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
Par 809 Page 78
FIGURE 8-5. CONVENTIONAL GEAR AIRCRAFT-LEAF SPRING TYPE TAILWHEEL
FIGURE 8-6. CONVENTIONAL GEAR AIRCRAFT-SHOCK STRUT TYPE TAILWHEEL
Par 809 Page 79
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
Par 809 Page 81
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
Par 903 Page 84
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.
Par 904 Page 85
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.
Par 906 Page 86
Par 908 Page 87 (and 88)
d. Buckles suitable for shoulder harness installations are defined by their release mechanism and come in three types:
(1) Lift lever,
(2) Push Button, and
(3) Rotary.
909. PARTS PRODUCED BY OWNERS/ OTHERS. Section 21.303(b)(2) allows owners or operators to produce parts for altering their own aircraft under certain conditions. Since approved restraints meeting TSO, PMA, or other standards are readily available, applying this regulation for the production of restraints is not advised, as owner produced restraints would also need to demonstrate proof of compliance with these standards. However, airframe attachment brackets or fittings might be candidates to be produced under this regulation. To qualify, the owner must have participated in controlling the design, manufacture, or quality of the part such as by:
a. Providing the manufacturer with the design or performance data from which to make the part.
The owner could provide the manufacturer with a part to be duplicated.
b. Providing the manufacturer with the materials from which to make the part.
c. Providing the manufacturer with fabrication processes or assembly methods to make the part.
d. Providing the quality control procedures to make the part.
e. Personally supervising the manufacture of the part.
(1) An example of this might be if the owner has discovered a desirable harness attachment bracket in an aircraft similar to his, and he creates a drawing to duplicate this bracket, specifying materials and performance standards. He could then either make the part himself or contract out for the manufacture of the part. When a mechanic installs the part, the maintenance record entry would include the installation information as required from the mechanic, and an entry by the owner that the part was produced under § 21.303(b)(2) by the owner for his aircraft. Note that this regulation provides no authority for the owner to install the part.
Furthermore, the mechanic will need to show that the installation meets minimum strength standards through static testing or stress analysis.
(2) Parts and kits to install harnesses are sometimes available from parts warehouses or individuals who supply components and instructions for harness installations without FAA approval such as STC, TSO, or PMA. The installer of these parts will need to obtain FAA approval, typically a field approval, for such installations.
910. THRU 912. RESERVED
Par 913 Page 89
SECTION 2. GEOMETRY AND ATTACHMENT
913. GENERAL.
a. Harness attachment points physically locate the shoulder harness relative to the occupant being restrained and establish the angles that will impose loads upon the aircraft structure. Careful selection of appropriate attachment points will maximize overall performance of the restraint system.
b. For best results, the restraint system should be anchored to the primary aircraft structure, defined as: “that structure which contributes significantly to resisting or transmitting flight or ground loads or may lead to an unsafe condition if failed.” 1 The structural attachment should be designed to spread the suddenly applied impact loads over as large an area of the structure as possible. The shoulder harness may be attached to selected secondary members that will deform slowly or collapse at a limited rate. This will assist in dissipating the high impact "G" loads to a level tolerable to the human body. However, the possibility of secondary members collapsing and creating a new impact hazard for the occupant, or making it difficult for an occupant to exit the aircraft, should not be overlooked.
914. MOUNTING CONFIGURATIONS. T h e type of shoulder restraint configuration acceptable for installation is dependent upon the attachments available in the aircraft. Basic harness mounting configurations are:
a. Seat mounted.
b. Airframe mounted.
(1) Side.
(2) Ceiling.
(3) Floor.
(4) Directly rearward.
1 AC 23-15A, Small Airplane Certification Compliance Program, p. 1.
915. BELT LENGTH. In all installations, it is desirable to keep the harness belt lengths as short as practical while still allowing for the required freedom of movement. Belt stretching, which reduces the effectiveness of the restraint by allowing slack during loading, increases as the belt length does. If too much slack is present, the occupant may contact the instrument panel or slide out of the harness during rapid decelerations.
916. ATTACHMENT GEOMETRY. The following are general guidelines for attachment of 3-point and 4/5-point harness configurations:
a. Single Diagonal 3-Point Harness. A proper installation for this type of harness positions the shoulder belt so that it passes over the midpoint of the shoulder, with the lower end fastened well to the opposite side of the occupant’s hip as shown in Figure 9-2. The optimum rearward attachment area for this type of harness is within an angle of 30-degrees above the horizontal measured from the midpoint on the occupant's shoulder as shown in Figure 9-3. Upper harness attachments should be located to the rear and outboard of the occupant’s neck. This mounting area is shown in Figure 9-4.
(1) Attachment points inboard of this area would allow the harness to impinge on the neck and could result in neck injury during crash impact. (See Figure 9-5.) In addition, the constant rubbing of the strap on the neck would be uncomfortable and, as a result, act as a distraction to the safe operation of the aircraft and a deterrent to use of the harness.
(2) Attachment points forward of this area would reduce the effectiveness of the harness, due to a reduction of contact between the harness and the upper torso of the occupant. (See Figure 9-6.) As a result, the potential for increased forward movement of the torso, increases the likelihood of head impact injuries from the instrument panel. Additionally, the chances for twisting out of the harness are significantly increased.