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
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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.
(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 cockpit floor. Howe ver, it may be more complicated to mount a retractor and maintain an unrestricted, straight-line entry and exit of the webbing.
b. Double Over-the-Shoulder 4/5-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 5-degree angle below the horizontal is also considered satisfactory, as shown in Figure 9-7.
These mounting angles may be used for either the dual independent or the “Y” type belts. Figures 9-8 and 9-9 depict correct harness positioning of 4/5-point harnesses.
(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 9-6.)
(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 poi nt that will prevent impingement on the neck. (See Figure 9-5.) Where the mounting structure is incapable of withstanding loads imposed by a “Y” type harness 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.
(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 9-4 and 9-10.) 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 9-9 shows a 5-point system.
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FIGURE 9-2. THE 3-POINT SINGLE HARNESS POSITIONING
FIGURE 9-3. SIDE MOUNTED SINGLE DIAGONAL TYPE HARNESS
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FIGURE 9-4. ACCEPTABLE HARNESS MOUNTING AREAS
FIGURE 9-5. UNDESIRABLE HORIZONTAL HARNESS POSITIONING
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Par 916 Page 93 FIGURE 9-6. UNDESIRABLE VERTICAL HARNESS POSITIONING
FIGURE 9-7. CEILING MOUNTED INERTIA REEL―DOUBLE OVER-THE-SHOULDER
TYPE HARNESS
FIGURE 9-8. THE 4-POINT INDEPENDENT HARNESSES POSITIONING
FIGURE 9-9. THE 5-POINT HARNESS POSITIONING
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Par 917 Page 95 FIGURE 9-10. “Y” TYPE RETRACTOR POSITIONING
917. AREA AND ANGLE DEVIATIONS. While the areas and angles given in the above paragraphs are intended to assist in the selection of attachment points, they should be considered the desirable optimum. Area and/or angle deviations could result in a decrease in the overall efficiency of the restraint system; however, they may be necessary in order to permit harness installation in an aircraft that cannot accommodate ideal harness geometry. As discussed in section 3, angles have a significant bearing on static test load requirements.
918. STRUCTURAL ATTACHMENT. This chapter presents only a few generic examples of structural attachment of harnesses that may be determined to be acceptable for a specific application. Refer to section 3 for strength requirements of the attachment.
a. Floor and Seat Attachments. The dual over-the-shoulder or Y type harness may be used with either floor or seat mounting points. Typical installations are illustrated in Figures 9-11, 9-12, and
9-13. Several factors need to be considered that may make this configuration undesirable:
(1) The floor, seat structure, and anchorages must be capable of withstanding the additional “G” loads imposed upon them by the restraints.
(2) The height of the seat back should at least be equal to the shoulder height of the seated occupant to reduce the possibility of spinal compression injuries.
(3) Harness guides may be necessary to maintain proper harness position over the occupant’s shoulders.
(4) Seat back strength is critical to performance of these installations. Folding seat backs must have a locking mechanism that can withstand the imposed loads; however, a locked seat back may impede emergency egress of other occupants and create an impact hazard for occupants seated aft of the locked seat back.
FIGURE 9-11. FLOOR MOUNTED INERTIA REEL―DOUBLE OVER-THE-SHOULDER
TYPE HARNESS
FIGURE 9-12. TYPICAL FLOOR MOUNTED INERTIA REEL INSTALLATION
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Par 918 Page 97 FIGURE 9-13. TYPICAL SEAT MOUNTED INERTIA REEL INSTALLATION
b. Airframe Attachments. The method used for the attachment of shoulder harness anchorages is dependent upon the construction features of the aircraft involved. When selecting a point of attachment, careful consideration should be given to the static strength and testing requirements found in section 3.
(1) Monocoque/Semimonocoque Type Constructions. Illustrations of typical aircraft members and installation methods are shown in Figures 9-14 through 9-18. Intercostal doublers and stiffeners are frequently added to provide increased strength. AC 43.13-1, Acceptable Methods, Techniques, and Practices-Aircraft Inspection and Repairs, chapter 4 (as amended), provides useful information on fabrication and installation of sheet metal repairs that may be applicable to doubler and stiffiner installations.
(2) Tube Type Construction. Various typical methods of attaching shoulder harness anchorages are shown in Figure 9-19. Attachment should be accomplished at the intersection of tubular members and not at the center of single unsupported tubes. When aircraft cable is used as a component in a shoulder harness anchorage, swage the cable terminals in accordance with the procedures contained in AC 43.13-1, chapter 7, as amended.
c. Structural Repair Instructions. In some instances, structural repair instructions are provided in the aircraft manufacturer’s maintenance manual.
While these instructions are primarily intended for use in repairing defective or damaged structure, they may also be used as reinforcement methods for shoulder harness attach fittings.
d. Flexible Attachments. Various aircraft are designed so that fuselage members and/or skin will flex or “work.” This type of structure should not be heavily reinforced for th e purpose of attaching shoulder harnesses, as this would defeat the design purpose. In these case s, use a localized
reinforcement such as that shown in Figure 9-18, at the attachment point. This will allow the fuselage to flex while still maintaining a collapsible structure to absorb the loads encountered in a crash.
919. THRU 922. RESERVED
FIGURE 9-14. TYPICAL WING CARRY-THROUGH INSTALLATION
FIGURE 9-15. TYPICAL HAT SECTION REINFORCEMENT INSTALLATION
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FIGURE 9-16. TYPICAL BULKHEAD REINFORCEMENT INSTALLATION
FIGURE 9-17. TYPICAL STRINGER SECTION REINFORCEMENT INSTALLATION
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