will result in a cable strength equal to five times the maximum external load.
Example: Maximum external load 850 pounds Minimum required sling-leg cable strength 850 x 2.5 = 2125 Minimum desired sling-leg cable strength 850 x 2.5 x 2 = 4250
(2) A 3/16-inch, nonflexible 19-wire cable (MIL-W-6940) provides a satisfactory cable strength. For convenience, the cable sizes desired for various loads have been calculated and are tabulated in Table 7-1 based on a factor of 5:
TABLE 7-1. CABLE LOAD TABLE Aircraft Cable Size For Each Cargo Sling Leg
Maximum External Load (pounds)
MIL-C-5693 and MIL-W-6940
MIL-W-1511
MIL-C-5424 100 200 300 400 500 600 700 800 900 1,000 1,200 1,400 1,600 1,800 2,000 1/16 3/32 7/64 1/8 5/32 5/32 3/16 3/16 3/16 7/32 7/32 ¼ 1/4 5/16 5/16 3/32 1/8 1/8 1/8 5/32 3/16 3/16 3/16 7/32 7/32 1/4 1/4 9/32 5/16 11/32 3/32 1/8 1/8 5/32 3/16 3/16 3/16 7/32 7/32 7/32 ¼ 9/32 5/16 5/16 3/8
d. Sling Installation.
(1) Attach the cargo sling to landing gear members or other structure capable of supporting the loads to be carried. Install the quick-release device in a level attitude with the throat opening facing the direction as indicated on the quick-release device.
When cables are used to support the quick-release device, make sure the cables are not twisted or allowed to twist in the direction to un-lay the cable.
(2) Some cargo release devices are provided with a fitting to permit installation of a guideline to assist in fully automatic engagement of the load target ring or load bridle. Secure the guideline to the quick-release device with a shear pin of a definite known value which will shear if a load becomes entangled on or over the guideline.
Provision should also be made for cable-supported slings to be drawn up against the fuselage into a stowage position to prevent striking or dragging the release on the ground when not in use.
e. Installation of Release Controls. See Figure 7-2 for typical wiring diagram of the electrical controls.
(1) Install a cargo release master switch, readily accessible to the pilot, to provide a means of deactivating the release circuit. The power for the electrical release circuit should originate at the primary bus. The "auto" position of the release master switch on some cargo release units provides for automatic release when the load contacts the ground and the load on the release is reduced to a preset value.
(2) Install the cargo release switch on one of the pilot's primary controls. It is usually installed on the cyclic stick to allow the pilot to release the load with minimum distraction after maneuvering the load into the release position.
Par 704 Page 63
FIGURE 7-2. TYPICAL CARGO SLING WIRING DIAGRAM
(3) Install the emergency manual release control in a suitable position that is readily accessible to the pilot or other crewmember. Allow sufficient slack in the control cable to permit complete cargo movement without tripping the cargo release.
(4) The manual ground release handle, a feature of some cargo release units, permits opening of the cargo release by ground personnel.
(5) Label or placard all release controls as to each function and operation.
f. Functional Test.
(1) Test the release action of each release control of the quick-release device with various loads up to and including the maximum external load. This may be done in a test fixture or while installed on the rotorcraft, if the necessary load can be applied.
(2) If the quick-release device incorporates an automatic release, the unit should not release the load when the master switch is placed in the "automatic" position until the load on the device is reduced to the preset value, usually 80 to 120 pounds.
FIGURE 7-3. TYPICAL CARGO SLING INSTALLATION NO. 1
Par 704 Page 64
g. Supplemental Flight Information. The aircraft may not be used in part 133 external-load operations until a Rotorcraft-Load Combination Flight Manual is prepared in accordance with § 133.47 of that part. Appropriate entries should also be made in the aircraft’s weight and balance data, equipment list, and logbooks. The FAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), should also be executed as required by part 43, § 43.5(a) and (b).
h. Inspection and Maintenance. Inspection of the complete installation should be accomplished prior to each lift for security and functionality.
Maintenance should be accomplished in accordance with the manufacturers instructions.
705. THRU 706. RESERVED
FIGURE 7-4. TYPICAL CARGO SLING INSTALLATION NO. 1 (SHOWING FUSELAGE ATTACHMENT FITTING)
FIGURE 7-5. TYPICAL CARGO SLING INSTALLATION NO. 1 (SHOWING FORE AFT LIMITING STOPS)
Par 704 Page 65
FIGURE 7-6. TYPICAL CARGO SLING INSTALLATION NO. 2 (CARGO HOOK ATTACHED
DIRECTLY TO UNDERSIDE OF FUSELAGE)
FIGURE 7-7. TYPICAL CARGO SLING INSTALLATION NO. 3 (4-LEG, CABLE SUSPENDED)
FIGURE 7-8. TYPICAL CARGO SLING INSTALLATION NO. 3 (SHOWING CABLE SLING
LEG ATTACHMENT TO LANDING GEAR CROSSTUBE FITTING)
Par 704 Page 66
Par 704 Page 67 (and 68) FIGURE 7-9. TYPICAL CARGO SLING INSTALLATION NO. 3 (SHOWING CARGO SLING
IN STOWED POSITION)
Par 707 Page 69 SECTION 2. CARGO RACKS
707. GENERAL. This section contains structural and design information for the fabrication and installation of a cargo rack used as an external-load attaching means for a Class A rotorcraft-load combination operation under part 133.
708. FABRICATION OF CARGO RACKS. The type of construction and method of attachment depends upon the material to be used and the configuration of the rotorcraft involved. Illustrations of typical construction and installation methods are shown in Figures 7-10 through 7-14.
FIGURE 7-10. TYPICAL CARGO RACK INSTALLATION NO. 1
709. STATIC TEST. The cargo rack installation must be able to withstand the static test load required by § 133.43(a). Conduct the test as outlined in chapter 1.
710. SUPPLEMENTAL FLIGHT INFORM- ATION. The aircraft may not be used in part 133 external-load operations until a rotorcraft-load combination flight manual is prepared in accordance with § 133.47.
711. THRU 799. RESERVED
FIGURE 7-11. TYPICAL CARGO RACK INSTALLATION NO. 1 (SHOWING ATTACHMENT DETAIL)
FIGURE 7-12. TYPICAL CARGO RACK INSTALLATION NO. 2
Par 710 Page 70
Par 710 Page 71 (and 72) FIGURE 7-13. TYPICAL CARGO RACK INSTALLATION NO. 2 (SHOWING RACK PARTIALLY INSTALLED)
FIGURE 7-14. TYPICAL CARGO RACK INSTALLATION NO. 3
CHAPTER 8. GLIDER AND BANNER TOW-HITCH
INSTALLATIONS
800. PURPOSE. This chapter contains design and installation information for banner and glider tow hitches. Guidance for inspection, service, and continuous airworthiness requirements for the hitches are also addressed in this chapter.
801. HAZARDS AND WARNING. The direction and maximum arc of displacement of banner towline loads occur within a more limited rearward cone of displacement than do g lider towline loads (see Figure 8-1 and 8-2). Hitches that meet the banner tow criteria of this chapter may not be suitable for glider towing. Due to the basic aerodynamic differences between the two objects being towed, glider and banner tow-hitch installations are treated separately with regard to loading angles.
802. REFERENCES. This chapter will occasionally reference manufacturer’s documents, certain parts of Title 14 of the Code of Federal Regulations (14 CFR), and certain FAA advisory circulars (AC). It is the installer’s responsibility to ensure the latest revisions of these documents are used as reference material. Refer to Civil Aviation Regulation 6, Rotorcraft Airworthiness; Normal Category, and Order 8900.1, Volume 8, Chapter 5, Section 2, Changes to Special Purpose for Restricted Aircraft.
803. INSTALLATION CONSIDERATIONS.
a. Weight and Balance. In most cases, the weight of the tow-hitch assembly will affect the aft center of gravity (c.g.) location. To assure that the possibility of an adverse effect caused by the installation has not been ignored, enter all pertinent computations in the aircraft weight and balance records, in accordance with the provisions contained in 14 CFR part 43, § 43.5(c). The requirements of § 43.5(a) and (b) should also be addressed, for maintenance record entries and repair and alteration forms.
b. Equipment List. The aircraft equipment list should be updated to reflect any tow equipment installations. Consideration should also be given to adding appropriate revisions to the aircraft’s Pilot Operating Handbook (POH), or Flight Manual, as required.
c. Corrosion Protection. Tow hitches are traditionally simple mechanical devices; however, improper care may lead to hazardous conditions for the aircraft operator. The aft, external location of tow hitches exposes them to the elements, and proper corrosion protection methods should be employed to prevent improper operation or malfunction.
Reference should be made to the manufacturers documents or current edition of AC 43.13-1, Acceptable Method, techniques, and Practices―Aircraft Inspection and Repair, chapter 6, for some additional corrosion protective measures that may be used.
804. FABRICATION AND INSTALLATION PROCEDURES.
a. Methods. Accomplish the installation of tow hitches using manufacturer’s data or that data previously approved by a representative of the Administrator, when available, such as a Supplemental Type Certificate (STC) or a Field Approval. Installations requiring fabrication of brackets, parts, fittings, etc. should be accomplished using data in the form of ACs such as AC 43.13-1 (current edition), to determine material requirements, load requirements, and type and size of hardware used.
b. Structural Requirements. The structural integrity of a tow-hitch installation on an aircraft is dependent upon its intended usage.
c. Attachment Points . Tow-hitch mechanisms are characteristically attached to, or at, Par 800 Page 73
tie down points or tail wheel brackets on the airframe. These are points where the design loadbearing qualities may be sufficient for towing loads.
Keep the length from the airframe attachment point to the tow hook at a minimum as the loads on the attachment bolts are multiplied by increases in the attachment arm.
d. Glider Tow Hitch Load Testing.
(1) Protection for the tow-plane is provided by requiring use of a towline assembly that will break prior to structural damage occurring to the tow plane. The normal to w load imposed on the hitch rarely exceeds 80 percent of the weight of the glider. Therefore, the towline assembly design load for a 1,000-pound glider could be estimated at 800 pounds. By multiplying the estimated design load by 1.5 (to provide a safety margin), we arrive at a limit load value of 1,200 pounds. The 1,200-pound limit load value is used in static testing or analysis procedures per paragraph 8-2 to prove the strength of the tow hook installation. When the hook and its attachment to the aircraft structure have been proven to withstand the limit load, 1,200 pounds in this example, then the “maximum” breaking strength of the towline assembly is established at the design load of 800 pounds. Thus, the towline will break well before structural damage will occur to the towplane.
FIGURE 8-1. GLIDER TOW ANGLE
FIGURE 8-2. BANNER TOW ANGLE
(2) Another approach can be applied if the limit load carrying capabilities of a tow hook and fuselage are known. In this case, the known load value can be divided by 1.5 to arrive at the design load limits. For example, if the tow hook and fuselage limit loads are known to be 1,800 pounds, divide this by 1.5 (1,800 ÷ 1.5 = 1,200) and we arrive at a design load value of 1,200 pounds. Thus, if a towline assembly rated at 1,200 pounds is used it will break before reaching the structural limits of the hook and attaching structure.
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(3) When considering tow hook installations, one may establish maximum towline breaking strength by:
(a) Dividing the known limit load capabilities of the fuselage and tow hook installation by 1.5; or
(b) Knowing the design load needs of the towline assembly and multiplying by 1.5 to arrive at a limit load. Then analysis or static testing, determine that the hook and fuselage are capable of towing the load.
(4) Banner Tow Hitches. Install and test the hitch to support a limit load equal to at least two times the operating weight of the banner.
(5) Multiple Hitches. Multiple tow hitches are sometimes used on banner tow aircraft.
These installations should be evaluated individually for approval.
805. STRUCTURAL TESTING. When installations are made on an aircraft using brackets that have not been previously approved, some structural testing may be required. Adequacy of the aircraft structure to withstand the required loads can be determined by either st atic test or structural analysis.
a. Static Testing. When using static tests to verify structural strength, subject the tow hitch to the anticipated limit load. Aircraft tow hitches used for banner towing shall be tested a minimum of 2 times the weight of the heaviest banner to be towed, using the cone angle shown in Figure 8-2. Aircraft towline used for glider towing shall be tested to a minimum load approximately 80 percent of the weight of the heaviest glider to be towed, using the cone angle shown in Figure 8-1. Static testing should to be done in accordance with the procedures in chapter 1, paragraph 106.
b. Structural Analysis. If the local fuselage structure is not substantiated by static test for the proposed tow load, using a method that experience has shown to be reliable, subject the fuselage to engineering analysis to determine that the local structure is adequate. Use a fitting factor of 1.15 or greater in the loads for this analysis.
806. ANGLES OF TOW. Tests should be conducted on the system at various tow angles to insure that:
a. There is no interference with the tail wheel or adjacent structure. Tow-hitches mounted to tail wheel springs or trusses, as in Figures 8-6 and 8-7, which travel up and down with the tail wheel, should be tested under load to ensure they don’t contact any control surfaces.
b. The towline clears all fixed and movable surfaces at the maximum lateral and vertical cone of displacement and full surface travel.
c. The tow hitch does not swivel. Experience has shown swiveling could result in fouling both the release line and towline during operations by the tow plane.
d. The opened jaw of the hitch does not strike any portion of the aircraft.
e. The hitch is able to release under load at all tow angles.
807. PLACARDS. A placard should be installed in a conspicuous place in the cockpit to notify the pilot of the structural design limits of the tow system.
The following are examples of placards to be installed:
a. For glider tow “Glider towline assembly breaking strength not to exceed _____*______ pounds.” (*Value established per paragraph 804d.)
b. For banner tow “Tow hitch limited to banner maximum wei ght of _____**_____ pounds.” (**Banner hitch limitations are one-half the load applied per paragraph 804d.)
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808. INSTALLATION PROCEDURES ― TYPES OF HITCHES. Two types of glider tow hooks are used in the United States: the Americanmade Schweizer and the European-made TOST brand, with Schweizer being the most commonlyused in the United States at the time of this publication. The FAA has coordinated with Schweizer in developing detailed inspection procedures and identification of life-limiting parts.
These procedures and specifications are identified in subparagraph a. The European-made TOST releases are German-manufactured and approved by the German Civil Aviation Authority. They are available in various types and configurations. Two types of TOST releases can be found on U.S.
operated gliders. The “E-type” release mechanism is commonly used as a nose release for sailplanes and as a tail release on towing aircraft. The “G-type” release mechanism is typically used as a center of gravity release for winch launching. TOST has provided installation, inspection, and maintenance instructions for its systems. An overview of these systems is identified in subparagraph b. Regardless of the brand used, all tow hooks should be inspected for proper operation daily, prior to tow activity.
a. Schweizer Hitch: Installation and Maintenance. The Schweizer is a simple over center L-hook type with a rubber tension block to preload the release lever (Figure 8-10). It is eligible for installation on several models of Cessna and Piper aircraft by STC, and has been installed on many other aircraft using the Field Approval process. It uses a tow rope with a single round steel ring attaching to the hook. While this hitch is a simple mechanism, proper maintenance and frequent inspection are necessary to ensure proper operation. All initial installations should be proof tested using the procedures outlined in the latest revision of Schweitzer Aircraft Corp. (SAC) Form F-236. The following additional procedures are recommended as a minimum, and should be performed at each 100-hour annual inspection, unless otherwise stated. All initial installations should be proof-tested using the procedures outlined in the latest revision of SAC Form F-236. This form is available through Schweizer and can be found at the following Web address:
http://www.sacusa.com/support/ServiceLetter/ F-236.pdf.
(1) Inspect the entire tow hook system for loose or worn pivot pins, damaged fasteners, elongated holes, cracks, corrosion, surface damage, excessive wear, deformed parts, frayed release cable, rubber block damage, and freedom of operation. The mounting location of the hitches leaves them exposed to sunlight and the elements. Ozone and heat can have a detrimental effect on the rubber block. Look for excessive hardness of the rubber block as well as a permanent indentation caused by the contact with the hook lever.
(2) Perform a closing check by verifying that a sufficient closing force is required to compress the rubber block with the pivot hook. The pivot hook should apply sufficient locking load against the latch arm after the latch arm is engaged. Verify that the movement of the latch arm toward the release position causes additional compression of the rubber block. The original shape of the rubber block must be maintained.
(3) Perform a no-load pull test at the release arm to verify that a load of 4 to 10 pounds of pull is required to release the lever. See SAC Form F-236 for this procedure. If the release load cannot be obtained within the specified range, the rubber block is worn or deformed and should be replaced with a serviceable block.
NOTE: The above procedures, when properly implemented, help to ensure proper operation of the hitch. When the glider under tow operates above a certain angle to the tow plane, the ring may slide upwards on the hook, causing excessive load on the hook, and difficulty in releasing the tow rope ring. The closing check and pull test should be performed with a standard tow ring in the hook.
(4) For a periodic inspection, a closing check should be performed during “each” hookup of Par 808 Page 76