(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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FIGURE 9-18. TYPICAL LOCALIZED STRINGER SECTION REINFORCEMENT
INSTALLATION
FIGURE 9-19. TYPICAL ATTACHMENTS TO TUBULAR MEMBERS
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Par 923 Page 101 SECTION 3. STATIC STRENGTH AND TESTING
923. PROOF OF COMPLIANCE. Retrofit harness installations must be proven to meet minimum strength requirements applicable to the aircraft certification basis; otherwise, their ability to provide adequate restraint under accident decelerations is undetermined. Static strength requirements can be demonstrated by static test or stress analysis.
a. Static tests are accomplished by application of the ultimate load to the harness attachment point.
Since TSO and PMA restraints have already been shown to comply with strength requirements, static tests are accomplished to substantiate the airframe attachment. Static testing in operational aircraft is undesirable because there is the risk of airframe damage; however, static testing in a conforming but unserviceable fuselage or cabin section of the same make and model aircraft is an acceptable alternative.
b. Stress analysis is accomplished through engineering data developed for the specific attachment point. This data can be developed by FAA Designated Engineering Representatives (DER) with aircraft structural approval authority.
The DER will perform the analysis, prepare a report, and complete FAA Form 8110-3, Statement of Compliance with the Federal Aviation Regulations, which demonstrates that a specific regulation has been met. The DER will list on FAA Form 8110-3 the data needed to show the structural adequacy of the installation of the harness. Since the DER is only approving the structural aspects of harness attachment, approval for return to service for the installation as a whole is accomplished by the authorized individual or agency on FAA Form 337.
To locate an appropriately authorized DER in your locality, contact the FAA Flight Standards District Office or Aircraft Certification Office having jurisdiction in your geographic area.
NOTE: It is up to the installer to determine which method will be the most efficient. For simple installations, it is possible that the DER could perform stress analysis in just a few hours. The DER may also be able to assist in the design of attachment points.
924. LOAD AND DISTRIBUTION ― AIRPLANES. Table 9-1 provides a reference to determine minimum static test loads based upon certification date, category, standard occupant weight, fitting factors, a nd harness factors. The numbers in each line are multiplied together to obtain the static test load minimum standard.
a. Load. For airplanes type-certificated prior to September 14, 1988, occupant weights were established as 170 pounds for normal category and 190 pounds for utility and acrobatic category airplanes. The additional 20 pounds accounts for a parachute. For aircraft type-certificated on or after this date, 215 pounds is the established weight for the 3 categories.
b. Distribution. In the assessment of a combined shoulder harness-safety belt restraint system, a forward static test load distribution of 40 percent to the shoulder harness and 60 percent to the safety belt has been an acceptable combined static test load distribution. Therefore, demonstration of harness attachment static strength is accomplished by applying 40 percent of the ultimate restraint system load to a single or “Y” type harness attachment, or 20 percent of the load simultaneously to each attachment of a dual independent harness.
Static strength test load requirements for the fifth belt of a 5-point system have not been established for aircraft.
c. Factor. The airplane must be designed to protect each occupant during emergency landing conditions. The critical ulti mate static load is established at 9.0 G’s in the forward direction. If the attachment can sustain this load, it is assumed the lesser loads in upward, sideward, and downward directions will be accommodated.
d. Fitting Factors. Regulations prescribe multiplying factors of safety, fitting factors, that