9-2
9.5.2 Structural Requirements. The structural integrity of a tow-hitch installation on an aircraft is dependent upon its intended usage.
9.5.3 Attachment Points. Tow-hitch mechanisms are characteristically attached to or at tiedown 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.
9.5.4 Glider Tow-Hitch Load Testing.
9.5.4.1 Towline Assembly. 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 tow 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 9.6 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 occurs to the tow-plane.
Figure 9-1. Glider Tow Angle Fig ure 9-2. Banner Tow Angle
9-3
9.5.4.2 Load Carrying Capabilities. 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.
9.5.4.3 Maximum Towline Breaking Strength. When considering tow hook installations, one may establish maximum towline breaking strength by:
1. Dividing the known limit load capabilities of the fuselage and tow hook installation by 1.5; or
2. Knowing the design load needs of the towline assembly and multiplying by 1.5 to arrive at a limit load. Then, through analysis or static testing, determine that the hook and fuselage are capable of towing the load.
9.5.4.4 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.
9.5.4.5 Multiple Hitches. Multiple tow hitches are sometimes used on banner tow aircraft. These installations should be evaluated individually for approval.
9.6 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 static testing or structural analysis.
9.6.1 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 two times the weight of the heaviest banner to be towed, using the cone angle shown in Figure 9-2. Aircraft towline used for glider towing shall be tested to a minimum load of approximately 80 percent of the weight of the heaviest glider to be towed using the cone angle shown in Figure 9-1. Static testing should be done in accordance with the procedures in paragraph 2.6.
9.6.2 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.
9.7 Angles of Tow. Tests should be conducted on the system at various tow angles to ensure that:
1. There is no interference with the tail wheel or adjacent structure. Tow hitches mounted to tail wheel springs or trusses, as in Figures 9-6, Conventional Gear
9-4 A ircraft—Shock Strut Type Tailwheel, and 9-7 , Conventional Gear Aircraft— Tubular Spring Type Tailwheel, which travel up and down with the tail wheel, should be tested under load to ensure they do not contact any control surfaces.
2.
The towline clears all fixed and movable surfaces at the maximum lateral and vertical cone of displacement and full surface travel.
3. 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.
4. The opened jaw of the hitch does not strike any portion of the aircraft.
5. The hitch is able to release under load at all tow angles.
9.8 Placards. A placard should be installed in a conspicuous place in the flight deck to notify the pilot of the structural design limits of the tow system. The following are examples of placards to be installed:
• For glider tow: “Glider towline assembly breaking strength not to exceed _____*______ pounds.” (*Value established per paragraph 9.5.4.)
• For banner tow: “Tow hitch limited to banner maximum weight of _____**_____ pounds.” (**Banner hitch limitations are one-half the load applied per paragraph 9.5.4.)
9.9 Installation Procedures―Types of Hitches. Two types of glider tow hooks are used in the United States: the American-made Schweizer and the European-made TOST brand.
The FAA has coordinated with Schweizer in developing detailed inspection procedures and identification of life-limiting parts. These procedures and specifications are identified in paragraph 9.9.1. The European-made TOST releases are German-manufactured and approved by the German Civil Aviation Authority (CAA). 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 CG release for winch launching. TOST has provided installation, inspection, and maintenance instructions for its systems. An overview of these systems is identified in paragraph 9.9.2. Regardless of the brand used, all tow hooks should be inspected for proper operation daily prior to tow activity.
9.9.1 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. 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 inspections are necessary to ensure proper operation. All initial installations should be proof tested. The proof test entails a total of two release-effort readings taken at the 1D112-8 arm assembly: one reading while a 600-pound load is applied to the hook and the other with a zero static load is applied to the hook. See Figure 9-10, Schweizer Type Hitch. The following additional procedures are
9-5 recommended as a minimum and should be performed at each 100-hour/annual inspection unless otherwise stated.
9.9.1.1 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 cables, 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.
9.9.1.2 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. A closing check should be performed during each hookup of the tow rope.
9.9.1.3 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 Figure 9-10. 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. Tow-plane and glider pilots should follow recommended launch and recovery procedures to help prevent this occurrence.
9.9.2 TOST Hitch. The TOST release is constructed using a steel housing that 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 for ease of tow release at high angles (see Figure 9-9, TOST Double Tow Rings). The life-limiting part is the release spring with a life expectancy of about 10,000 actuations or 2,000 launches. This assumes about five actuations in normal flight 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 the 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 the manufacturer’s brackets where possible. Use of any other installation equipment may
9-6 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.
9.10 Tow Release Mechanism.
9.10.1 Release Lever. A placard indicating the direction of operation should be installed to allay the possibility of confusion or inadvertent operation, and the design of the release lever should provide the following:
9.10.1.1 Convenience in operation.
9.10.1.2 Smooth and positive release operation.
9.10.1.3 Positioned so as to permit the pilot to easily exert a straight pull on the release handle.
9.10.1.4 Sufficient handle travel to allow for normal slack and stretch of the release cable.
9.10.1.5 A sufficient handle/lever ratio to ensure adequate release force when the tow line is under high loads (see Figure 9-3, Typical Tow-Hitch Release Handle).
9.10.1.6 Protection of cables from hazards, such as:
• Wear and abrasion during normal operation.
• Binding where cables pass through fairleads, pulleys, etc.
• Accidental release.
• Interference with or by other aircraft components.
• Freezing and moisture accumulation when fixed or flexible tubing guides are used.
9-7 Figure 9-3. Typical Tow-Hitch Release Handle
9.10.2 T est 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 with the procedure in paragraph 9.9.
9.10.3 Release Cable. Representative size and strength characteristics of steel release cable are as shown in Table 9-1; however, it is recommended that all internally installed release cables be 1/16 inch or larger.
Table 9-1. Representative Steel Cable Qualities Diameter Inches Nonflexible Carbon Steel 1 x 7 and 1 x 19 (MIL-DTL-87161) Flexible Carbon Steel 7 x 7 and 7 x 19 (MIL-DTL-83420) Breaking Strength (pounds) Pounds 100 Feet Breaking Strength (pounds) Pounds 100 Feet 1/32 185 0.25 - - 3/64 375 0.55 - - 1/16 500 0.85 480 0.75 5/64 800 1.40 - - 3/32 1,200 2.00 920 1.60
9-8 Figure 9-4. Tricycle Gear Aircraft Fig ure 9-5. Conventional Gear Aircraft—Leaf Spring Type Tailwheel
9-9 Figure 9-6. Conventional Gear Aircraft—Shock Strut Type Tailwheel Fig ure 9-7. Conventional Gear Aircraft—Tubular Spring Type Tailwheel
9-10 Figure 9-8. Typical TOST Hitch Fig ure 9-9. TOST Double Tow Rings
Figure 9-10. Schweizer Type Hitch MEASURE RELEASE EFFORT -→
- 1D112-8 ARM ASSY
2.5"
PROOF AND NO + 42AD PULL TEST.
1D112-16 ASSY- 1D112-15 ASSY
- 1.75" -
4.0" Test Load Release Effort Proof test 600 Ibs. 60 Ibs. (max.)
Proof and no-load pull test 0 (zero) Ibs. 4 Ibs. (min.) - 10 (max.)
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9-12 Figure 9-11. Typical Multiple Hitch for Banner Tow Fig ure 9-12. Typical TOST Mount for Tricycle Gear Aircraft
9-13 Figure 9-13. Typical TOST Conventional Leaf Spring Mount