(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
FIGURE 8-13. TYPICAL TOST CONVENTIONAL LEAF SPRING MOUNT
CHAPTER 9. SHOULDER HARNESS INSTALLATIONS SECTION 1. GENERAL
900. 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 airc raft manufacturer or Supplemental Type Cer tificate (STC) retrofit installations have not been developed.
901. 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 Section 3, Table 9-1, Static Test Requirements.
902. ADDITIONAL REFERENCES. The following references (current editions) provide additional information for shoulder harness installations.
a. Advisory Circular (AC) 21-34, Shoulder Harness-Safety Belt Installations.
b. AC 23-17, Systems and Equipment Guide for Certification of Part 23 Airplanes and Airships.
c. AC 91-65, Use of Shoulder Harnesses in Passenger Seats.
d. Civil Aviation Regulation 6, Rotorcraft Airworthiness; Normal Category.
e. TSO-C22, Safety Belts.
f. TSO-C114, Torso Restraint Systems.
g. Aerospace Standard SAE, AS8043, Restraint Systems for Civil Aircraft.
903. INSTALLATION METHODS. Shoulder harness installations can be performed by minor or by major alterations to the type design, depending on the complexity.
a. 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 9-1.)
Two examples of minor alterations for shoulder harnesses are:
(1) Some aircraft ma nufacturers have included hard-points 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 as required.
(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. Refer to chapter 1 to determine design loads.
b. Major alterations can be accomplished by one or more of the fo llowing methods and will Par 900 Page 83
require completion of the FAA Form 337 using approved data.
(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 of aircraft. A listing of STCs can be searched for applicability on the FAA Web site.
(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 make /model, with a similar mounting configuration, but the STC does not cover the aircraft by year of manufacture or serial number.
(3) Designated Engineering Representatives (DER) Data. DERs can evaluate structural attachments and provide approved data on FAA Form 8110-3. 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.
(4) Manufacturer Data. Some manufacturers have developed service kits or service instructions that are FAA-approved for harness retrofit installations. 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 installati on 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.
(5) Other FAA-Approved Data. As specified in the Introduction, 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.
FIGURE 9-1. MAJOR VS. MINOR ALTERATIONS
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904. RESTRAINT SYSTEM CONFIGUR- ATIONS. Restraint systems incorporating a shoulder harness are available in three configurations:
a. The 3-point system consists of a single shoulder belt that is positioned diagonally across the occupant’s upper torso. (See Figure 9-2.)
b. The 4-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 tw o 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 9-8.)
c. The 5-point system is similar to the 4-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 5-point system is the 6-point system where 2 belts pass between the occupant’s legs.
(See Figure 9-9.)
905. ADVANTAGES OF DIFFERENT CON- FIGURATIONS.
a. The 3-point single diagonal shoulder harness in combination with a lap belt is the leastcost, 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.
b. The 4-point dual shoulder harness works well for both longitudinal and lateral decelerations.
c. The 5-point system, incorporating the negative-G strap, resists the upward motion of the buckle during loading and limits submarining, which is the tendency for the occupant to slide underneath the lap belt during rapid decelerations. The 5-point system has proven to be very effective and has been adopted for many commercial, agricultural, military, and aerobatic operations.
906. MANUFACTURING STANDARDS. There are several standards that aircraft restraint system may be manufactured to.
a. Restraint Systems Produced by or for the Aircraft Manufacturer. These Original Equipment Manufacturer (OEM) harnesses may bear the manufacturer’s part number or other identification.
b. Restraint systems produced under a Parts Manufacturing Approval (PMA) with specific aircraft eligibility as listed in the PMA. Restraints will bear the PMA markings required by § 45.15(a).
c. Technical Standard Order (TSO) Restraints. Two TSOs are applicable to the performance standards of restraint systems. These TSOs do not approve the restraint for installation in the aircraft. Restraints approved to a TSO will be marked in accordance with § 21.607(d) and the specific TSO marking instructions.TSO restraints will be marked in accordance with § 21.607(d).
(1) TSO-C22G, Safety Belts, prescribes the minimum performance st andards that safety belts must meet in order to be identified with the applicable TSO marking. This TSO applies to the pelvic or lap belt portion of a restraint system. Belts bearing an earlier revision TSO marking (e.g., C22f) are acceptable for continued use if their condition remains satisfactory.
(2) TSO-C114, Torso Restraint Systems, prescribes the minimum performance standards that torso restraint systems must meet in order to be identified with the applicable TSO marking. This TSO applies to pelvic and upper torso restraints and includes the fifth belt of a 5-point system. Harnesses manufactured prior to March 27, 1987, the effective date of this TSO, will not be marked as meeting this TSO.
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d. Restraint systems produced under a Military Specification (MIL-SPEC) such as MIL-R-81729. These restraints are marked in accordance with MIL-STD-130. FAA-approved data will need to be obtained for installation of restraints designed to MIL-SPEC standards.
907. COMPLIANCE WITH STANDARDS.
a. Prior to March 27, 1987, the TSO-C114 shoulder harness standards were established.
Lacking such standards, harnesses manufactured prior to this date often were not identified with any markings. These harnes ses are acceptable for existing installations if they were installed before the effective date of this TSO and remain in satisfactory condition.
b. For harness installations performed under this AC, use only those restraints that are properly marked and traceable to one of the above standards.
There are several restraint manufacturers who can custom-build restraint system components that are manufactured to and identified with TSO-C114 markings. There are also several companies that can manufacture restraints under a PMA for specific applications.
c. It is an acceptable practice to replace existing restraints, including lap belts, with OEM, TSO, or PMA units afte r compatibility has been determined. This can be performed as preventive maintenance, defined under part 43, appendix A, paragraph (c)(14), by the aircraft owner/operator, along with the required maintenance record entry.
However, if the aircraft is operated under part 121, 129, or 135, the work mu st be accomplished by appropriately rated mechanic.
d. Used TSO restraints, typically obtained through salvage companies, must be overhauled by an FAA-approved facility in accordance with an FAA-approved specification prior to installation, as these harnesses may have been exposed to unknown environmental conditions or accident loads.
908. MATERIALS. For a more thorough discussion of restraint assembly materials, refer to AC 21-34, Shoulder Harness― Safety Belt Installations, as amended.
a. Webbing, the woven fabric portion of the restraint, is made from synthetic materials such as nylon or polyester. Minimum breaking strength is determined by the standard under which the restraint was manufactured.
b. Attachment hardware must conform to AN, MS, NAS, or other acceptable industry standards or specifications, and be able to withstand the loads it will be subjected to.
c. Retractors are frequently incorporated into the shoulder harnesses, and sometimes into pelvic restraints. Some current production aircraft incorporate retractors into the harness and the lap belt. Retractors function to provide for adjustment in length and allow the occupant additional freedom of movement when compared to fixed harnesses. They are available in two categories:
(1) Automatic locking retractors provide automatic retraction of webbing for length adjustment and stowage of webbing. Their mechanism permits free webbing extension for coupling of the belt, but the moment any webbing is automatically retracted, the locking mechanism locks to prevent further webbing extension.
(2) Emergency locking retractors are frequently called “inertia reels” because their mechanism provides positive restraint only when inertial forces are experienced. The most common type of inertia reel appropriate for aircraft use is known as the webbing sensitive reel. It produces locking by a change in the rate (acceleration) of webbing withdrawal from the retractor, which is functional for occupant accelerations in any direction producing extension of the webbing.
Emergency locking retractors may be equipped with a mechanism that will allow the user to manually lock the reel when a deceleration is anticipated. This feature relieves the possibility of a malfunctioning inertia mechanism.
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