate of strain. (Appendix A) The laboratory method is useful at the design stage of a pavement.3. Standard Test Method for the Bearing Ratio of Soils in Place is the field CBR test method. Field CBR tests are intended for the measurement of pavement foundations that have been in place for several years. The degree of soil saturation (percentage of voids filled with water) must be 80% or greater. This method is also applicable to the measurement of a granular surface layer for which CBR is the desired parameter. Materials must not be significantly affected by moisture. but is of limited use for operational purposes. which is expressed as CBR 100. where moisture has been allowed to reach an equilibrium condition. Such materials include coarse grained or cohesionless soils. This method is applicable to sub-base and base course materials.0 3. 3.1 MEASURING SURFACE STRENGTH CALIFORNIA BEARING RATION (CBR) The CBR is the ratio of the load bearing capability of a given sample of soil to that of crushed limestone. 12 .S. CBR tests may be conducted in the laboratory or the field. 2. Limestone has a CBR value of 100. The CBR test is considered to be a measure of the confined strength of a soil. A soil with CBR 10 has 10% of the bearing strength of crushed limestone. C ORPS OF ENGINEERS) CBR TEST METHOD (APPENDIX A) This method is the standard test method used to determine the CBR of soils in place. A test pit may have to be opened for the measurement of these layers. Field CBR tests can also used to measure the strength of the surface layer of the gravel pavement. then the test must be conducted under the following conditions: 1. ASTM D 1883 Bearing Ratio of Laboratory Compacted Soils is the laboratory CBR test method. ASTM D 4429. The ASTM D 4429 method is of greater interest for the measurement of gravel runway surface strength and may be considered to be the definitive CBR test method. The CBR is calculated by dividing the penetration stress at a 0. The test apparatus is similar to that of the ASTM method. The soil must not have been modified by construction activity in the two years prior to the test.3 BOEING HIGH LOAD PENETROMETER (APPENDIX B) The Boeing High Load Penetrometer test is used for the measurement of surface strength and may also be used to measure the subgrade strength of paved and unpaved runways. dial gauges. The test apparatus consists of a mechanical screw jack. In this test procedure. proving rings. This value is the applied force (as measured on load cell) divided by the piston area.025 inch increments to a depth of 0. surcharge weights.1 in and 0. Generally a pressure reading is taken 30 seconds after movement of the penetrometer has stopped at the 4 inch depth. Friction and shear forces at the side of the 13 . the cone reference point is driven at a steady rate to a four inch depth into the surface. This method is not a confined test as is the case with the ASTM method. This test method allows for tolerances in CBR value. Construction activities such as compacting or grading subsequent to the bearing test will invalidate the test.1 penetration but 0. The test points for the ASTM method should be spaced at a minimum of 7 in apart for cohesive soils and 15 in for non-cohesive soils. a soil sample is obtained to determine the water content. penetration piston. Tolerances increase with increasing CBR values. Pressure is read off of the hand pump to calculate the load applied on the penetrometer. Penetration is recorded at . A curve is derived of penetration stress versus depth of penetration.2 in depth to the same penetration stress for the standard material (crushed limestone). The ASTM method measures the soil reaction pressure when a 2 inch diameter (nominally 3 sq in) piston is driven up to 0.2 in may be used if the resulting CBR value is higher and results are more consistent.05 inch per minute rate of penetration.3. Soil failure pressure is derived from the penetrometer force divided by the projected area of the 2 inch diameter penetrometer cone point. by the application of pressure through the hand pump. The test procedure involves preparation of a test area. A hand pump is used instead of a mechanical screw jack. the Boeing High Load Penetrometer test method does not provide any guidance on the correction of soil strength due to moisture. Test results may be invalidated by the presence of a rock or voids beneath the penetration piston. Penetrometer pressure is taken when a condition of equilibrium is reached between the hydraulic pressure and the surface reaction pressure. jacks and a large reaction load (Figure 12). Normally the CBR is in the value derived from 0.50 inches. Unlike the ASTM method. At the end of the test. 3. Reaction loads used are typically large vehicles or construction equipment.5 inches deep into a confined soil sample. placing the apparatus under the reaction load. soil disturbance or construction activity. and applying a load to the penetration piston to achieve a 0. The test probe is a conical projection at the end of a cylinder. This measuring device consists of a hydraulic cylinder to provide a large penetration force at a test probe (Figure 13). Penetration stress is computed at each increment of penetration. 5 to 15. This is indicated by a sudden large increase in penetrometer pressure readings during pressure application. Boeing claims that this method has the advantages of accuracy. 14 . For the Boeing test method the surface material must be homogeneous to a depth beyond the cone point tip. The shock penetrometer consists of a long rod with a cone in contact with the soil and a sliding weight. the CBR values from this method are approximately 70% of those of the Boeing method for the same soil reaction pressure (Figure 15). 3. versatility. The range of CBR values derived by this method are from a CBR of 2. speed of use and may be performed by relatively untrained personnel. Both methods are similar. grassed or gravel runways. (Figure 14) The manufacturer states that this test method is applicable to “laterite”. a CBR value of 10 by this method corresponds to a CBR of 14 by the Boeing method. Boeing has provided a curve of the CBR versus Soil Failure Pressure (Boeing High Load Penetrometer) which indicates that the CBR value derived from the Boeing Penetrometer generally corresponds to that of the ASTM method at a penetration of 0. The CBR is read from a chart of CBR versus Soil Reaction Pressure.cone point are included in the total force. CBR is read off of a calibration chart as a function of the number of drops.Breguet Aviation (AMD-BA) and Aerospatiale for the determination of runway CBR. Inaccuracies may also result from friction in the jack and sides of the penetrometer if any tilting occurs. but Boeing states that the presence of a large stone will be clearly evident to the operator during measurement. This method does not specify the spacing of test points.50 inches (Figure 15). but because of available information. the presence of large stones may introduce errors.4 SHOCK PENETROMETER (APPENDIX C) The shock penetrometer has been employed by Avions Marcel Dassault .2 in. The CBR values from the Boeing method are on the average 10% less than that of the ASTM method for the same soil reaction pressure.1 or 0. The penetrometer is driven into the soil to a depth of 10 cm by the release of a sliding drop weight along a handle. The CBR value is determined by a formula relating the CBR to the soil failure pressure. Relocation to another test position several feet away is recommended. This test method does not provide any correction to CBR values due to moisture or soil disturbance. When this curve is compared against a similar curve produced by the Boeing method. As an example. The soil reaction pressure to CBR conversion appears to be only applicable to soils having CBR values below 20. the Aerospatiale method is described here in greater detail. The soil strength is indicated by the number of drops required to achieve the 10 cm penetration. This test method determines a soil reaction pressure derived from the number of drops of the falling weight divided by the projected surface area of the cone. However it is noted that the ASTM method normally requires the CBR to be calculated at a depth of 0. The assumption behind this method is that the minimum depth of soil for aircraft operation is 10 cm (4 in). if any other units or methods are used to express the strength of a runway. Because of the errors associated with using the results of various test equipment and methods to estimate the standard CBR value of the soil.3. the principle forces reacting against these cone penetrometers are the shear strength and bearing reaction of the soil.25 sq in) being driven into the soil to an approximate depth of 4 in. however. Aerospatiale provides a similar chart for their Shock penetrometer. Here. the soil reaction pressure is the penetration force divided by the contact area of the penetrometer device. 3. the ASTM method and Boeing penetrometer test methods would yield a CBR value of approximately 14. like the other two penetrometer methods. The ASTM method using a 3 sq in piston directly measures the bearing strength of the soil. 15 . The shock penetrometer is driven into the soil by a series of impacts. Similarly. the CBR value derived from the ASTM method will be considered as the standard to which the other methods will be compared. aircraft operations may take place on a surface that is weaker than expected. This chart indicates that for soils with CBR’s less than 40. For this reason the AFM should state the measuring technique associated with CBR value. For example. The Boeing penetrometer and Shock penetrometer have cones of similar projected areas (3. they should be clearly stated along with the strength value and the layer of the runway to which they apply. Shear reaction to this test method is minimal because of the confinement of the soil and the small depth of penetration. The Shock penetrometer may not correlate well to CBR because it is an impact test.14 sq in vs.5 inch penetration). The ASTM method and Boeing penetrometer also work on the principle of a steady application of pressure to drive the piston/cone into the soil. which is overlaid on this chart. rather than a constant rate of loading. it is important that the test method be identified. A comparison of surface strength measurements using the Shock penetrometer has identified significant discrepancies in CBR values as compared to the Boeing penetrometer when measured on the same runways. but the method of measurement is usually not indicated. if the soil reaction pressure is 500 psi. For example. the Boeing High Load Penetrometer will yield CBR estimates that agree very closely with results of the ASTm method (0. whereas the Shock penetrometer derivation of soil failure pressure would give a CBR value in the range of 10.5 COMPARISON OF CBR STRENGTH MEASUREMENT METHODS For the purposes of this discussion.6 and Appendix D) Figure 15 suggests that the application of CBR values for operational use without stating the test method may be misleading. CBR would be expressed as “CBR 14 as determined by the Boeing Penetrometer”. (See paragraph 3. Figure 15 is a plot of CBR versus oil reaction pressure provided by Boeing to show the correlation of their penetrometer to the ASTM method. The geometry of these cone penetrometers are different from each other. Without knowing the method. Current practice is to state a minimum CBR value in the Airplane Flight Manual (AFM). CBR values estimated from Shock penetrometer do not correlate closely to either the ASTM or Boeing methods. 3.The Boeing and Shock penetrometer methods do not provide any CBR correction factors to account for soil moisture content. The range indicates the variability of CBR values for a particular runway. CBR values are presented as average for the runway and the range from minimum to maximum. appears to be reasonable. As an unpaved surface may be significantly weaker following spring thaw or heavy precipitation. before performing any tests. clay and fine gravel such as Hall Beach (NWT). The composition of the various runways tested was found to vary considerably. The Boeing and Shock penetrometer test methods also do not provide any information on the precision and bias of the CBR results. The locations specified for the Transport Canada plate test. dry or wet conditions and lateral location with respect to the centerline.6 SURVEY OF SEVERAL RUNWAY CBR M EASUREMENTS (APPENDIX D) Appendix D is a summary of CBR measurements of various runways in support of Canadian runway measurement and aircraft certification programs. the condition under which the measurements were made should be recorded and provided in the AFM. Soils in general. CBR correlation data should be obtained from other equipment manufacturers to qualify their equipment. Figure 16 is an example of a strength survey conducted at Baker Lake by Boeing using their penetrometer. The contact area should equate as closely as possible to the tire contact area of a typical aircraft operating on the gravel surface. do not have well defined failure points. The Boeing penetrometer has been calibrated by making tests on various soil samples for which the CBR has been determined using conventional CBR test methods.Bearing Strength. The plate diameter proposed is 150mm (6 in). the degree of soil saturation. The DOT surveys provided an average CBR measurement and did not provide a range of measurements. Figure 17 is an illustration of the test set-up. the materials in the surface and the date of measurement. and is sized to approximate the wheel loads imposed by a single aircraft tire. where measurements are taken in the aircraft wheel tracks at specified intervals. The plate diameter may be varied to account for the failure criteria of the soil and the available size of the reactive load. Report AK-67-09-280 discusses the modification of a Boeing penetrometer by replacing the cone point with a small flat plate. Soil failure would be expressed as the pressure required to indent the soil to a specified depth. Only a few runways such as Rankin Inlet were composed entirely of gravel. as the definition of soil failure is a critical consideration. This proposal would require further investigation and testing. These should be similar to those specified for the ASTM method and include. The CBR measurements taken by the Canadian DOT Gravel Runway Survey did not specify the measurement method used although it is believed it was the Boeing High Load Penetrometer. Many runways were composed of sand and fine gravel such as Churchill (Manitoba) or sand. Details are also provided (if available) on the date of the measurement. More detailed information may be obtained by review of the references used. 16 . AK-68-31-000 Airport Pavement Evaluation . The above test methods do not specify where the tests should be performed on the runway. This same test method gave an average CBR of 15 for the Kuujuaq runway which appeared to be more highly compacted. AMD-BA explains that the low CBR values were attained at Kuujuarapik because the runway was composed of sandy soil with a lot of free gravels. These particular runways are all composed of sand and gravel. CBR measurements recorded by AMD-BA were approximately one third of the magnitude of those recorded by other agencies. wet and dry CBR values varied by a factor 2. Kuujuaq (Quebec) and Nanivisik (NWT) provided CBR values for wet and dry runways. Kuujuaq and Nanisivik showed no apparent reduction in CBR for wet runways as compared to dry. Based on approximate CBR strengths of materials present in these runways. The Shock penetrometer CBR measurements for the Kuujuarapik runway gave an average CBR of 10. CBR measurements taken were consistent in value with the exception of those taken by AMD-BA for the Falcon 900 program using the Shock penetrometer. 17 . At Churchill. There were no wet runway CBR values obtained at Hall Beach. the AMD derived values for Kuujuaq would still be low for either poorly graded sand or gravel runways (See figure 3). As a comparison. Arizona was described by one manufacturer as being composed of soft sandy areas on a runway constructed of deep loose gravel with large stones. which would have been interesting considering the presence of clay in this runway. A low CBR would be expected here but measurements taken by the Boeing penetrometer on different occasions had indicated a minimum single CBR value of 34 and minimum average CBR of 50.Test results for Churchill (Manitoba). the runway at Lake Havasu. handling. systems and powerplant operation. Reducing tire pressure when on these soils has little effect. The result is reduced aircraft acceleration on takeoff from a gravel surface resulting in increased take-off and accelerate-stop distances. A useful parameter against which to correlate rolling friction is the ratio of aircraft tire pressure divided by the runway surface CBR value (Figure 18). A less rigid tire tends to flatten when under load allowing it o create shallower rolling tracks which results in reduced rolling friction. This action extracts energy from the wheel motion and causes an increased rolling coefficient of friction. thereby reducing shear stresses in the runway surface. the lower the strength of the surface. Tire rigidity should also be considered in its effect on the rolling coefficient of friction. more effort is required to push sandy soils away by the tires. it is desirable to have a combination of low tire pressure and reduced tire rigidity on surfaces having a low CBR value. Another method of reducing this ratio is by the selection of oversize tires inflated to a lower pressure. Excessive tire pressures may cause shear failures of the surface and deflections in the form of rutting. 4. as well as the effect of damage from debris.0 4. the rolling coefficient of friction varies with moisture depending on the specific soil. This characteristic is independent of tire pressure. because when dry. On weak runway surfaces. Loose sandy soils exhibit a decrease in rolling coefficient of friction with increasing moisture content. structure. Once in motion. The minimum permissible tire pressure is constrained by limits on tire size and tire deflection for a given wheel load.4. Ideally.2 ROLLING COEFFICIENT OF FRICTION The relatively weaker surface of gravel runways have an adverse effect on aircraft rolling resistance. then the higher the rolling coefficient of friction. A reduction in tire pressure reduces this ratio and in turn the rolling coefficient of friction. This includes consideration of aircraft performance. Reducing wheel load (aircraft weight) is a method of limiting tire pressure without exceeding tire deflection limits. The rolling coefficient of friction is increased considerably with moisture for clay and silt soils because these soils tend to stick to tires and increase their rolling friction. The rolling coefficient of friction may also be a function of tire rotation speed for soft soils. For a given aircraft tire pressure. 18 . This causes an increase in tire contact pressure causing the tire to sink deeper into the soil thereby increasing the rolling coefficient of friction. On a soft soil.1 DESIGN CONSIDERATIONS FOR OPERATIONS OF AIRCRAFT ON GRAVEL RUNWAYS GENERAL Operation of aircraft from gravel runways should be as safe as that from paved runways. the soil beneath a wheel has little time to move which prohibits the tire from sinking in. and results in a reduction in the rolling coefficient of friction with tire speed increase. the static equilibrium is disturbed on initial motion resulting in the total load being distributed over a smaller area. This results in the wheel load being distributed over a larger surface area. the front wheel does the main work in forming a track. Significant degradation in braking was reported by one manufacturer for tests conducted on a wet gravel runway which had experienced considerable loss of gravel due to weathering. Similarly. For surfaces having CBR values ranging from 10 to 20. a worst case surface should be used during performance testing to obtain conservative flight manual data. when the surface layer is bonded by frozen moisture. On surfaces having a CBR of 30 and above. Uneven surfaces however will degrade anti-skid response. Anti-skid systems are desirable on gravel runways since they compensate for changes in tire adhesion to the runway surface and result in shorter deceleration distances. Generally. Accumulations of fine sandy soils may also cause a reduction in the braking coefficient.075. Tire adhesion may improve under some conditions such as frozen well graded gravel surfaced runways. especially for aircraft without effective anti-skid systems. One manufacturer has concluded that the response of aircraft anti-skid performance is a function of runway surface condition rather than the CBR strength. at a tire pressure of 150 psi. the rolling coefficient of friction remains fairly constant as the tire pressure increases and may actually trend slightly downwards. The dependence of braking performance on surface characteristics requires that the surface be defined to ensure that predicted braking response will be achieved. Surface slipperiness is also significant when the upper layers of a frozen runway thaw. 19 . In tandem arrangements. Tandem wheels have a lower rolling coefficient of friction when compared to side by side wheels. low pressure tires have better adhesion on wet soil surfaces than high pressure tires and also adhere better to surfaces containing any vegetation.Wheel arrangement has an effect on the rolling coefficient of friction. which directly affects the braking coefficient of friction. reduced adhesion increases the possibility of skidding during braking. In all cases. Corps of Engineers. The rear wheel runs on a smoothed level track and experiences less rolling resistance. Initial design configurations may not anticipate future gravel runway operations. The wheel configuration of the aircraft will be fixed prior to certification for unpaved runway operations. The Unified soil classification system may represent a method of identifying runway surfaces for aircraft performance charts. Soil properties and moisture are the primary factors affecting the adhesion between the surface and the tire. Because of the variability of gravel runway surfaces. resulting in reduced adhesion to the surface. the rolling coefficient of friction increases as the tire pressure increases to a value of about 0. Fine soils tend to be more slippery than coarse grained soils and may plug tire treads.S. 4. Figure 20 is a graph of tire pressure versus the rolling coefficient of friction from test results conducted by Boeing and the U. while the lower layers remain frozen. accumulations of loose and smooth gravel particles may reduce tire adhesion.3 BRAKING Test results indicate that the braking coefficient is a function of the surface characteristics and is independent of runway CBR. Regardless of whether a plate load or CBR procedure is employed.4. In the case of the plate load test. The allowable tire pressure may be estimated by applying a factor of two to the soil failure pressure. it may also be necessary to apply a further strength reduction factor to account for weakening of the pavement during spring thaw. a series of tests should be made along the length of the runway. The failure load could then be determined and the failure pressure calculated as the failure load divided by the contact area of the test plate. The factor of safety is to account for the effects of tire motion which are not present in the static test.S. the U. braking. An essential step is to define and determine the soil failure pressure which provides an indication of the shear strength of the surface. aircraft testing is still necessary to ensure that this value is adequate.0-0. Depending on the surface and/or subgrade soil type. As this value is only a static approximation. generally in the aircraft wheel paths. Army Corps of Engineers have applied a method of estimating allowable tire pressure based on the rolling coefficient of friction on a low 20 . The plate contact area should approximate closely as possible to the aircraft tire contact area. uneven wheel loads and surface degradation can produce shear stresses 30% greater than those of the average tire contact pressure.4 ESTIMATION OF MAXIMUM ALLOWABLE TIRE PRESSURE Report AK-67-09-280 provides some general guidelines for estimating the maximum allowable tire pressure for a gravel surfaced pavement. Suggested spring reduction factors based on soil type are given in Figure 19. localized tire contact pressure. the results should be averaged and a statistical measure such as lower quartile point or standard deviation applied to account for strength variations along the runway length. Tire motion effects such as tire scrub. In another approach. An example calculation for the estimation of allowable tire pressure on a gravel surfaced pavement is given below: Parameter Soil Failure Pressure Value 400 psi Note as measured from a plate load test or converted from a CBR value factor of safety of 2 applied Spring Reduction Factor of 25% applied for a soil of type GC -”Gravel with Clay fines” Allowable Tire Pressure Allowable Tire Pressure (Spring) 400/2 = 200 psi 200*(1. This may be measured in place by a small 150 mm (6 in) diameter static plate load test or by the Boeing penetrometer test. it would be necessary to standardize the test deflection which would constitute the “failure” for the size of the plate used.25)=150 psi The estimated maximum allowable tire pressure for the gravel surface would therefore be 150 psi. On retractable landing gear. Corps of Engineers. A criteria was established for determining a tire pressure to give a rolling coefficient of friction of . external lights and antennas. Tire deflection depends on the ratio of aircraft weight to tire pressure. b) Protection of Aircraft Surfaces The gravel kit may require the installation of protective panels or the bonding of materials to the skin of the aircraft to protect surfaces from the effects of flying debris.3 of a transport category aircraft and 25% of maximum thrust required to start the aircraft rolling from a stopped condition on a weak surface.03 Mpa (150 psi). The . the allowable tire pressure would be 1. Flap extension angles may have to be decreased for take-off and landing to minimize damage caused by debris spray from the wheels.strength surface. (.075 is calculated as the product of . the thrust required to start the aircraft moving was measured.075 and a CBR of 20. One manufacturer has employed mud guard type nose wheel deflectors on their aircraft. Maximum allowable aircraft weight will correspond to the tire pressure tested. inboard flap panels. In the tests conducted by the U. 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 gravel deflectors. mechanisms may be necessary for the fairing of these deflectors into the profile of the aircraft to reduce aerodynamic drag. For example assuming a rolling coefficient of friction of . If test surfaces have a different strength than the minimum proposed. 4. which are typically flat plate shields attached to the landing gear between.075. External light protection may include the requirement that belly lights be retractable or covered in a protective wire mesh. Other methods include the installation of tires with chines facing inboard to prevent the debris spray from impinging on the gear struts. debris and stones.075 value was based on the typical thrust to weight ratio of . then individual tire pressures for each surface strength tested will have to be established.3 and 25%). 21 . Protection systems are typically characterized as “Gravel Runway Kits” and are comprehensive modifications to minimize the adverse effects of gravel runway operations. a range of tire pressures were evaluated to establish this rolling coefficient of friction on surfaces with varying CBR. Areas to be protected include belly surfaces. around or behind the wheels. Aircraft weight may have to be reduced in proportion to tire pressure to satisfy tire deflection limitations.S. cables and pipes routed on the landing gear.5 PROTECTION OF AIRCRAFT Operations on gravel runway surfaces require protecting the aircraft from the effects of flying dust. For a given tire pressure. Note however 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 may result in an optimistic value of the maximum allowable tire pressure. Test results are shown in Figure 20. The most critical condition for blade damage generally occurs at high propeller RMP and a low aircraft ground speed. especially gas turbine engines because of the possibility of severe engine damage occurring. and may eventually increase the possibility of flight controls jamming. Limitation against the operation of air conditioning systems or the prohibition of specific bleed air configurations while on the ground may be required. Both types of propulsion must avoid the ingestion of debris. 22 . Damage is minimized by gradual application of power until the aircraft achieves a minimal ground speed. Tires are also vulnerable to cutting and penetration from sharp stones that may be found on gravel surfaced runways. Jet engines mounted close to the ground have suction effects similar to those found on propeller driven aircraft. The abrasive qualities of dust may promote the erosion of paint surfaces and the crazing of windows if correct cleaning procedures are not applied. ducts. Power levers may also have to be set to fine pitch during taxi. Devices such as vortex dissipators which extract bleed air may also reduce available take-off thrust. Gravel deflectors may be required to prevent this. Jet engines are primarily susceptible to the ingestion of debris from wheel spray. The infiltration of dust into mechanical linkages may promote increased wear. Propeller driven aircraft are susceptible to impact damage caused by the close proximity of the blade tip to the runway surface. especially for engines mounted close to the ground. In these conditions. This may be more critical at lower speeds. which are probes at the front of the engine designed to destroy a vortex created by low pressure at the engine intake.c) Protection of Engines Jet and propeller driven aircraft have unique protection requirements for operation on gravel surfaces. but may also occur during static operation at ground idle. The Boeing 737 gravel kit incorporates engine vortex dissipators. Braking application may have to be reduced to minimize tire damage from inadvertent skidding which in turn will have an adverse effect on braking performance. f) Increased Structural Loading Aircraft landing gear and tire structures will likely be exposed to greater static and dynamic loadings on gravel runways as compared to hard surfaced runways. drains and air data sources. The vortex tends to raise debris especially at low ground speeds. The probes extract engine bleed air and direct it at the vortex to destroy it. the suction effect from the propeller has the greatest possibility of raising debris. Take-off distances will increase when the application of take-off power or thrust is scheduled by a gradual rate of application or a rolling take-off is performed. The relatively rougher surfaces of gravel runways may allow large forces to be transmitted to landing gear. d) Tires Increased tire wear is common on unpaved surfaces because of the rough texture of unpaved surfaces. The ability to apply maximum takeoff thrust while the aircraft is stationary is usually not possible on an unpaved runway. e) Effects of Dust and Debris on Systems Flying debris and dust may result in the gradual blockage of intakes. tires. especially during turning manoeuvres and during the passage of the nose gear through deposits of softer materials. Operations on gravel runways require maintenance programs to be modified and approved for such operations. An approved Maintenance Manual Supplement or specific maintenance instructions may be necessary. The frequency of cleaning of the aircraft and the lubrication of linkages should also be increased because of the increased dust in the gravel runway environment. 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.Lateral loads are considerably higher on low CBR runways. Since tire deflection will be increased under the influence of a reduced tire pressure for a given weight. 23 . landing gear systems. It may be necessary to perform an inspection before each flight for any damage from the previous flight. Propellers. filters and proturbances should be checked more frequently for dust and debris damage. g) Inspection and Maintenance Increased inspection requirements are necessary for gravel runway operations. depending on the aircraft basis of certification. compressor blades. Minor paint touch-ups and repairs in the field may be necessary before more permanent repairs can be made at a maintenance base. it may be necessary to limit aircraft weight. engine intakes. aircraft surfaces. 0 5.4 discusses the determination of maximum allowable tire pressure. It will however still be necessary to ensure that the pavement has sufficient overall strength to support the load of the aircraft. the strength of the surface should be measured prior to conducting any aircraft gravel runway performance testing. Construction records or other sources giving the maximum pavement strength load ratings (such as the ICAO ACN/PCN system) should be referred to at this stage. For the purpose of certifying aircraft for operation on gravel surfaces. runway conditions.1 AIRCRAFT CERTIFICATION TEST PROGRAM FOR OPERATIONS ON GRAVEL RUNWAYS GENERAL The objective of a certification test program is to produce a set of limitations.2 a) TEST SURFACES Measurement of Runway Strength Surface strength for each test surface should be measured at the start of the test program. Flight test demonstrations should be conducted on both wet and dry runway surfaces because of the effects of moisture on rolling resistance and braking performance. 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 maximum allowable tire pressure for this strength level. The method used to measure the surface strength should be recorded as well as the locations of the test measurements. presence of frost and other effects should also be recorded. rather than the bearing strength of the entire foundation. procedures and performance appropriate to operation on gravel runways. The date of the measurement. Section 4. then the tests must cover worst case conditions. For conservative aircraft performance data. b) Soil Analysis A soil analysis of each test surface should be conducted using a standard method such as the Unified system.5. restricting measurement to the shear strength to the surface. 5. 24 . without causing subgrade failure. Braking performance may also be affected by the soil composition of the surface. whether it is wetness following spring thaw conditions or as a result of long lasting precipitation. Criteria may have to be established on the exact definition of a wet surface. since the passage of the aircraft during the test program may further weaken the surface. If it is intended that performance data cover all likely operating conditions. is generally sufficient. the moisture conditions. Strength of the gravel runway surface measured in the summer and fall may also be higher than the spring because of moisture from the spring thaw. It is therefore important to note the time and conditions under which the strength measurements were obtained. This data would be used to define the composition of the runway surface tested for the AFM and for the determination of seasonal reduction factors for tire pressure. The higher of the two rolling friction values obtained should be applied to the proposed runway strength for conservative results. depending on the external configuration. maximum aircraft weight may be constrained by deflection limits of the tire. spoilers or reverse thrust. such as the soil group descriptions found in the Unified soil classification system (Figure 3). Performance data for takeoff distance and climb performance may require adjustment. should be assessed. Surface characteristics of the test runway should be defined in terms of a soil classification system. It may not be possible to find a gravel runway with the minimum strength proposed for the operation. In general the braking performance of at least three different runways typical of expected operation. As mentioned before. Indicated airspeed and altitude may also require calibration. Therefore a limiting configuration should be defined by the aircraft weight over tire pressure ratio. Weights should be varied from minimum to the maximum for the gravel runway.5. b) Braking Coefficient of Friction Braking performance is primarily a function of the characteristics of the braking surface rather than the surface strength. Testing under wet conditions may result in the worst possible braking performance. Braking tests should therefore be conducted on surfaces having the characteristics that will result in the worst braking performance. Tests should also be conducted to establish procedures for scheduling thrust or power and establish limits for thrust reverse. The aircraft should be allowed to decelerate to the lowest practical speed with the engines at an idle setting and without using brakes. Landing touchdown characteristics should be evaluated during the landing performance tests. Data for testing should be obtained for the worst case of tire pressure divided by runway strength (CBR) ratio. 25 . Braking coefficient data should be collected for the take-off and landing configurations. Factors may have to be applied to braking distance to account for reduced braking performance under wet conditions. d) Drag from Gravel Kit Installation Tests to determine drag polars and assessment of handling qualities due to the addition of gravel kits should be conducted. c) Performance Take-offs and Landings Performance take-offs and landings should be conducted to verify the performance data. Tests should then be conducted on two runways of differing strength using corresponding tire pressures to obtain the rolling coefficient of friction for each surface strength. Tests may be conducted by accelerate-coasts or landing on the unpaved runway using takeoff flap setting. Braking data may be obtained during accelerate-stop and/or landing tests with spoilers deployed and full braking.3 a) PERFORMANCE Rolling Coefficient of Friction Rolling coefficient of friction is a function of tire pressure and runway surface strength. The AFM should include the following information as applicable: a) Limitations Section 1. Minimum and maximum tire pressures and tire types 4. Continuous ignition on for take-off and landing 9. spoilers/lift dumpers and anti-skid to be operative for take-off and landing 8. Wheel brakes. Particular attention should be made to ground handling with low pressure or oversized tires for controllability during the take-off and landing rolls.4 a) HANDLING Ground Handling Characteristics Taxi tests should be conducted to determine lowest runway strength and minimum allowable tire pressures necessary for acceptable handling characteristics. The ground handling characteristics should be checked on the various types of surfaces on which they are intended to operate. Prohibition of reduced thrust take-offs 7. “Operations from Unpaved Surfaces” provides guidance for information to be included in the AFM.6 AIRCRAFT FLIGHT MANUAL (AFM) Aircraft Flight Manual information will be required for aircraft gravel runway airworthiness approval. Specified gravel protection system installed and operative 5. Flight testing of deflectors includes determining any increase in the drag coefficient caused by the installation of these devices. by using water to simulate the debris spray pattern. Increased gear retraction time as a result of the installation of gravel deflectors may require adjustments to take-off performance data. the information is included in an AFM supplement. and turns while taxiing. Nose wheel steering operative 10. 5. Transport Canada Airworthiness Manual Advisory (AMA) 525/4. Any limits on taxi operation should be determined. and checking that there are no adverse effects on the handling qualities of the aircraft.5. Weight and centre of gravity limitations 2.5 a) STRUCTURAL INTEGRITY AND SYSTEMS OPERATION Protection Functional performance of protection equipment must be tested including the effectiveness in minimizing debris spray and satisfactory operation during gear retraction and extension. Water trough testing may be employed in some cases. 5. Normally. Minimum strength of runway surface and approved types of unpaved runways 6. Approved take-off and landing configurations 3. Any other applicable limitation 26 . The use of soil classification system group symbols to describe gravel 27 . It is pertinent to consider the preceding examples on their degree of usefulness for operational use. “Take-off and landing field lengths shown in this section were determined on a wet gravel runway” “The runway surface should have a uniform covering of gravel that is graded smooth and kept free from ruts to avoid collection of undrained water during periods of precipitation”. 4. The surface definition of gravel runways has been typically specified in the performance section of the AFM supplement instead of the limitations section. 3. 3. “Runway subsurfaces constructed of materials impervious to water should be graded to facilitate water drainage”. No credit for clearway and/or stopway should be allowed. The application of distance factors may be necessary for wet gravel runways especially those with standing water. the failure of the bleed source to an engine vortex dissipator may require diversion to another airport having a paved runway. 6. c) Normal Procedures Section Procedures for taxiing. rejected take-off and landing on gravel runways Setting of thrust or power on take-off Selection of reverse thrust on landing and minimum speeds for operation Procedures for operation of braking system Note on minimum turn radius available Note that engine runups should be avoided over surfaces composed of loose material Abnormal Procedures and Emergency Procedures Section There are typically no changes to the emergency procedures section with the incorporation of gravel runway kits or for gravel runway operations. 2.b) 1. The following are examples of surface definitions applied in some AFM supplements: 1. “The subbase strength at a depth of 8 inches below the runway surface capable of supporting 260 psi at 0. “Gravel runway should be inspected at a frequency dictated by local conditions to assure that it is in a satisfactory condition”. well compacted and with a California Bearing Ratio of at least 30. For example.1 inch penetration when tested by the in place CBR method or by equivalent method”. “Surface material at least 6 inches thick. d) Performance Section This section includes all applicable charts pertaining to aircraft field performance and other parameters affected by the gravel kit installation. Abnormal procedures may however require a review. 4. 2. 6. take-off. 5. and no areas of deep loose gravel deficient in fines”. 5. Unless specifically evaluated there should be no dispatch with inoperative equipment.runway surfaces in AFM performance charts may be clearer in providing a definition of the type of surface the performance chart is based on. The AFM supplement may include accompanying information on the corresponding properties of gravel runways.7 MASTER MINIMUM EQUIPMENT LIST (MMEL) Specific gravel protection equipment will usually lack redundancy for dispatch with failures. This would provide flight crews and operators with general information of the probable behavior of gravel runway surfaces under varying conditions affecting aircraft performance. 28 . This would also provide a link between the identification of gravel runway surfaces used during certification testing to those used operationally. 5. It is recognized that a certain amount of damage from gravel runway operations may occur as long as this does not affect flight safety. Acceptable soil types as defined by the Unified group symbols would be listed and described here. Gravel surfaces are susceptible to weakening from moisture penetration and frost action. Measurement of surface shear strength. Large discrepancies in measured CBR values have been identified dependent on measurement technique. The lower strength and less uniform properties of gravel surfaces results in significant differences in rolling friction and braking performance. the most common operational problems result from the shear failure of the surface (rutting) caused by excessive aircraft tire loading. Although there is a minimum surface strength requirement for a given aircraft weight and tire loading. Performance data based on a hard surfaced smooth dry runway is usually not valid when applied to a gravel runway.0 SUMMARY A gravel runway is essentially a flexible pavement with a surface course of unbound granular material. Airworthiness approval requires consideration of aircraft performance. The same CBR method should be applied for operational measurements. handling characteristics and systems operation on gravel runway surfaces. a description of the surface composition is an important parameter to relay to the operator/crew to ensure that the performance data obtained during testing is appropriate for aircraft operations. Braking performance is usually most affected by surface soil properties rather than surface strength.6. The AFM should describe the characteristics of the gravel runway surface for which the performance testing was conducted. The rough texture of gravel surfaces contribute to increased tire wear. Loose material associated with gravel runway surfaces also results in the requirement to protect the aircraft from debris. that will provide a practical aid in the identification of the runway surface for operational use. On a given runway. procedures. The specific CBR method used for gravel runway certification testing should be identified in the AFM. Because of the variability of gravel runways. compaction and aggregate interlock. For conservative performance data. tests should be conducted on surfaces having the most adverse effects on rolling resistance and braking performance. structural and systems aspects. gradation. rather than the bearing strength of the entire foundation is generally sufficient for certification testing. moisture content. and performance information. the CBR value is dependent on the test method used. CBR also indicates runway surface shear strength if a penetrometer method is used. California Bearing Ratio (CBR) is a common parameter used to express soil bearing strength. Test programs are necessary to determine aircraft performance. handling. 29 . This information must be presented in a manner. and the provision of AFM limitations. Gravel runway surface strength depends on the surface composition. aircraft configurations and acceptable criteria should be provided in this document. The terminology used to identify soil types should be such that it is readily understandable. Airworthiness approval should be a prerequisite for any operational approval. This should include the conditions under which the testing was conducted and the specific runway layers on which measurements were taken. The ASTM method and Boeing High Load Penetrometer should be the only approved methods for determining gravel runway surface CBR for the time being. 2. that will aid in the identification of the runway for operational use.7. The AFM performance section should provide a description of the gravel runway surface on which performance testing was conducted. material and strength specifications continue to be met for operational use. frost and weathering of specific soil types on the performance and handling of the aircraft should be provided. The method of strength measurement used for certification testing should be stated. The establishment of an approved Maintenance Manual Supplement or specific aircraft maintenance instructions should be considered for gravel runway airworthiness approval. Information regarding the influence of moisture. Operations on Unpaved Runways should be reviewed and updated to incorporate the above recommendations. Specific tests. 30 . This would serve as an aid in identifying gravel surfaces for the operator when performing their own assessment. 8. The Shock penetrometer results may be unreliable in predicting CBR and should not be approved for use until differences can be resolved 4.0 RECOMMENDATIONS 1. 7. structural integrity and systems operation. Airworthiness Manual Advisory. 5. The AFM should express the strength of the runway using a strength classification system that can be easily interpreted by the flight crew and aircraft operator. 3. The AFM should include a description of the soil types tested. This information must be presented in a manner. Safe gravel runway operation requires an airworthiness approval because of significant effects on performance. handling. AMA 525/4. Operational approval should consider the need for gravel runways to undergo continued inspection and maintenance to ensure that runway thickness. 6. and the material discussed and referenced in this report. F. September 15. Avions Marcel Dassault-Breguet Aviation report. Gravel Runway Certification Results. dated 4/5//84. AMA 525/4. Section or Addendum No. Mystere-Falcon 900. REFERENCES AGARDograph 45. Public Works Canada. Airports and Construction. Principles and Practices. July 1992. 14. ASTM D 4429-93. Flight Test Guide for the Certification of Transport Category Airplanes. 12. 9. 16. Safety and Technical Services. Operations from Unprepared and Semi-prepared Airfields. Rev. High Load Penetrometer Soil Strength Tester. 15-1. Washington. Airport Pavement Design and Evaluation. Standardized Method of Reporting Airport Pavement Strength-PCN. Lockheed Aeronautical Systems Company Marietta. Advisory Circular AC No. 2. Manual of Pavement Structural Design. Standard Test Method for CBR (California Bearing Ratio) of Soils in Place.Bearing Strength. Advisory Circular AC 25-7-X. AC No: 150//5320-6D. 11. September 1960. September 1987. The Boeing Company. dated June 1984. Operation on Unpaved Runways. Gravel Runways Condition Reporting Procedures and Surface Stability Test Methods. F. 10. Architectural and Engineering Services.0 1. Boeing Document No. F. dated 15 February 1985. Boeing Model 737-200 Document No. 31 .8. Seattle.A. AK-68-31-000. Operations on Unpaved. E. 150//5335-5. Canadair Challenger. ASG-a9 (AK-69-12). 6. 1996. Jan. 13. D6-32021.A.A. 7. 1988. Public Works Canada. Advisory Circular.A. 15. dated 11-5-68. 8. 3. Worthington. Currey. AK-67-09-280.A. Transport Canada Air. Georgia. 30. 4. Transport Canada Airworthiness Manual Advisory. Air Transportation. Aircraft Landing Gear Design. Transport Canada Airports. Airport Pavement Evaluation . 5. D6-24555.A. Forest W. MIL-STD-612A Method 101. California Bearing Ratio of Soils. Boeing Model 737 Advanced Low Pressure Tire Gravel Runway Certification Program. MAA 601-138: Performance and Handling of the CL-601 Equipped to Operate on Gravel Runways. Norman S. 32 .