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.
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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.
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10.6.2 The four-point dual shoulder harness works well for both longitudinal and lateral decelerations.
10.6.3 The five-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 five-point system has proven to be very effective and has been adopted for many commercial, agricultural, military, and aerobatic operations.
10.7 Manufacturing Standards. There are several standards that aircraft restraint systems may be manufactured to.
10.7.1 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.
10.7.2 Restraint Systems Produced Under a Parts Manufacturer Approval (PMA). Restraint systems produced under a PMA, with specific aircraft eligibility listed in the PMA, will bear the PMA markings required by 14 CFR § 45.15(a).
10.7.3 TSO Restraints. Two TSOs are applicable to the performance standards of restraint systems. These TSOs do not approve the restraint system for installation in the aircraft.
Restraint systems approved to a TSO will be marked in accordance with 14 CFR § 45.15(b) and the specific TSO marking instructions.
10.7.3.1 TSO-C22g, Safety Belts, prescribes the minimum performance standards 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.
10.7.3.2 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 five-point system. Harnesses manufactured prior to March 27, 1987, the effective date of this TSO, will not be marked as meeting this TSO.
10.7.4 Restraint Systems Produced Under a Military Specification (MIL-SPEC). Restraint systems produced under a MIL-SPEC, such as MIL-R-81729, 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.
10.8 Compliance With Standards.
10.8.1 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 harnesses are acceptable for existing installations if
10-5 they were installed before the effective date of this TSO and remain in satisfactory condition.
10.8.2 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.
10.8.3 It is acceptable to replace existing restraints, including lap belts, with OEM, TSO, or PMA units after compatibility has been determined. The aircraft owner/operator can perform this as preventive maintenance, defined under 14 CFR part 43 appendix A, paragraph (c)(14), along with the required maintenance record entry. However, if the aircraft is operated under 14 CFR part 121, 129, or 135, the work must be accomplished by an appropriately rated mechanic.
10.8.4 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.
10.9 Materials. For a more thorough discussion of restraint assembly materials, refer to AC 21-34.
10.9.1 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.
10.9.2 Attachment hardware must conform to Army/Navy and Air Force/Navy (AN), Military Standard (MS), National Aerospace Standards (NAS), or other acceptable industry standards or specifications and be able to withstand the loads it will be subjected to.
10.9.3 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.
10.9.3.1 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.
10.9.3.2 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
10-6 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.
10.9.4 Buckles suitable for shoulder harness installations are defined by their release mechanism and come in three types:
• Lift lever,
• Push button, and
• Rotary.
10.10 Parts Produced by Owners/Others. Title 14 CFR § 21.9 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:
10.10.1 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.
10.10.2 Providing the manufacturer with the materials from which to make the part.
10.10.3 Providing the manufacturer with fabrication processes or assembly methods to make the part.
10.10.4 Providing the quality control procedures to make the part.
10.10.5 Personally supervising the manufacture of the part.
10.10.5.1 An example of this might be if the owner has discovered a desirable harness attachment bracket in an aircraft similar to their own, and they create a drawing to duplicate this bracket, specifying materials and performance standards. They could then either make the part themself 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 14 CFR § 21.9 by the owner for their 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.
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10.10.5.2 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.
10-8 Section 2. Geometry and Attachment
10.11 General.
10.11.1 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 the restraint system’s overall performance.
10.11.2 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.
1 AC 23-15, Small Airplane Certification Compliance Program, subparagraph 1a.
10.12 Mounting Configurations. The type of shoulder restraint configuration acceptable for installation is dependent upon the attachments available in the aircraft. Basic harness mounting configurations are:
10.12.1 Seat mounted.
10.12.2 Airframe mounted:
• Side.
• Ceiling.
• Floor.
• Directly rearward.
10.13 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 increases. If too much slack is present, the occupant may contact the instrument panel or slide out of the harness during rapid decelerations.
10.14 Attachment Geometry. The following are general guidelines for attachment of three-point and four/five-point harness configurations:
10.14.1 Single Diagonal Three-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
10-9 lower end fastened well to the opposite side of the occupant’s hip, as shown in Figure 10-2. 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 10-3, Side Mounted Single Diagonal Type Harness. Upper harness attachments should be located to the rear and outboard of the occupant’s neck.
This mounting area is shown in Figure 10-4, Acceptable Harness Mounting Areas.
10.14.1.1 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 10-5, Undesirable Horizontal Harness Positioning). 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.
10.14.1.2 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 10-6, Undesirable Vertical Harness Positioning).
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 of twisting out of the harness are significantly increased.
10.14.1.3 A retractor can be used with the diagonal harness. It has the added benefit of allowing for more freedom of movement, especially when controls such as fuel selectors are located on the flight deck floor. However, it may be more complicated to mount a retractor and maintain an unrestricted, straight-line entry and exit of the webbing.
10.14.2 Double Over-the-Shoulder Four/Five-Point Harness. This type of harness should be mounted either directly rearward or to the ceiling. Ideally, the mount should be within the 30-degree vertical angle, or, because of the limited number of rearward shoulder harness attachment points in many aircraft, a five-degree angle below the horizontal is also considered satisfactory, as shown in Figure 10-7, Ceiling Mounted Inertia Reel―Double Over-the-Shoulder Type Harness. These mounting angles may be used for either the dual independent or the “Y” type belts. Figures 10-8 and 10-9 depict correct harness positioning of four/five-point harnesses.
10.14.2.1 If the harness attachment is located more than 5 degrees below the horizontal angle measured from the midpoint on the occupant’s shoulder, there is an increased risk of spinal compression caused by the vertical component induced during impact deceleration (see Figure 10-6).
10.14.2.2 For dual independent harnesses, the outboard limit must be established to provide sufficient contact that will prevent the belt from slipping off the shoulder, and the maximum inboard angle is limited to a point that will prevent impingement on the neck (see Figure 10-5). Where the mounting structure is incapable of withstanding loads imposed by a “Y” type harness
10-10 single retractor, two retractors used with dual independent harnesses may spread the load enough to satisfy strength requirements. Dual independent harnesses that are crossed aft of the occupant’s head will need careful evaluation to preclude neck impingement.
10.14.2.3 Retractors are often used with “Y” type and dual independent harnesses. The single retractor used with a “Y” type harness is mounted aft of the center vertical plane of the occupant (see Figures 10-4 and 10-10, “Y” Type Retractor Positioning). The fifth belt, if used, is attached to the seat or airframe so that it joins the buckle perpendicular to the lap belt in the centerline of the seat. Figure 10-9 shows a five-point system.
Figure 10-2. The Three-Point Single Harness Positioning
10-11 Figure 10-3. Side Mounted Single Diagonal Type Harness Fig ure 10-4. Acceptable Harness Mounting Areas