10-7
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
10-12 Figure 10-5. Undesirable Horizontal Harness Positioning Fig ure 10-6. Undesirable Vertical Harness Positioning
10-13 Figure 10-7. Ceiling Mounted Inertia Reel―Double Over-the-Shoulder Type Harness Fig ure 10-8. The Four-Point Independent Harnesses Positioning
10-14 Figure 10-9. The Five-Point Harness Positioning Fig ure 10-10. “Y” Type Retractor Positioning
10-15
10.15 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 or angle deviations, or both, could decrease 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, Static Strength and Testing, angles have a significant bearing on static test load requirements.
10.16 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. See Section 3 for strength requirements of the attachment.
10.16.1 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 10-11, Floor Mounted Inertia Reel―Double Over-the-Shoulder Type Harness;
10-12, Typical Floor Mounted Inertia Reel Installation; and 10-13, Typical Seat Mounted Inertia Reel Installation. Several factors need to be considered that may make this configuration undesirable.
10.16.1.1 The floor, seat structure, and anchorages must be capable of withstanding the additional “G” loads imposed upon them by the restraints.
10.16.1.2 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.
10.16.1.3 Harness guides may be necessary to maintain proper harness position over the occupant’s shoulders.
10.16.1.4 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.
10-16 Figure 10-11. Floor Mounted Inertia Reel―Double Over-the-Shoulder Type Harness Fig ure 10-12. Typical Floor Mounted Inertia Reel Installation
10-17 Figure 10-13. Typical Seat Mounted Inertia Reel Installation
10.16.2 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.
10.16.2.1 Monocoque/Semimonocoque Type Constructions. Illustrations of typical aircraft members and installation methods are shown in Figures 10-14, Typical Wing Carry-Through Installation, through 10-18, Typical Localized Stringer Section Reinforcement Installation. Intercostal doublers and stiffeners are frequently added to provide increased strength. AC 43.13-1, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair, Chapter 4, Metal Structure, Welding, and Brazing, provides useful information on fabrication and installation of sheet metal repairs that may be applicable to doubler and stiffener installations.
10.16.2.2 Tube Type Construction. Various typical methods of attaching shoulder harness anchorages are shown in Figure 10-19, Typical Attachments to Tubular Members. 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, Aircraft Hardware, Control Cables, and Turnbuckles.
10-18
10.16.3 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.
10.16.4 Flexible Attachments. Various aircraft are designed so that fuselage members or skin, or both, will flex or “work.” This type of structure should not be heavily reinforced for the purpose of attaching shoulder harnesses, as this would defeat the design purpose. In these cases, use a localized reinforcement, such as that shown in Figure 10-18, 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.
Figure 10-14. Typical Wing Carry-Through Installation Fig ure 10-15. Typical Hat Section Reinforcement Installation