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.)
9-11
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
10-1 CHAPTER 10. SHOULDER HARNESS INSTALLATIONS Section 1. General
10.1 Purpose. The purpose of this chapter is to provide guidance for retrofit shoulder harness installations. Information contained herein may be adaptable for the installation of shoulder harnesses in aircraft for which aircraft manufacturer or Supplemental Type Certificate (STC) retrofit installations have not been developed.
10.2 Hazards and Warnings. Installations that do not meet the minimum standards prescribed by regulations are not acceptable. At no time should a retrofit shoulder harness installation perform at less than the static test load requirements specified in Table 10-1, Static Test Requirements.
10.3 Additional References. The following references provide additional information for shoulder harness installations.
• AC 21-34, Shoulder Harness-Safety Belt Installations.
• AC 23-17, Systems and Equipment Guide for Certification of Part 23 Airplanes and Airships.
• AC 91-65, Use of Shoulder Harness in Passenger Seats.
• Civil Air Regulation (CAR) 6, Rotorcraft Airworthiness; Normal Category.
• Technical Standard Order (TSO)-C22g, Safety Belts.
• TSO-C114, Torso Restraint Systems.
• Society of Automotive Engineers (SAE) Aerospace Standard (AS) AS8043, Restraint Systems for Civil Aircraft.
10.4 Installation Methods. Shoulder harness installations can be performed by minor or by major alterations to the type design depending on the complexity.
10.4.1 Minor alterations are limited to those where no change in the aircraft structure is required for mounting the harness. If the installation does not require operations such as drilling holes into or welding brackets onto the primary structure, it could be classified as a minor alteration (see Figure 10-1, Major Versus Minor Alterations). Two examples of minor alterations for shoulder harnesses are:
10.4.1.1 Some aircraft manufacturers have included hardpoints in the type design, such as nutplates or predrilled holes, for the mounting of harnesses. Some also provide service kits or service instructions that include parts and instructions necessary to install harnesses. If the harness installation does not involve modification of primary structures, it can be returned to service as a minor alteration unless otherwise specified in the installation instructions. The authorized mechanic needs only to complete a maintenance record entry and update the equipment list and Weight and Balance (W&B) as required.
10-2
10.4.1.2 In some instances, a cable or a bracket can be secured around a structural member without altering the structure, which will accommodate attachment of the harness. Truss tube construction is most commonly retrofitted with harnesses using this method. After performing static load tests or obtaining stress analysis documentation, the record entries as described above are completed. See Chapter 2, Structural Data, to determine design loads.
10.4.2 Major alterations can be accomplished by one or more of the following methods and will require completion of the FAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), using approved data. Refer to 14 CFR part 43.
10.4.2.1 STC. There are many STCs issued for installation of shoulder harnesses in a variety of aircraft. The STCs are issued for specific makes and models (M/M) of aircraft. A listing of STCs can be searched for applicability on the FAA website.
10.4.2.2 Field Approval. A properly trained and certified FAA Airworthiness aviation safety inspector (ASI) can field approve data under certain conditions. ASIs are not engineers, so unless the case for field approval is supported with adequate data to evaluate the installation, the request for field approval may be turned down. An example of justification for field approval might be if an STC exists for the same M/M with a similar mounting configuration but the STC does not cover the aircraft by year of manufacture or serial number.
10.4.2.3 Designated Engineering Representatives (DER) Data. DERs can evaluate structural attachments and provide approved data on FAA Form 8110-3, Determination of Compliance With Airworthiness Standards. This data can be substituted for static tests, allowing the installation to be approved for return to service on the FAA Form 337 by an appropriately authorized person.
10.4.2.4 Manufacturer Data. Some manufacturers have developed service kits or service instructions that are FAA-approved for harness retrofit installations.
Depending on the complexity, these installations may be performed as major or minor alterations. For example, if the installation consists of affixing harness assemblies to existing hard points, such as nutplates, the installation could be classified as minor with no requirement to complete FAA Form 337.
If the kit is FAA-approved but results in modification to the airframe structure, FAA Form 337 must be completed, referencing the approved kit/instructions, with no additional approval required.
10.4.2.5 Other FAA-Approved Data. As specified in the Chapter 1, General, the data contained herein may conditionally be used as approved data. Data such as static tests, attachments, and materials could be used to show that the installation complies with regulations. Previously performed static tests meeting the minimum standards specified in this AC may be applicable for a retrofit installation if adequately documented.
10-3 Figure 10-1. Major Versus Minor Alterations
10.5 Restraint System Configurations. Restraint systems incorporating a shoulder harness are available in three configurations.
10.5.1 The three-point system consists of a single shoulder belt that is positioned diagonally across the occupant’s upper torso (see Figure 10-2, The Three-Point Single Harness Positioning).
10.5.2 The four-point system consists of dual shoulder belts with one belt passing over each of the occupant’s shoulders. Upper end attachment is accomplished at either two locations aft of the occupant’s shoulders or at one location aft of the occupant’s head after the belts have been joined together in a “Y.” Lower end attachment is accomplished at a buckle centered on the occupant’s lap belt or symmetrically at each side of the occupant (see Figure 10-8, The Four-Point Independent Harnesses Positioning).
10.5.3 The five-point system is similar to the four-point system except that an additional belt, the negative-G strap, commonly referred to as a “crotch strap,” is passed between the occupant’s legs, attaching one end at the lap belt and the other end at the front edge of the seat or to the airframe under the seat. A variation of the five-point system is the six-point system where two belts pass between the occupant’s legs (see Figure 10-9, The Five-Point Harness Positioning).
10.6 Advantages of Different Configurations.
10.6.1 The three-point single diagonal shoulder harness in combination with a lap belt is the least-cost, most-simple restraint system and has been proven to work effectively for longitudinal (forward) decelerations. However, during lateral (sideward) decelerations away from the shoulder harness, an occupant in this type of harness has a tendency to slip out from the harness even when it fits snugly.