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
Par 918 Page 96
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
Par 918 Page 98
FIGURE 9-16. TYPICAL BULKHEAD REINFORCEMENT INSTALLATION
FIGURE 9-17. TYPICAL STRINGER SECTION REINFORCEMENT INSTALLATION
Par 918 Page 99
FIGURE 9-18. TYPICAL LOCALIZED STRINGER SECTION REINFORCEMENT
INSTALLATION
FIGURE 9-19. TYPICAL ATTACHMENTS TO TUBULAR MEMBERS
Par 918 Page 100
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
structural attachments must be subjected to in order to prove minimu m load bearing capabilities. A
1.15 fitting factor for restraint attachments in small aircraft type-certificated prior to September 14, 1969, must be used. For aircraft type-certificated on or after this date, a fitting factor of 1.33 must be used.
925. LOAD AND DISTRIBUTION ― NORMAL CATEGORY HELICOPTERS.
a. Load. For helicopters type-certificated in the normal category, occupant weights are established as 170 pounds.
b. Distribution. In the assessment of a combined shoulder harness-safety belt restraint system, for helicopters type-certificated prior to December 13, 1989, a forw ard static test load distribution of 40 percent to the shoulder harness and 60 percent to the safety belt is the required combined static test load distribution.
c. G Factor. For helicopters type-certificated prior to December 13, 1989, the critical ultimate static load is established at 4.0 G’s in the forward direction. If the attachment can sustain this load, it is assumed the loads in upward, sideward, and downward directions will be accommodated.
NOTE: Due to increased G factors and limited applicability for retrofit harnesses installations, helicopters type-certificated on or after December 13, 1989, are not addressed in this chapter. Additionally, airplanes seat/restraint systems typecertificated after September 14, 1988, must undergo dynamic testing, as prescribed by § 23.562, which is not covered in this chapter.
926. DEMONSTRATING STATIC LOADS.
Two methods are available to demonstrate compliance with static load requirements. One method involves the fabrication of test blocks and application of the ultimate load to the blocks that are being restrained by the harness. The other method involves use of an equation to calculate and simulate the ultimate load at the harness attachment fitting, compensating for horizontal and vertical angles as a result of restraint attachment geometry.
a. Test Blocks. Although AC 23-4, Static Strength Substantiation of Attachment Points for Occupant Restraint System Installations, was cancelled on January 27, 2003, it is still available in various archives and can be used as an acceptable method for fabricating test blocks and applying static loads to restraints. Using this method, deceleration loads imposed upon the restraint by the occupant are accurately duplicated, and the load figures in Table 9-1 can be applied directly to the test blocks. Because of the time-consuming nature of fabricating test blocks for this procedure, it may be desirable to use the second method described below that, in effect, replaces the test blocks with an equation.
b. Calculation. The alternate method is to perform a mathematical calculation to account for the angles that the attachment points are subjected to during decelerations. Increases in angles can result in a significant increase in the load that must be applied to simulate belt loading during occupant decelerations. It is not uncommon for a harness to impose both vertical and horizontal angles that must be compensated for by use of the equation. (See Figures 9-20 and 9-21.) In the case of a “Y” type harness, the horizontal angle is eliminated as long as the attachment is located along the vertical centerline of the occupant.
(1) The equation used to perform the calculation is:
Test Load = [Static Test Minimum] [1/Cosine(vertical angle)] [1/Cosine(horizontal angle)]
(2) Table 9-2 provides 1/Cosine numbers for various angles. These numbers can be inserted into the equation to determine the static load that needs to be applied. For example, using Table 9-1 and Table 9-2, if the airplane is normal category, post-September 14, 1969, pre-September 14, 1988, Par 924 Page 102
Par 926 Page 103 the test load is 814 lbs. If the horizontal angle is 10o and the vertical angle is 15 o, the belt load is calculated as follows.
Test Load = [814][1/Cos(15)][1/Cos(10)] = [814][1.035][1.015] = 855 lbs.
This is not a large increase, but if both angles were 30o, the belt load would be 1086 lbs.
[814][1.155][1.155] = 1086 If there is no angle pr esent in one of the directions, that portio n of the equation is eliminated. For an airplane type-certificated in the normal category prior to September 14, 1969, with a vertical angle of 25 degrees, the equation would be: [704][1.103] = 777 lbs.
c. After obtaining the test load value, the load is applied for a minimu m of 3 seconds, forward (horizontally) when using test blocks, or in the direction of the restraint angles when applying the load to the attachment point(s).
d. Equipment used for measuring test loads must be calibrated to a standard acceptable to the FAA. Load-cells are capable of performing these measurements; however, other equivalent equipment as determined by the user may be acceptable.
927. THRU 930. RESERVED
TABLE 9-1. STATIC TEST REQUIREMENTS TC Basis
Airplanes Category* G Factor Weight Factor (lbs) Fitting Factor Harness Factor (40%) Static Test Minimum Load (lbs) Effective 09/14/88 and on N, U, A 9 215 1.33 .4 1029 N 9 170 1.33 .4 814 Effective 09/14/69 U, A 9 190 1.33 .4 910 N 9 170 1.15 .4 704 Pre 09/14/69 U, A 9 190 1.15 .4 787 Helicopter Pre 12/16/84 N 4 170 1.15 .6 469 Helicopter Post 12/16/84 Pre 12/13/89 N 4 170 1.33 .6 543
• N=Normal, U=Utility, A=Acrobatic
• ** Airplane seat/restraint systems type certificated after September 14, 1988, must undergo dynamic testing, as prescribed by § 23.562, which is not covered in this chapter.
TABLE 9-2. CALCULATED 1/COSINE VALUES Degrees 5 10 15 20 25 30 35 40 50 60 1/Cos Multiplier
1.004 1.015 1.035 1.064 1.103 1.155 1.221 1.305 1.556 2.0
FIGURE 9-20. VERTICAL ANGLE
FIGURE 9-21. HORIZONTAL ANGLE
Par 926 Page 104
Par 931 Page 105 SECTION 4. INSTALLATION AND INSPECTION CHECKLISTS
931. GENERAL. The following checklists are not all-inclusive, but may provide a quick reference for factors to consider when installing retrofit shoulder harnesses and for establishing instructions for continued airworthiness of retrofit harness installations. If available, manufacturer’s instructions take precedence over this AC.
a. Shoulder Harness Installation Checklist.
(1) Locate potential attach points.
(2) Evaluate the geometry of attach points in relation to the occupant and determine the best harness configuration. Short belt lengths are desirable.
(3) Ensure that the configuration will allow unimpeded egress for all occupants. Singlepoint release is mandatory. There should be a means to secure belts and harnesses when not in use to avoid interfering with operation of the airplane or egress from it.
(4) Restraints must allow crewmembers to perform all necessary flight operation functions while seat belts and harnesses are fastened.
(5) Utility and acrobatic category airplane restraints must accommodate an occupant wearing a parachute.
(6) If retractors will be used, ensure that mounting will provide straight-line entry and exit of webbing to prevent binding or frictional drag.
Retractor loading should not impose bending forces upon retractor mounting brackets.
(7) If harness installation will impose loads on the seat back or seat attachment, additional evaluation will be needed for structural integrity.
(8) Refer to the aircraft type certificate to find the certification date and basis to establish minimum static load requirements. Determine the feasibility of performing static tests or stress analysis. Static testing on operational aircraft is not recommended.
(9) Select only restraints that meet OEM, TSO, PMA, or other established industry standards or specifications.
(10) Install restraints using AN, MS, NAS, or other acceptable aircraft hardware with adequate strength properties.
(11) Perform a check to ensure all components function properly.
(12) Complete record entries as required, including FAA Form 3 37, logbook, weight and balance, and equipment list.
b. Shoulder Harness Inspection Checklist.
Inspect restraint system for condition and function at each annual or each required inspection of cabin or cockpit equipment. If questions arise regarding any of the conditions listed below, contact the restraint manufacturer for specific limits.
(1) Inspect stitching on webbing for broken or missing stitches.
(2) Inspect webbing for fraying, fading or cuts. Fraying that causes binding in the retractor is excessive. Fading caused by exposure to sunlight or chemicals may indicate a reduction in strength.
(3) Ensure that TSO, PMA, OEM, or other required identification tags are present and legible on each belt assembly. Tags may not be required for harnesses manufactured prior to March 27, 1987. Contact the aircraft or restraint manufacturer for specific requirements.
(4) If so equipped, check the harness post on the lap belt to make sure the nylon bushing or grommet is present and functional to provide positive harness end fitting engagement. Do not replace the bushing with nylon bundle ties. Contact
the belt manufacturer for replacement bushings/grommets and instructions.
(5) Check the belt buckle for latching and release functions.
(6) Check the retractors to ensure that locking mechanisms engage when webbing is positioned at the normal operating length. Inertia reels should lock when the webbing is accelerated quickly out of the reel. Automatic locking retractors should lock at approximately one-inch increments as the webbing retracts back into the reel.
(7) Inspect buckles, connectors, and fittings for corrosion, cr acks, or other damage.
Check mounting hardware for security and ensure that airframe attachment of end fittings self-align and do not bind.
(8) Inspect quick release end fittings for cotter pins.
(9) If equipped with manual locking inertia reel, check for proper operation and condition.
932. THRU 999. RESERVED
Par 931 Page 106