Restraint load distribution. The FAR specify that the restraint strength capacity be sufficient to restrain a 170-pound occupant exposed to the ultimate inertia forces given in table 1 of this AC. 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. In addition, the safety belt alone should be able to carry 100 percent of the total ultimate static test forward load.
1.33 fitting factor for the attachments in small aircraft type certificated after November 1, 1949, should be used.
d. Qualification of installation. The FAR permit three methods of qualifying a shoulder harness-safety belt system installed in an aircraft after production. These methods are static test, stress analysis, or a combination of stress analysis and static test. Unless original design data are available for the existing airframe, qualification by stress analysis should be based on conservative assumptions (elongation, deformation, stress concentration, load distribution) because of the unknowns that may be involved with the original structural integrity at attachment points.
Static testing to ultimate loads in an operational aircraft is undesirable because there is a high risk of permanent airframe damage in some aircraft; however, static testing in a conforming fuselage or cabin section of the same make and model aircraft is an acceptable alternative. The static strength substantiation for restraint system installations is discussed in AC 23-4, Static Strength Substantiation of Attachment Points for Occupant Restraint System Installations.
7. STRUCTURAL ATTACHMENTS.
Preferred method. Some aircraft manufacturers have developed kits for some of their inservice aircraft. The use of a manufacturer's kit for structural attachment of shoulder harness installations is preferred. Before undertaking modification of an aircraft in service, it is prudent to determine if a shoulder harness installation kit is available from the aircraft manufacturer. Kits approved by a Supplemental Type Certificate (STC) may also be available from a third person. The local FAA Flight Standards District Office (FSDO) or Manufacturing Inspection District Office (MIDO) will provide information on STC kits. Otherwise, an FAA Aircraft Certification Office should be requested to assist in evaluating the data necessary for and independent shoulder harness design and installation.
Scope and intent. Recognizing that it is virtually impossible to conceive of every situation that can be encountered, this AC presents only a few general design concepts of attaching shoulder harness-safety belt systems.
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These few design concepts are intended to create an understanding of the features needed in the attachments.
(1) Concept 1. The first concept is to spread attachment loads into as much surrounding structure as possible and as gradually as possible. Gradual dissipation of loads minimizes stress concentrations at abrupt changes in material cross section which promote local failures, either immediately or upon a subsequent accident load cycle.
(2) Concept 2. The second concept is to minimize local structural bending by attachment loads. Semimonocoque structure generally offers poor resistance to bending, but is good in tension and shear applications. Airframe bending, buckling, or collapse adds to forward movement of the occupant.
Concept 3. The third concept is to ensure that fastener type, strength, and number are adequate in tension, shear, and bending, depending on the application.
Airframe buckling under restraint loads will result in compound loading of connector plates as well as fasteners.
Concurrently, material thickness is important in preventing fastener pull-out, and continued security (safety wire or equivalent) of threaded fasteners should be considered.
Details of attachment. Some existing aircraft will already have shoulder harness attachment points, often called "hard points", which were installed during production. As an alternative, it is fortunate to be able to attach shoulder belts to reasonably rigid structure where only a doubler may be needed to replace the material removed for fastener holes.
Most often, it is necessary to attach shoulder belts to relatively thin formed sections, or even skin panels, of semimonocoque construction to achieve a satisfactory geometric configuration of the belts when in use. In most cases, attachment points need reinforcement. Attachments to welded tube and wood frame construction present a special problem in selecting the attachment point and the hardware for attachment of shoulder belts.
Bulkhead attachments. It is necessary that a bulkhead be a structural bulkhead rather than a cabin partition to offer a suitable structural attachment for a shoulder harness. Figure 15 offers generic examples of how a shoulder harness might be attached to a bulkhead. The reinforcement hat stiffener on the aft side of the bulkhead.
and the intercostal doubler on the forward side, illustrate one means of distributing restraint load to the bulkhead in two directions with a minimum of added weight. Although not as effective, the added hat stiffener and intercostal doubler can be installed on reverse sides of the bulkhead when the bulkhead stiffeners are on the aft side of the bulkhead.
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-Stool Houns Brocket Nanad stres — Doubter Added Eye Bod Cable Clarie or Wabbing Gulde Typical Alloch up 2 bulkhead rach side are ar baskhadd Formed a Accted ixiruded th Figure 15. Possible Bulkhead Attachment and Reinforcement Chap 2 Par 7 33
e. Wing carry-through and beltframe attachments. Wing carry-through structure and beltframes offer reasonably substantial structure for attaching the upper end of a shoulder harness. Figure 16 illustrates the type of reinforcement needed for restraint load distribution at an attachment to wing carry-through structure. Reinforcement of a beltframe should be considered in the same manner. The reinforcements, illustrated in figure 16, provide a better design if they form intercostals between the wing carrythrough member and a beltframe or bulkhead. Tapered reinforcements can offer similar fore and aft stability to the local attachment structure as well as load distribution to skin panels. Again, the goal is to provide local structural stability and load distribution to minimize local stress concentrations.
iSTor mont added Load Transfer Rainporcement Added - Mobbing Figure 16. Possible Wing Carry-Through or Beltframe Attachment and Reintorcement 34 Chap
£. Stringer attachments. Direct attachment of a shoulder harness to body stringers is discouraged, because he strin stringers are intended to carry tension loads only, and it is difficult to reinforce the stringer to support bending stresses likely to be introduced by restraint loads.
Intercostals between beitframes offer the best approach to attaching a shoulder harness between beltframes.
g. Floor attachments. As mentioned previously in this AC, special precautions are necessary in attaching a shoulder harness to the floor behind a seat, like the illustration of figure 17. Seat back strength is critical to performance of the shoulder harness, and will introduce additional loads to the remainder of the seat. These aspects need evaluation before proceeding with attaching the shoulder harness to the floor behind a seat. Recall, also, that a dual shoulder harness is needed for this installation, and the seat back needs to be near average midshoulder height (about 25 inches above cushion) to minimize spinal compression loads. Two aspects should be considered in designing these attachments:
Figure 17. Dual Shoulder Harness Attachment to Floor Par 7 35
(1) Reinforcement. The shoulder harness attachments at the floor may consist of an eye bolt or a connector plate; however, simple attachment to the floor panel is usually insufficient to support restraint loads.
a floor beam or other primary floor support structure is not available at the point necessary for attaching the shoulder harness, the addition of an intercostal between floor beams is warranted for connection of the eye bolt or connector plate. The intercostal will provide a beam to carry the local vertical restraint loads at the shoulder harness attachment.
(2) Retractor mounting. Recalling that most retractor frames are designed to transfer restraint loads through shear of the attachment fasteners, it may be necessary to install a vertically oriented connector plate which is anchored to a floor beam or an intercostal. The vertical plate may also be needed if the bolt-on end fitting of figure 9 is used on the shoulder harness assembly.
Attachment to welded tube structure. In attaching a shoulder harness to welded tube construction, remember that the primary occupant inertial force is directed forward and downward. The cross section of most tubing used in small airplanes offers an insufficient moment of inertia for resisting bending from restraint loads. Therefore, it is best to make shoulder harness attachments at a point offering a better 3-dimensional stability, as illustrated in figure 18(a), rather than at some intermediate point between Joints. The lateral gusset illustrated in figure 18(a) provides a means for aligning the webbing entry and exit angle of a retractor. The retractor could also be attached to the underneath side of the gusset. The lateral gusset also provides a mounting for various other types of shoulder harness end fittings and cable attachments. As a cautionary note, the addition of welded tube reinforcements may affect the heat treatment and fatigue characteristics of the tube structure. figure 18(b) illustrates a possible alternative bolt-on fixture for attaching shoulder harness retractors and end fittings or a cable clevis.
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(•) (b> Figure 18. Attachment and Reinforcement of Welded Tube Structure
1. Attachment to wooden structure. Experience with shoulder harness attachments to wooden structures is scarce.
The goal, however, is the same as for metal structures.
Distribute the restraint load from the attachment to other structure to avoid local stress concentration failure.
ussets, cross braces, and intercostals are the techniques while it is difficult to predict the structural confiquratior that might be available for attaching a shoulder harness in a wooden structure airplane, some techniques peculiar to wooden structure are worthy of mention:
(1) Nuts and bolts. The use of lag bolts or wood screws to resist restraint loads in tension is poor practice.
Boits through the structural member is the preferred method.
(2) Bolt head backup. Use metal backup plates for all attachment bolts to avoid compression of the wood.
Chap 2 Par 7 37
AC 21-34 6/4/93
(3) Wood splitting. A wooden beam tends to split grain by a shear bolt through the beam This effect can be in the grain when loads are applied perpendicular to the Bolts loaded in single shear are the worst offenders.
minimized by locating the bolt hole to permit as much wood depth as possible in the direction of the applied load.
(4) Avoid glued joint. It is good practice to avoid attaching shoulder harness mounts where restraint loads produce bending in a glued splice. If this practice is unavoidable, metal reinforcement of the splice is a suitable alternative for preventing splice separation, or bending fractures at the edge of the splice, under restraint loading.
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CHAPTER 3. CONCLUSIONS
8. SUMMARY.
a. Intent of AC. The intent of this AC is to provide guidance for achieving an effective shoulder harness-safety belt installation in the dynamic loading environment of an accident. It is recognized, however, that compromises are necessary in some aircraft due to the absence of sufficient structure at the ideal attachment points. Factors of hardware, geometric ramifications, strength and attachment techniques are presented to assist in making intelligent decisions in selecting and installing a shoulder harnesssafety belt system. A checklist of factors to consider fOllowS:
b. Shoulder Harness-Safety Belt Checklist.
(1) Width of webbing in contact with occupant, nominally 2.0 inches or more;
(2) Minimize webbing length for less webbing stretch;
(3) Flexible cable in lieu of guy wire cable;
(4) Buckle release force should be 12 pounds or less in the unloaded condition;
(5) Single buckle for release and escape;
(6) Webbing properly looped on length adjusters;
(70 Tilt lock adjusters lock at 30 degrees or more;
(8) Emergency locking retractors (inertia reels) engage at 0.75 to 1.5 G's;
(9) Automatic locking and emergency locking retractors mounted to provide straight line entry and exit angle of webbing (very important If webbing guide not present);
(10) Retractor mounts designed to resist restraint loads by shear of the retractor mounting bolts;
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(11) Webbing wrapped on retractor spool does not Interfere with retractor locking;
(12) Retractor spring tension sufficient to overcome webbing drag over seats and webbing guides for webbing retraction and stowage;
(13) Quick disconnect end fittings safety pinned and protected from keeper damage;
(14) Attachment position of safety belt permits a belt angle of 45 to 55 degrees for all seat positions;
(15) Webbing guides position dual shoulder belts at middle of occupant's shoulder;
(16) Lower attachment of single diagonal shoulder belt positioned to the side of occupant's hip;
(17) Upper attachment of single diagonal shoulder belt provides belt angle across the torso and over the approximate middle of the shoulder for various sizes of occupant and various seat positions; and
(18) Elevation angle of the shoulder belts) extending behind the occupant is between -5 and +30 degrees from the longitudinal to avoid spinal compression loads and/or amplified forces in shoulder belts.
Special consideration for engineering. A strength evaluation should be accomplished for the following:
(1) Review for false security or possible occupant injury due to shoulder harness geometry;
(2) Integrity of existing safety belt attachments verified for change in restraint force magnitude and direction resulting from addition of shoulder harness;
(3) Integrity of rear seat leg attachments to floor, including seat tracks, relative to loads introduced by the shoulder harness;
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Special evaluation of entire seat strength when upper end of shoulder harness is attached in a manner which applies restraint loads to the seat back;
(5) Structural attachment of shoulder harness to the airframe reinforced to add stability at the attachment point and distribute restraint loads to other structure;
(6) Attachment plates and fasteners adequate for initial direction of restraint loads and +10 degrees to either side;
(7) Connector plates and fasteners adequate for compound stresses from fuselage buckling when attaching a shoulder harness to overhead structure (assume a change in the direction of restraint loads of at least + 30-degree change in the vertical plane and +l0-degree change in the horizontal plane); and aircraft in service is undesirable, but static test in a conforming fuselage or cabin section is desirable.
Conservative stress analysis is an acceptable alternative.
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