FIGURE 7-11. TYPICAL CARGO RACK INSTALLATION NO. 1 (SHOWING ATTACHMENT DETAIL)
FIGURE 7-12. TYPICAL CARGO RACK INSTALLATION NO. 2
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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
Par 808 Page 77 the tow rope. Tow plane and glider pilots should follow recommended launch and recovery procedures to help prevent this occurrence.
b. TOST Hitch. The TOST release is constructed using a steel housing which contains the hook mechanism surrounded by a steel ring. The tow rope attaches to a connecting ring pair: an oblong ring and a round ring, looped through each other. This ring pair must meet required specifications. Welded ring pairs are prohibited. The ring surrounding the hook mechanism, along with the double rings on the tow rope, allows ease of tow release at high angles (see Figure 8-9). The lifelimiting part is the release spring, with a life expectancy of about 10,000 actuations or 2000 launches. This assumes about five actuations in normal night operations. If more actuations occur per launch, the time between overhaul is reduced accordingly. Once life expectancy is met, the release must be returned to th e manufacturer or TOST representative for complete reconditioning and regulatory check. TOST recommends a general overhaul after 4 years: Its safety and operating life can only be guaranteed by keeping the prescribed maintenance intervals. Installation of the TOST release should be accomplished using manufacturer's brackets where possible. Use of any other installation equipment may require additional evaluation. In all cases, the installation, adjustment, maintenance, inspection, and overhaul intervals should be accomplished in accordance with the latest revisions of the TOST Installation and operating manuals for tow releases.
WARNING NOTE: Because of the difference in the size of the steel rings, under no circumstances should the Schweizer single ring rope be used with the TOST hook, nor the TOST double ring rope with the Schweizer hook.
809. TOW RELEASE MECHANISM.
a. Release Lever. A placard indicating the direction of operation s hould be installed to allay the possibility of confusion or inadvertent operation, and the design of the release lever should provide the following:
(1) Convenience in operation.
(2) Smooth and positive release operation.
(3) Positioned so as to permit the pilot to easily exert a straight pull on the release handle.
(4) Sufficient handle travel to allow for normal slack and stretch of the release cable.
(5) A sufficient handle/lever ratio to assure adequate release force when the tow line is under high loads. (See Figure 8-3.)
(6) Protection of cables from hazards such as:
(a) Wear and abrasion during normal operation.
(b) Binding where cables pass through fairleads, pulleys, etc.
(c) Accidental release.
(d) Interference with or by other aircraft components.
(e) Freezing and moisture accumulation when fixed or flexible tubing guides are used.
FIGURE 8-3. TYPICAL TOW-HITCH
RELEASE HANDLE
b. Test of Release Lever. Depending on the installation, the Schweizer type hitch is susceptible to excessive release loads when the tow cable is at high positive angles, and should be tested before each tow in accordance w ith the procedure in paragraph 808.
c. Release Cable. Representative size and strength characteristics of steel release cable are as shown in Table 8-1; however, it is recommended that all internally inst alled release cables be 1/16-inch or larger.
TABLE 8-1. REPRESENTATIVE STEEL
CABLE QUALITIES Nonflexible Carbon Steel 1 x 7 and 1 x 19 (MIL-W-6904B) Flexible Carbon Steel 7 x 7 and 7 x 19 (MIL-W-1511A and MIL-C-5424A)
Diameter inches Breaking strength (lbs.)
Pounds 100 ft.
Breaking Strength (lbs.)
Pounds 100 ft.
1/32 185 .25 --- --- 3/64 375 .55 --- --- 1/16 500 .85 480 .75 5/64 800 1.40 --- --- 3/32 1,200 2.00 920 1.60
810. THRU 899. RESERVED
FIGURE 8-4. TRICYCLE GEAR AIRCRAFT
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FIGURE 8-5. CONVENTIONAL GEAR AIRCRAFT-LEAF SPRING TYPE TAILWHEEL
FIGURE 8-6. CONVENTIONAL GEAR AIRCRAFT-SHOCK STRUT TYPE TAILWHEEL
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Par 809 Page 80 FIGURE 8-7. CONVENTIONAL GEAR AIRCRAFT – TUBULAR SPRING TYPE TAILWHEEL
FIGURE 8-8. TYPICAL TOST HITCH FIGURE 8-9. TOST DOUBLE TOW RINGS
FIGURE 8-10. SCWEIZER WELD ON TYPE HITCH
FIGURE 8-11. TYPICAL MULTIPLE HITCH FOR BANNER TOW
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Par 809 Page 82 FIGURE 8-12. TYPICAL TOST MOUNT FOR TRICYCLE GEAR AIRCRAFT