DESIGN AND CONSTRUCTION OF HELIPORTS AND HELIPADS
This chapter presents information on the design and construction of operational facilities for rotary-wing aircraft. The first step in the design of such facilities is to identify the types and amount of traffic that will use the heliport or helipad. Next, establish requirements for geometric dimensions, surface types, and service facilities. Determine subgrade strength, design load, and design life to determine the proper surface type. The proper surface also depends on expedient matting, expedient membrane, and soil-stabilization requirements. Procedures for marking and lighting heliports and helipads are also discussed.
Army helicopters are classed as observation. utility, cargo, and attack helicopters. Important characteristics of current Army helicopters are shown in Table 13-1. Design criteria given later in this chapter are based on use by the most critical aircraft (greatest pavement load).
- Observation helicopters (OHs) are used for visual, photographic, or electronic observations and the adjustment of fires. OHs are also used for command and control; reconnaissance; surveillance; aerial wire laying; and a limited amount of resupply, evacuation, and aerial fire support.
- Utility helicopters (UHs) are used for missions such as troop and cargo lift, passenger transport, patient movement, command and control, and dissemination of material during psychological operations.
- Cargo helicopters (CHs) are used to support air-movement operations and to transport troops, equipment, and supplies within the battle area. They are also used for refueling tankers and evacuating patient, prisoners, or damaged equipment. Cargo aircraft with vertical takeoff and landing capabilities can transport surface vehicles and other heavy equipment for short distances over natural or manufactured obstacles.
- Attack helicopters (AHs) provide direct aerial fires and escort troop-carrying helicopters and provide a suitable platform for various weapons.
The size and configuration of heliports are accommodated by the facility. The dictated by the type and number of helicopters accommodate by the facility. The size and configuration change as the tactical and environmental conditions change. Under some circumstances, several hundred helicopters may be based at a single location. Another set of conditions may require that aircraft density be limited to 25 helicopters. Developed area requirements are for dispersed heliports with densities as low as 25 helicopter per site.
Requirements in the underdeveloped areas of the world are usually satisfied by locating heliports with large aircraft capacities at a fixed-wing airfield. This density is determined by security requirements, responsiveness to supported units, and reduction of airfield construction effort.
TYPES OF HELIPORTS
The four levels of heliport development in the TO are LZs of opportunity, austere forward area fields, substandard but operational support area fields, and deliberate rear area fields.
Landing Zone of Opportunity
This type facility, normally located in the close battle area, represents the minimum cleared area at which a helicopter can land to discharge or pick up passengers or cargo under conditions existing at the time of use. Geometric requirements are kept to the absolute minimum and do not exceed those for a support area helipad or heliport. If the helicopter is to remain in the area, there should be a minimum disturbance of the natural terrain. No construction effort, other than clearing, is expended at LZs of opportunity.
Austere Support Area Field
This construction standard is the minimum acceptable in safety and efficiency for aircraft operations. The heliport is usually limited to a grass or soil surface (preferably grass) with appropriate expedient treatment to permit operations under most weather conditions. The ground should be sufficiently firm, horizontal or nearly so, and clear of any objects likely to be blown about by the rotor.
These heliports are established on a temporary basis for support of a particular operation. The duration of use, determined by the tactical situation, is usually two weeks. Routine organizational maintenance and limited field maintenance are done.
The location of an austere support area airfield is dictated primarily by the tactical situation. It is not necessarily the best location from the standpoint of efficiency of flight operations. Any work required to develop this site is usually done by the unit occupying the site, except for such effort as may be available from an engineer combat battalion. Heliport maintenance is only enough to accomplish the mission.
Support Area Field
This construction standard provides conventional safety and efficiency of operations. Heliport traffic areas are normally surfaced with membrane or landing mat, anti aircraft operate under most weather conditions. Facilities at this site provide for POL resupply and extended maintenance. The field is usually located in the corps area and is constructed by engineer combat or combat heavy battalions.
The magnitude of operations at these fields is far greater than at shaping area fields. The site should be located near flying operations. To economize on the construction effort and to maximize the use of available sites, these heliports may be combined with fixed-wing airfields. Because support area heliports normally operate over a long period of time and may eventually be fitted into the overall theater airfield scheme, maintenance is progressive and vigorous.
Rear Area Field
These heliports are designed and constructed for all-weather operations. A rear area field has a well-graded, thoroughly compacted base and an expedient or conventional surface. The field is usually in the COMMZ and is located at a fixed-wing airfield. Construction is by engineer combat heavy battalions. The location will stress operational efficiency. A high standard of heliport maintenance is provided because of the magnitude of operations and the size of aircraft involved.
Helipads are constructed for aircraft that do not require a TGR to become airborne. They are most advantageous where a limited number of helicopters are to be located or at heliports that handle a large volume of traffic where separate landing and takeoff operations are desired. Helipad layouts have been developed for the various helicopters.
FACTORS INFLUENCING DESIGN
The basic TO heliport complex as visualized in development of design criteria for close battle support and rear area heliports, is shown in Figure 10-2,. Each heliport shown is included in the complex for a specific purpose. The design of each heliport is based on the requirements for the aircraft listed in Table 13-1. Additionally, geometric requirements and minimum area requirements for each kind of helipad/heliport considered are shown in Tables 13-2, 13-3, and 13-4.
Factors that influence the development of heliport design criteria are helicopter characteristics, operational considerations, and requirements for expedient airfield surfacing and rustproofing, Location, traffic area, and safety also affect the design of heliports.
Helicopter characteristics that influence heliport strength requirements are weight, landing-gear configuration, and tire pressure. Ground run and dimension characteristics affect heliport geometric layouts,
Heliport surfaces are designed to withstand the load applied by the helicopter. This load is distributed to the heliport surface at several points in a pattern determined by the landing gear configuration. The loading for each wheel is determined by the total wheel load and the dimensions of the tire area in contact with the heliport surface. The contact area dimensions are influenced by tire pressure. The heliport surface must have sufficient strength to resist repeated applications of maximum unit loads.
Sortie rate, tonnage to be handled, estimated heliport life, and the number of helicopters to be accommodated are the operational considerations that influence heliport criteria. Sortie rates determine the number of landings per unit time applied to the heliport surface. Design life indicates the total number of loadings the surface will sustain. The number of helicopters to be accommodated and tonnage to be handled establish taxiway, parking, and other hardstand requirements.
Surfacing and Dustproofing
Heliport traffic areas are brought to design strength by removing and replacing inadequate soils, compacting soil, and applying a bituminous pavement. Landing mats eliminate the need for these operations or reduce the time required to perform them. The mats are placed on low-strength soils to provide support for helicopter operations.
Membranes provide waterproofing and dustproofing on soils that have adequate strength for airfield traffic areas.
See FM 5-410 for soil stabilization methods that add strength to soils with low bearing values.
Dust-control materials are applied to unsurfaced traffic areas to limit the safety hazards and maintenance problems caused by dust. These materials also deny the enemy heliport intelligence gained through observation of traffic-induced dust.
The level of development to which a heliport is constructed most often depends on its location. As the heliport location approaches the forward edge of the battle area, austerity of construction increases. In close battle and support areas, criteria reflect the requirement for haste in construction, short heliport life, and greater reliance on helicopter performance characteristics. In rear areas, helicopter support facilities are provided, greater numbers of helicopters are accommodated, and heliport dimensions reflect less reliance on helicopter performance characteristics.
Traffic Area Requirements
Locating heliports at logistical airfield complexes assures continued supply of POL and necessary aircraft maintenance parts and material. To avoid saturating or overloading the airfield, limit the total helicopter aircraft to approximately 30 to 100, which is equivalent to one to three companies. Each company or equivalent unit operates from its own area or dispersed hardstands located or satellite in the airfield operational area (Figure 13-1). Traffic areas for helicopters are assumed to carry the same requirements (medium-duty mat, light-duty mat, or membrane) as the parking areas of the basic airfield complex.
The tactical situation may necessitate deviations from the safety criteria established in technical manuals. The degree of departure from established criteria are dictated by the degree of risk that the command is willing to accept for that particular situation. If these standards must be compromised, the criteria in this manual are regarded as minimum.
The geometric design requirements for helicopter landing areas can be simplified into four basic types: helipads, heliports with taxi-hoverlanes, heliports with runways, and mixed battalion heliports.
NOTE: If the tactical situation warrants, helicopter parking dispersion should be increased to the maximum extent possible and protective revetments should be built. The tables and figures in this chapter state the minimum dimensions allowable. Chapter 14 addresses design, construction, and maintenance of fortifications to protect parked Army aircraft from hostile fire and associated damage from exploding fuel and ammunition.
The geometric layout and section views of a helipad are shown in Figure 13-2. The minimum dimensional (geometric) requirements are in Table 13-2. The circled numbers in Figure 13-2 identify the item numbers listed in Table 13-2. Use Figure 13-2 with Table 13-2 to determine the geometric requirements for each critical helicopter and each type of landing pad (close battle area, support area, and rear area).
Heliports with Taxi-Hoverlanes
The geometric layout and section views of a heliport with a taxi-hoverlane are shown in Figure 13-3. Parking and landing pads are offset midway across from each other on both sides of a taxi-hoverIane. Helicopters approach and depart this heliport via the hoverlane and approach/departure zone. The circled numbers refer to the item numbers in Tables 13-2 and 13-3, for the various types of heliports.
Heliports with Runways
The geometric layout and section views of a heliport with a runway are shown in Figure 13-4,. This heliport, normally located in support or rear areas, is for the heavier wheeled cargo helicopters. The circled numbers in Figure 13-4 identify the item numbers in Table 13-3. Use these numbers to determine the geometric requirements for this type of heliport. Parking pads are offset midway across from each other on both sides of the taxilane. Helicopters normally approach and depart this heliport via the runway.
Mixed Battalion Heliport
The size of the mixed battalion heliport shown in Figure 13-5, is not standard but varies according to the number and types of helicopters that occupy it. This heliport may have a maintenance apron and multiple types of heliports in its layout. Each type heliport must be separately designed to the requirements indicated previously. This facility normally is located only in support or rear areas.
The runway lengths at sea level and 59°F for all helicopters considered are shown in Table 13-3. Runways are not shown in layouts for skid-type helicopters or wheel-mounted helicopters in the close battle area, where the taxi-hoverlane is used for this purpose.
Increase the runway length by 10 percent for each 1,000 feet in altitude above 1,000 feet. Make a temperature correction of 4 percent. for each 10 degrees above 59° F in mean temperatures for the warmest period during which operations will be conducted. In no case is the length of the runway less than the minimum length shown in Table 13-3 for each type heliport.
Heliport runways normally are oriented according to the local prevailing winds. This orientation minimizes the detrimental effect. of crosswinds on aircraft operation. Determine this orientation after thoroughly studying wind through graphic analysis, often called a surface-wind rose analysis. (See Chapter 11.) Other factors permitting, align the runway as closely as possible with the prevailing wind.
Geometric requirements for close battle, support, and rear area heliports and helipads are shown in Tables 13-2 and 13-3. In general, heliport development consists of arranging a series of individual helipads together at the spacing required to accommodate the type of helicopters expected to operate from the facility. Tables 13-2 and 13-3 show the requirements for parking pads, taxiways, and runways (minimum length, width, and gradient). Related airfield elements such as shoulders, clear areas, overruns, lateral safety zone, clear zone, and approach zone are included in these tables. These requirements are based on the operational characteristics of the aircraft considered. Therefore, variation in these requirements beyond or outside the limits indicated should not be allowed except where sufficient evidence justifies the change.
Minimum area requirements for elements of a heliport are shown in Table 13-4. If dimensions must be changed because of existing conditions, the affected areas must be recalculated. The total traffic area is the sum of the parking pad area, taxiway area, and runway area for wheel-mounted helicopters in the support and rear areas. It is equal to only the parking pact areas for skid-mounted and wheel-mounted helicopters in the forward area.
Dustproofing and waterproofing areas for heliports where no landing mat is required are equal to the total area within the lateral clearance line, plus any area around the perimeter of the heliport that is within the area affected by the rotor downwash. The diameter of the area affected by the rotor downwash is shown in Table 13-5. These values are for loose soil. Where other soil conditions exist, the requirements for dustproofing will be smaller.
The strength of the subgrade soil must be known to determine the best type of heliport and helipad surface. The type and number soil test required depend on the characteristics, Generally, sieve analysis, specific gravity, hydrometer analysis, Atterberg limits. And CBR analysis test are required. Recommended procedures for treating material used in the layers of the pavement beneath surface course are presented in Chapter 5, FM 5-430-00-l/AFPAM 32-8013, Vol 1, and FM 5-410.
Unsurfaced areas such as deserts, dry lake beds, and flat valley floors serve as possible heliport sites. Special procedures must be exercised to ensure adequate dust control is used. Dust control is described in Chapter 12 and later sections of this chapter. Site reconnaissance and smoothness requirements for heliports are the same as those described in Chapter 12 for airfields.
Surfacing requirements for various subgrade strengths (CBR) for both unsurfaced areas and areas to be surfaced with landing mat are shown in Table 13-6, for traffic areas (runway, taxiway, and apron) and service roads. Overruns are unsurfaced and steps are not usually taken to improve the existing soil strength. The approximate number of traffic passes that a particular helipad or heliport can sustain, if built according to these soil strength requirements, is determined from the subgrade strength curves for specific aircraft.
Predicted traffic volume is a prime factor in determining surface requirements. However, a considerably larger volume of traffic may occur at a heliport than was estimated when the subgrade strength requirements were developed. In addition, helicopters may be operated at gross weights different from those in Table 13-6. In such situations, the basic soil strength requirements in Table 13-6 no longer apply. In these cases, the required soil strengths for all reasonable combinations of gross weight and traffic volume are determined through use of the subgrade strength requirements curve in Figure 13-6.
When developing TO heliports, use soil-stabilization materials and processes to improve engineering characteristics and performance of existing soils. Soil-stabilization processes and materials described in FM 5-410 can help the engineer select and use appropriate methods of soil stabilization for specific operational and functional needs. The same criteria and principles of stabilization pertinent to the construction of airfields generally apply to heliports. Because differences of design and usage exist, this chapter presents information specifically for heliport development, This information and information in FM 5-410 describe the effective use of soil stabilization n techniques.
For TO heliports, soil stabilization may be used to accomplish one or a combination of three primary functions: strength improvement, dust control, and soil waterproofing.
Runways, taxiways, aprons, and parking pads may need stabilization to improve strength. The use of soil-stabilization methods applies primarily to heliports and helipads designed to support operations of the CH-47, CH-54, UH-lH, and AH-64 helicopters. Sometimes it may be desirable and justifiable to construct an improved landing pad for the OH-58 or UH-60 helicopter even though the existing soil may have the required minimum strength to support these aircraft.
Where stabilization for strength improvement is needed, certain basic design requirements must be met, in terms of strength and thickness of a stabilized soil layer on a given subgrade. The minimum strength and thicknesses of the stabilized soil layer shown in Table 13-7, are based on the traffic demands for operations in the areas by the CH-47 and CH-54 helicopters.
No strength or thickness requirements are shown for helipads for the OH-58 or UH1H helicopters because these skid-equipped machines can operate satisfactorily on unsurfaced soils that have very low strength (Table 13-6). If the existing soil strength is less than the indicated minimum requirements for the OH-58 or UH-1H helicopter, the condition of the soil generally is such that much construction effort is required to achieve an acceptable stabilized facility.
Some soils have sufficient strength to support the OH-58 and UH-1H helicopters but are weak enough to create a nuisance in the form of mud. Where such a condition exists and select borrow material can be obtained conveniently, consider improving the parking pad area by placing a blanket of the select borrow material on the lowstrength soil surface. Additionally, drainage must be improved in the local area. A 6- to 8-inch layer of crowned quality soil is usually sufficient to provide a firm parking pad and stable working area.
Proper evaluation of the subgrade is essential. When evaluating the subgrade for stabilization, establish a representative CBR strength profile to a depth that will avoid overstress at any point in the underlying subgrade. The depth of a necessary strength profile depends on the particular heliport and using helicopters, the pattern of the profile, and the manner in which stabilization is achieved.
Use the thickness data in Table 13-7 to establish an adequate strength profile.
Generally, a profile to a depth of 24 inches is sufficient to indicate the strength profile pattern. The use of Table 13-7, to establish design requirements for soil stabilization is similar to that described in Chapter 12 for airfields.
Stabilization to improve strength of an existing soil can be accomplished by mechanical or chemical methods. Mechanical stabilization methods include compaction of an existing soil or blending of soils to obtain an improved quality soil. The chemical stabilization method involves blending soil with some type of stabilizing material to achieve a more firm and durable soil layer. The stabilizers most commonly used in the construction of heliports are portland cement, lime, and bituminous materials.
Two general methods for applying soil stabilizers are admix application and surfacepenetration application. An admix application blends existing soil with another material to achieve a uniform mixture. Admix applications may be mixed in place or off site. The admix application technique is used primarily to incorporate stabilizing materials for strength improvement.
With surface-penetration application, a soil treatment material is placed directly on the ground surface by spraying or other means of distribution. This method is used only for the placement of dust-control agents and soil waterproofers.
Dust is a major problem in helicopter operations during dry weather. Dust can come from almost any unsurfaced area of a heliport complex. Therefore, it is necessary to provide dust control for all areas of a heliport. Membranes and landing mats often are used to cover the primary traffic areas of a heliport such as runways, taxiways, aprons, and parking pads. Where this is the case, dust-control materials are used to control dust on all remaining areas. Without a membrane or landing mat, it may be necessary to use dust-control materials in traffic areas. Materials selected for traffic areas should provide dust control and waterproof the soil surface to prevent loss of strength during wet weather operations.
Dust-control and soil waterproofing materials are described in Table 12-7, and FM 5-410. Recommendations for their use and guidance in selecting appropriate materials for traffic and nontraffic areas of a heliport are discussed in FM 5-410. When estimating material requirements, use Table 13-4, to determine the areas for each heliport element.
Areas that may require dust control include runways, overruns, taxiways, aprons, taxi-hoverlanes, parking pads, roadways, and all peripheral areas that are subjected to the downwash from helicopter rotors. (See Table 13-5.) All exposed ground within the entire heliport complex should be dust-free, and any area not protected by membrane or landing mat requires dust-control treatment.
Areas that may require waterproofing to maintain soil strength include runways, overruns, taxiways, aprons, parking pads, and roadways for support vehicles.
Expedient Surface Design
Design Steps. The design steps for an expedient surface heliport or helipad are as follows:
1. Determine the heliport location. Normally this is given in the mission statement as close battle area, support area, or rear area.
3. Determine the using aircraft and associated gross weight. Again, this information will be presented in the mission statement. Gross weights for aircraft are shown in Table 13-1.
NOTE: Use 15 kips as the minimum weight for aircraft gross weight.
4. Determine the strength of the subgrade in terms of critical CBR. This information is normally extracted from the battalion soils analyst or Air Force CCTs or given as part of the mission statement. Determination of the critical CBR is described in detail in Chapter 12. Ensure the subgrade thickness requirements from Table 13-7 are met.
5. Determine surface requirements from Table 13-6 based on critical CBR.
6. Determine the required strength from Figure 13-6 in terms of CBR based on the anticipated traffic passes for design aircraft (from mission order) and surface requirements from step 5. Compare the required CBR to the critical CBR in step 4. If the required CBR is less than the critical CBR requirements, the site is suitable. If it exceeds the critical CBR, existing soil CBR must be increased or a new site selected.
Determine if a site is suitable for 200 passes for a UH-60 Blackhawk heliport in the support area. Also determine if soil strength is uniform with depth, yielding an average CBR of 7 in the top 12 inches.
1. Heliport location: Support area.
2. Helipad geometric requirements: Given in support area UH60 (Table 13-2).
3. Using aircraft: UH-60 Blackhawk
Gross weight: From Table 13-1, 20,250 pounds (20.25 kips)
4. Critical CBR = 7 for uniform strength profile (given).
5. From Table 13-6, a light mat must be used unless the existing soil can be improved to a CBR greater than 17.
6. Required CBR based on 200 anticipated traffic passes: From Figure 13-6, the required CBR is 4, less than the critical CBR: therefore, this site is suitable for the mission.
Many aircraft require the use of mats or membrane placement is contained membranes to operate successfully in areas in Appendices L, M, and N. of low subgrade strengths as previously discussed. Membrane is used for dustproofing and waterproofing. Table 13-6 details basic surfacing requirements for the critical aircraft in the three heliport locations. Table 13-4 details the total area required for heliport construction based on criteria in Tables 13-2 and 13-3. Table 13-8 shows the material requirements of heavy-duty membrane-surfaced heliports and helipads. A thorough discussion on mat and membrane placement is contained Appendices L, M, and N.
It is possible to calculate and tabulate expedient matting requirements for specific facilities because heliport and helipad design are standardized. Use the guide on to locate information required for this process.
Membrane requirements for specific heliports and helipads can also be calculated. The following tabulation shows what tables are used in this process:
The design procedure for flexible surfaces for heliports and helipads is almost identical to that of discussed airfields as in Chapter 12.
1. Determine the heliport/helipad location
2. Determine the design aircraft gross weight
3. Check soils and construction aggregates.
4. Determine the number of passes required.
5. Determine the total surface thickness and cover requirements.
6. Complete the temperate thickness design.
7. Adjust thickness design for frost susceptibility.
8. Determine compaction requirements and subgrade depth.
9. Draw the final design profile.
STEP 1. DETERMINE THE HELIPORT/HELIPAD LOCATION
Flexible pavements are only constructed for heliports/helipads in the rear area.
STEP 2. DETERMINE THE DESIGN AIRCRAFT AND GROSS WEIGHT
Flexible-pavement structures have the capability to support large cargo helicopters with tremendous gross weights as well as small helicopters with large tire pressures. As with fixed-wing aircraft, it is logical to design the heliport/helipad for only the most constraining aircraft, the CH-47D Chinook. If the designer knows, however, that the heliport/helipad will be used only by a specific aircraft, its load can be used for the design. The CH-47D has a design gross weight of 50 kips.
STEP 3. CHECK SOILS AND CONSTRUCTION AGGREGATES
The procedure for evaluating materials for flexible-pavement structures is the same as for fixed-wing airfield structures. First, locate borrow sites and evaluate them for suitability as select and subbase courses. Use Table 12-9, to check soil characteristics and strength against the specifications for each layer. Second, check the strength and gradation of the base course. The strength of a known material is determined from Table 12-10 while the gradation of a soil must meet the specifications in Table 12-11, based on the MSA. Third, check the materials above the compacted subgrade for frost susceptibility. Frost susceptible borrow materials cannot be used in the design. If the subgrade is frost susceptible, determine the frost group and soil support index from Tables 12-12 and 12-13.
STEP 4. DETERMINE THE NUMBER OF PASSES REQUIRED
Since the rear area is considered temporary construction (6-24 months), design flexible pavement heliports/helipads to sustain an appropriate number of passes. Remember, one pass refers to one takeoff and one landing.
STEP 5. DETERMINE THE TOTAL SURFACE THICKNESS AND COVER REQUIREMENTS
Enter the curve, Figure 13-7, for the soil CBR and number of required passes. The resulting thickness is the cover required above that particular soil layer to protect it from shear failure. Second, the apshalt thickness (inches) is a function of the strength of the base course as follows:
STEP 6. COMPLETE THE TEMPERATE THICKNESS DESIGN
Same as for fixed-wing aircraft.
STEP 7. ADJUST THICKNESS DESIGN FOR FROST SUSCEPTIBILITY
These design steps are the same as previously discussed for fixed-wing airfields. See Chapter 12 for a review.
STEP 8. DETERMINE THE COMPACTION REQUIREMENTS AND SUBGRADE DEPTH
Depth (inches) of required subgrade compaction below the surface of all areas of a heliport/helipad is 24 inches for cohesive soils and 30 inches for cohesionless soils. At a minimum, subgrade will be compacted to 6 inches.
STEP 9. DRAW THE FINAL DESIGN PROFILE
Draw the final design profile as previously shown for fixed-wing airfields.
Design pavement for the parking/loading area of a rear area heliport in Central America, capable of handling 5,000 passes of a CH-47D aircraft. The soil layers have already been determined by the soils analyst.
Subgrade: Clay, PI = 12, LL = 20; natural CBR = 4; compacted CBR = 5.
Borrow A: Select material CBR = 15, PI = 7.
Borrow B: Subbase material CBR = 40, PI = 4.
Base course (limestone): CBR = 80, PI = 4. Meets gradation specifications for MSA (2-inch) (Table 12-12).
Step 1. Airfield location (given) = rear area/Type C traffic area.
Step 2. Design aircraft = CH-47/50 kips.
Step 3. Check soils and construction aggregates:
a. Select and subbase (given).
Borrow A: Select material CBR = 15.
Borrow B: Subbase CBR = 40.
b. Base course: Limestone, CBR = 80; meets gradation.
c. Frost is not a concern in Central America.
Step 5. Determine the thickness requirements from Figure 13-7.
Step 6. Complete the temperate thickness design
See Table 12-15 with the traffic area (C) and the base course CBR (80) to find that the thickness of the AC pavement = 4 inches. See Table 12-16, for a further breakdown of the specific course in the pavement design. Next, from Step 4, calculate the layer thicknesses. For instance, the cover required over the select material is 6 inches. With the base course and the AC pavement combined, the thickness is already 10 inches; therefore, a subbase is not required. To meet the cover requirement over the select material, the thickness of the subbase must be at least 3 inches; 6 inches is used because it is the minimum size layer thickness.
Step 7. Frost adjustment not applicable.
Step 8. Determine subgrade depth and compaction requirements. From Table 12-17, determine the required depth of subgrade compaction. Since the subgrade is cohesive (PI = 5), the depth required is 24 inches. The total design thickness is 16 inches; therefore, the depth of subgrade compaction is 8 inches. Next, determine the compaction requirements for each layer from Table 12-18.
Step 9. Draw the final design profile.
Special airfield flexible-pavement design considerations, such as designing for frost areas, designing for arid areas, and using stabilized soil layers discussed in Chapter 12, apply to flexible-pavement heliports and helipads ports as well. As such, no further made discussion will be made on these areas. Evaluation of flexible pavements of heliport and helipads follow the exact same procedure as detailed in Chapter 12 Evaluation or Airfield Pavements. As such, no further discussion will be on this subject.
Depending on the tactical situation. the marking pattern defined here is placed on all surfaced helipads or helicopter runways, whether the surfacing material is concrete, asphalt, mat, or membrane. The lighting specified is provided by generator-powered lighting units if the traffic areas are surfaced; battery-powered lighting units are acceptable otherwise.
The touchdown area marker for helipads is shown in Figure 13-8. The dimensions of the pattern compared with the pad size are also shown. On all helipads, the center of the marking pattern is placed at the center of the pad. The vertical bars of the letter H should be parallel to two opposite sides of the helipad. The marking pattern is also placed on both ends of all runways and taxi-hoverlanes used for landings. This pattern indicates a safe touchdown point. It is not placed at parking areas or where helicopters do not normally land or takeoff.
The marking pattern should be either white paint or tape, and it should be edged in black when placed on a light-colored surface. The broken-line border around the perimeter of the pad is included on all helipads.
MARKING OF TEMPORARY HELICOPTER LANDING FIELDS
Temporary airfields are not usually marked. When they are marked, use the following procedures:
Mark the four corners with regulation panels--0.50 by 0.65 meter (20 by 26 inches) or 1.80 by 0.66 meters (71 by 26 inches)--or by improvised panels of comparable size that are a different color than the ground.
As far as possible, mark telephone wires, electric wires, and similar objects near the area. The direction of the sun's rays in relation to the direction of landing or takeoff may make them difficult for the pilot to see.
Indication of Wind Direction
This indication is of primary importance. The following methods can be used:
- Place a wind sock outside the area.
- Place smoke machines or fires emitting clearly visible smoke outside the area. Arrange them to avoid all risk of fire.
- Use a staff experienced in helicopter landings to stand with their backs to the wind, arms raised in a V-shape close to the spot where the helicopter is to touch down.
Identification of the Unit
The unit's identity signals must be arranged outside the area near the four corners of the field. If possible, the signals should be legible from the landing direction and be on the right of the landing area.
LIGHTING FOR HELIPADS
The following discussion concerns helipad lighting for visual meteorological conditions (VMC). Helipad lighting for instrument meteorological conditions (IMC) is beyond the scope of TO construction. IMC helipad lighting is discussed in STANAG 3684.
Each helipad must be surrounded by a perimeter of aviation yellow, omnidirectional lights, preferably not more than 13 inches in height. The lights will be placed in accordance with the following requirements:
- Lights will not be less than 1 foot nor more than 3 feet from the border of the landing pad and will be an equal distance apart on parallel sides.
- The separation between lights on a side will not be less than 15 feet nor more than 25 feet. This separation will be equal on a given side and will not differ by more than 5 feet between adjacent sides. Lights on parallel sides will be placed opposite each other.
Table 13-9 gives the number of lights required as a function of the length of the line of lights on a side, using the shortest distance permitted by these requirements. If the dimensions of the landing pad differ by more than 10 feet between adjacent sides, the long side will contain at least one more light than the short side. Table 13-10 shows the application of these requirements to the helipads developed for the control helicopters used in this manual.
LIGHTING FOR HELICOPTER RUNWAYS
Lines of aviation yellow, bidirectional lights will be located on each side of the runway at a distance of not less than 3 feet and nor more than 5 feet from the surfaced edge of the runway. The spacing within the line of lights will be approximately 40 feet apart but not less than 35 feet nor more than 45 feet.
These lines of lights will be extended past the ends of the runway to intersect lines of aviation yellow, bidirectional lights placed not less than 20 feet and not more than 25 feet from the surfaced end of the runway. The spacing within these lines will be approximately 10 feet, but not less than 5 feet nor more than 15 feet. Lines of green, threshold lights will be placed not less than 5 feet nor more than 10 feet from the surfaced end of the runway. The spacing within these lines will be approximately 10 feet but not less than 5 feet nor more than 15 feet. There will be no fewer than six threshold lights in each line.
LIGHTING FOR TAXI-HOVERLANES
Heliports for company-size or larger units will normally be designed to permit mass landings on the taxi-hoverlane between the parking pads (Figure 13-3). The lighting will be as follows:
- Place a landing pad at each side of the taxi-hoverlane that has a common centerline with the taxi-hoverlane. The size of the helipad will be determined by the largest control aircraft using the facility. Mark and light the landing pad in accordance with the requirements given previously in this chapter.
- Place aviation yellow lights that are not more than 13 inches in height and not less than 1 foot or more than 3 feet from the edge of the taxi-hoverlane and midway between parking pads. These lights will not exceed one-half the intensity used on the landing pad perimeter lights.
- Place two aviation red lights at the two corners of each parking pad farthest from the taxi-hoverlane. These lights will not exceed 13 inches in height and will have approximately 10 percent of the intensity used for the landing pad perimeter lights.
The following taxiway lighting system is required to designate paths followed by the helicopter in going between landing/takeoff, service, and parking areas. This lighting system will not be used on taxi-hoverlanes used for mass landings.
The basic taxiway lighting system will consist of a line of elevated or semiflush, blue guidance lights on each side of the taxiway, defining the lateral limits and direction of the taxiway. Taxiway lights will not be installed in those sections where surfaced aprons adjoin the taxiways. The lines of taxiway lights normally will be between 1 and 3 feet from the paved edge of the taxiway.
Straight Sections of Taxiways
On straight sections of taxiways, the pairs of lights will be uniformly spaced on centers approximating 40 feet but not less than 35 feet nor more than 45 feet apart. The longitudinal spacing of the pairs of lights will be calculated from the nearest point of tangency (PT) of the fillet, curve, or corner at one end of the section to the nearest PT of the fillet, curve, or corner at the other end of the section. Companion lights on opposite sides of a taxiway will be located on lines perpendicular to the centerline of the taxiway. Where it is practicable to light only a single straight edge of taxiway section, the lights will be uniformly spaced between the PTs or corners or between points opposite the PTs or corners, as applicable.
Curved Sections of Taxiways
On curved sections of taxiways, taxiway lights will be uniformly spaced on radial lines from the center of the curve. The spacing will be determined by the radius of the applicable curved edge of the taxiway. The taxiway lights will be spaced approximately 13 feet apart on the periphery of the curve, but not less than 10 feet nor more than 16 feet apart, except that no curve will have fewer than three light locations, including those at the PTs.
Available equipment recommended to meet the lighting requirements in this manual is listed under Federal Aviation Authority (FAA) numbers. The recommended equipment is to be used in series circuits controlled by constant current regulators having five-step brightness controls. The intensity of different elements (such as pad perimeter, runway, and taxiway) of a lighting system should be controlled separately.
The lights in each element of a system are to be coupled to the series circuit through direct-burial, insulating transformers (L834 or L-844, depending on the regulator). A transformer and an L-823 connector kit will be required for each light fixture. The cable to be used can be Number 8, stranded, 5 kilovolts (kv), cross-linked polyethylene. The light fixtures recommended for each element in the lighting system are as follows:
- Helipad. The fixture recommended for the helipad perimeter light is the stake-mounted, L-810 base with yellow L-810 lens in place of red. In the L-810 base, a medium prefocus socket is used for 6.6 ampere series circuit, 10 and 20 lumen lamps.
- Taxi-Hoverlane. The fixture recommended for the taxi-hoverlane taxiway and landing lighting is the stakemounted L-822 with standard blue L822 lens.
- Obstruction. The fixture recommended for obstruction lighting is the L-810 base with standard red L-810 lens.
- Runway. Until bidirectional lights with an appropriate beam spread for helicopters become available, the same fixtures may be used on runways that are used for heliports. The threshold lights should have green lenses in place of red.
Occasionally, situations develop that require clearing a helipad in a wooded area too dense to permit air landing of a clearing crew. In these conditions, personnel, equipment, and technique of operation employed by an engineer squad rappelling from a hovering helicopter to clear an expedient helipad are described in the following paragraphs. The procedure requires two helicopters to transport the clearing squad and to carry engineer equipment in underslung boxes.
The squad consists of a noncommissioned officer in charge (NCOIC) and two teams (A and B). Each team is composed of a noncommissioned officer and five other soldiers (two chain-saw operators, two ax operators, and one brush-hook operator). The weight of each individual is assumed to be 200 pounds.
The weight of the box, loaded with equipment, is approximately 333 pounds. The following equipment is contained in the box:
- 2 chain saws.
- 1 brush hook.
- 3 axes.
- 1 block-and-tackle set (1 single block and 1 double block).
- 1 set of climbers with safety straps.
- 1 can of gas.
- Sixteen 2-1/2-pound blocks of C-4.
- 2 oil cans.
- 250 feet of demolition cord.
- 1 galvanometer.
- One 10-cap blasting machine.
- 20 electric caps (should be carried by personnel and will be stored separate from explosives and fuel).
- 1 brace and bit.
- 1 sledgehammer.
- 2 wedges.
- 2 screwdrivers.
- 2 pliers.
The procedure is divided into two phases--delivery of equipment and personnel and preparation of the helipad.
Delivery of Equipment and Personnel
Equipment is delivered to the proposed helipad area by lowering it in a box designed to protect and offer ready access to the contents. The equipment requirements may be changed to suit the expected area of use.
The box is slung beneath the helicopter by the aircraft cargo hook. Rappelling ropes are attached to the box and secured to the floor D-rings within the helicopter to prevent oscillation. In the event of in-flight emergency because the pilot cannot jettison the external load, engineers within the cargo compartment are responsible for cutting or releasing ropes upon direction by the pilot or copilot. To lower the box to the ground, the cargo hook is released and the box is lowered by hand using the attached rappelling ropes.
Rappelling of personnel from the helicopter is performed as taught by Army service schools using the Swiss seat with snap links. For a complete discussion of rappelling, see FM 90-4.
Preparation of the Helipad
Personnel in the team are equipped with field equipment, machetes, weapons, and other items that can be easily carried on the person but would not interfere with rappelling activities. Other field gear, if needed, is enclosed in the equipment box lowered from the helicopter.
The first person on the ground removes the rappelling rope from the equipment box. The NCOIC, who is either the first or second person on the ground, will start laying out precut strips of engineer tape to mark the perimeter of the proposed helipad. The amount of tape laid out to define the area depends greatly on the terrain and vegetation encountered. Figure 13-9, shows the tape layout and the configuration of the helipad.
Personnel are organized as described previously. Two teams, one per helicopter, with an NCOIC are the desired composition for the accomplishment of the mission within the time allotted. One team is fully capable of preparing a helipad, but clearing time makes this undesirable.
The chain-saw, ax, and brush-hook crews move into the proposed helipad area and begin clearing the undergrowth. The next step is felling and clearing trees and other vegetation within the periphery of the tapemarked helipad.
Trees are felled as close as possible to the ground level, with the necessary limbing and bucking performed for easy removal. When felling and cutting, any vegetation that may be sucked up into the helicopter blades must be removed from the helipad proper. Vegetation should not be burned. When time permits or in marshy areas, the felled timbers may be used to prepare a hardened landing pad.
Landing pad logs are leveled to ensure a satisfactory surface upon which the helicopter skids can rest without danger of bending. The perimeter of the helipad must be checked to ensure vertical clearance.
In densely wooded areas and jungle forests, it is necessary to fell additional trees to provide an approach and departure zone. These zones are necessary to provide adequate clearance of obstacles 50 feet in height. (The normal time for clearing such a helipad in tropical zone forests by welltrained soldiers should not exceed three hours if trees do not exceed 12 inches in diameter.)
Before a helicopter lands in a forested helipad, landing reference panels are placed adjacent to the desired helicopter touchdown point. The landing reference panels serve as a visual guidance system during approaches and must be carefully positioned and firmly secured before a helicopter lands. Figure 13-10, shows the correct placement of landing reference panels on the ground.
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