(a) (b) Seat reference point (typical)
2.0m. +0.25
(c) (d) Figure 10. Safety Belt Installation Angle Chap 2 Par 4 19
D. Dual shoulder harness. One of the more common "dual" shoulder harness configurations is similar to the system illustrated in Figure 1(d), with all segments joined by the buckle in the center of the safety belt. This design allows rapid exit from the restraint by releasing only one with a shallow salety beit angle permits the shoulder harness a system installec to pull the safety belt up, off the pelvic area, into the abdominal region as illustrated in figure 11(a). In addition to the injury potential discussed in subparagraph 4a(1), this action also introduces a slack in the shoulder harness which tends to defeat the purpose of the shoulder harness in preventing upper torso rotation into the instrument panel or other structure.
(c)
(d) Figure 1l. Occupant Reactions with Dual Shoulder Harness
(1) Optimum harness geometry. Correct installation for the dual belt, single point release, system uses safety belt attachment points which provide a belt angle sufficient to minimize upward movement of the buckles as illustrated in figure 11(b). Again, a 45 to 55-degree belt angle relative to the longitudinal axis of the aircraft is most appropriate. This angle permits the safety belt to react to the upward pull of the shoulder harness. The shorter safety belt length in this figure also reduces stretching for better control of occupant movement.
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(2) The negative-G strap. Another method of reducing upward movement of the dual belt, single point release, system uses the "negative-G strap" (often called a crotch strap) as illustrated in figure 11(c). A properly installed negative-G strap is attached at one end of the buckle, and at the other end to the front edge of the seat or to the airframe under the seat. The length is such that no slack exists in the strap when the lap belt is properly positioned in the pelvic region. In this position, the negative-G strap acts to resist the upward pull of the shoulder harness. This method has proven very effective and has been adopted for many commercial and acrobatic pilot crew seats.
(3) Alternative dual shoulder belts. An alternative dual shoulder harness system is illustrated in figure ll(d). This system attaches the shoulder belts in the same general area as the safety belt attachments. Special evaluation of the attachment is necessary when attaching the lower end of this type of dual shoulder harness to existing safety belt attachments, because of the change in direction and magnitude of the ultimate forces applied to the fittings in an accident. This system avoids any upward pull at the buckle from the shoulder harness, and the safety belt is more likely to remain in the pelvic region during an accident;
however, releasing the buckle does not release the shoulder harness. Use of emergency locking retractors on the upper end of the shoulder harness will simplify length adjustment and exit from this type of dual shoulder harness.
(1) A precaution regarding the dual shoulder harness system 1llustrated in Figure 11(d) is the need for the shoulder belts to be separate and distinct from the safety belt straps, so that failure of one does not produce loss of restraint from the other. The concept of making one side of the safety belt and one of the shoulder straps from one continuous webbing looped through the attached fitting at the seat or floor, as in Figure 12(a), is undesirable for two reasons:
Figure 12.
(a) UNDESIRADLE
(b) ACCEPTABLE Continuous Loop Safety Belt - Shoulder Harness Chap 2 Par 4 21
(A) Failure of either the safety belt portion or the shoulder belt portion releases the entire restraint system; and (B) Since such a continuous loop system will apply equal load to the body in all belt segments, attempts to tighten the safety belt will place uncomfortable loads on the shoulder/clavicles. This will encourage a loosely worn restraint and creates a situation for submarining. Structural deformation of the cabin during an accident can also cause slack in the safety belt with the same effect; however, a method of extending the lower end of the shoulder strap through a slot in the safety belt attachment fitting, and permanently attaching (sewing) it to the safety belt near the buckle, as in Figure 12(b), is acceptable, and will provide slack in the shoulder harness for exiting when the buckle is released. Special evaluation of the buckle and attachment fitting strength is necessary
(11) A second precaution regarding the shoulder harness system of Figure l1(d) is the need to use the "Inverted I" type shoulder harness, or to have each shoulder belt attached to the seat or a bulkhead directly behind the occupant's head (See figure 1). The union of the "Y" type shoulder straps needs to be within 3 to 6 inches behind the occupant's neck to retain the shoulder belts in the proper position on the occupant's shoulders. Too large a separation may allow upper torso slippage through the straps by a narrow-shouldered person.
(1) Webbing guides. Dual shoulder belts with lengths of webbing extending more than 12 inches behind the shoulders need webbing guides positioned to hold the belts in the approximate midpoint of the occupant's shoulder. The guides should pemit no more than 1/2 inch of lateral movement of the webbing. Without the guides, occupant movement can position the shoulder belts at the outside of one or both shoulders, such that inertial forces in an accident cause the shoulder(s) to slip from the shoulder restraint with loss of upper torso protection and possible violent twisting injury of the spinal column.
22 Par Car 42
(a) (b)
(d) Figure 13. Single Diagonal Shoulder Harness Geometry
c. Single diagonal shoulder harness. Diagonal shoulder harness systems have been used in automobiles for a number of years, and accident experience with those systems indicates they perform well. Most automobile restraint systems are the result of careful pianning, extensive testing, anc considerable experience. vithout this type of experience, certain precautions are necessary in selecting and installing a diagonal shoulder harness system.
Chap 2 23 Par 4
(1) Proper installation. A proper installation geometry for the diagonal shoulder harness positions the shoulder belt so that it passes over the midpoint of the Sccupant's hip as 1llustrated in Figure 13(a). Attachment of with the lower end fastened well to the side of the the lower end of the shoulder harness to the safety belt is an acceptable convention, as long as the proper geometry is maintained, but caution is needed to ensure that the portion of webbing common to both the shoulder belt and the safety belt has sufficient strength to sustain restraint loads from both segments. Alternatively, the lower end of the shoulder belt may be attached separately with an emergency locking retractor incorporated in the upper end for length adjustment and exit purposes. In either case, the geometric arrangement of the shoulder belt is important in performing the upper torso restraint function.
(2) Improper lower attachment. Figure 13(b) illustrates an improper attachment of the diagonal shoulder belt to a buckle situated near the center of the pelvic region. The shoulder belt is no longer diagonal across the occupant's body. The shoulder belt passes low and to the side of the center of mass of the upper torso, so that in a severe accident the torso may twist around the belt and even slide out of the belt.
Upper end attachment. In selecting the upper end attachment point for a diagonal shoulder harness, precautions should be taken to avoid critical problems associated with variations in occupant size. Figure 13(c) illustrates the situation in which the shoulder belt bears against the neck or the side of the head of a short occupant.
A similar situation is expected, even for an average size occupant, when the upper attachment point is located too near the vertical centerline plane of the seat. This installation geometry is aggravating and generally discourages use of the shoulder harness. An excessive elevation angle on the shoulder belt may have the same effect (Reference figure 14(a)). Figure 13(d) illustrates how the shoulder belt may tend to fall off the shoulder of a tall occupant, or when the upper attachment point is too far outboard or too low with respect to the midpoint of the occupant's shoulder.
This geometry is also aggravating and discourages use of the shoulder harness. When the shoulder belt falls below the center of mass of the upper torso, the body may rotate around the shoulder belt, and a severe accident may produce enough body motion to allow the head to contact the instrument Chap 2
• 24 Par 4
Occupant height. General anthropomorphic data indicate that an adult occupant sitting height to the midshoulder can range from about 21.5 inches for a small female to 27.5 inches for a large male. A midshoulder height of 25 inches approximates a large female and an average male and is an appropriate starting point for selecting shoulder harness attachment points.
d. Spinal compression. Additionally, for both dual and single diagonal shoulder harnesses, compression of the spinal column by the shoulder belts(s) should be avoided. Spinal compression can generally be avoided if the upper attachment point of the shoulder harness is selected to provide a geometry where the trailing length behind the occupant does not fall below an angle of 5 degrees below the longitudinal tangent to the occupant's shoulder, as illustrated in Figure 14(a). Spinal compression is likely to occur when the upper end of the shoulder belt is mounted an excessive amount below the occupant's shoulder level. With the configuration 1llustrated in figure 14(b), the shoulder belt pulls down and back on the torso as it resists the forward motion of the occupant. The resultant restraint force, as depicted in figure 14(c), will place the spinal column in compression, and will add to the stresses in the column caused by the vertical component of the impact deceleration force.
Chap 2 Par 4 25
Excessivo Acceptal
(b) Figure 14. Should Harness Attachment Elevation
- Low attachment creates spinal compression;
- High attachment creates additional structural loads and poor restraint.
26 Par 4 Chap 2
Added structural loads. Upper shoulder harness attachment points should be selected to avoid installations which amplify stresses in the shoulder harness and its structural attachments. In an accident, tension in a shoulder harness is increased over that of the occupant inertial force by an amount depending on the angle between the direction of the occupant inertial force and the reacting tension force in the shoulder belt.
(1) Excessive angle - excessive loads. for both dual and single diagonal shoulder harnesses, general experience indicates that the motion of the occupant under accident conditions tends to cancel the increase in load when the belt elevation angle relative to the tangent at the occupant's shoulder is less than 30 degrees, as illustrated in figure 14(a). The excessive angle position of figure 14(a) provides a large mechanical advantage for webbing failure, collapse of the airframe, or separation of shoulder harness attachments.
(2) Excessive angle - excessive motion.
excessive elevation angle also provides little resistance to forward motion of the occupant's upper torso before impact with the instrument panel or controls. Neck injuries are also possible as mentioned in paragraph 4c(3). Since the shoulder belt in tension only to loads, significant forward movement of the torso is necessary before the shoulder belt is aligned to a position where it can react to the occupant inertial force. Again, the 30-degree elevation angle is about the maximum angle which will provide effective restraint by the shoulder belt.
(3) Webbing quides. When a suitable attachment point is not available for the upper end of the shoulder harness, webbing guides for the shoulder harness can sometimes provide an acceptable pull-off elevation angle behind the occupant's shoulder; however, the strength of the webbing guide attachment needs careful evaluation. If the guide is attached to the seat back, an evaluation of seat strength is also needed, because longitudinal and vertical forces will be applied to the seat back by the restraint forces in the shoulder harness in an accident. A seat back not designed for these forces may experience early failure and defeat the purpose of the shoulder belt guide.
side racing seats. side racing seats, althougn not required by FAR Section 23.785 to have shoulder harnesssafety belt systems, should be provided with restraints that distribute the load to hard skeletal portions and muscular masses of the body, such as the pelvis, shoulder, and thigh.
Chap 2 Par 4 27
Supplementary lateral (reference to occupant) supports, such as padded bulkheads, nets, etc., may be valuable in distributing inertial forces over a maximum body area and in reducing tendencies of the body to twist. Belts should not impinge on the neck.
Installation compliance. Compliance with the strength requirements of the FAR for a combined shoulder harness-safety belt system added to an aircraft in service may be shown by stress analysis, static tests, or a combination of the two methods. Conducting a static ultimate load test in an operational aircraft is not recommended.
Refer to subparagraph 6d.
5. SHOULDER HARNESS EFFECTS ON SEAT INTEGRITY.
Need for seat integrity. When any part of a combined shoulder harness-safety belt system is attached to a seat, the seat is automatically a significant link in the occupant restraint function. Therefore, although this AC is primarily concerned with the shoulder harness-safety belt installation, it is also important to consider the ramifications of attaching a shoulder harness to the seat.
The ramifications are a complex subject in themselves and are related to the dynamic response characteristics of the occupant, the seat, and the safety restraint system under various impact conditions. Attention to certain aspects of the seat design can minimize the possibility of false security because of a weakness in the seat design.
aspects involve the case of adding a lower shoulder harness attachment to an existing seat and the case of attaching both ends of a shoulder harness to a seat.
Lower shoulder harness attachment to seat.
addition and attachment of the lower end of a shoulder harness to an existing seat needs special evaluation of the attachment strength. Due to previous occupant protection and seat design requirements, the attachments and local seat structure may lack the inherent structural integrity sufficient to sustain the change in magnitude and direction of the restraint forces imposed by the shoulder harness.
point to evaluate on a forward-facing seat is the method of fastening and the local seat structure at the shoulder harness-safety belt attachment. A stress analysis, test, or combination of test and analysis should be accomplished to ensure the attachment integrity.
(2) Rear seat leg attachments. The second point to evaluate on a forward-facing seat is the rear seat leg attachment to the aircraft. This point is often the weak link in the occupant restraint system, especially for those Chap 2 28 Par 4
fittings made to slide on a track. The track is also a suspect for weakness in the total system, especially if degraded through wear or other conditions. Stress analysis, test, or a combination of test and analysis of the seat attachments, as well as the restraint attachment to the seat, is necessary.
Shoulder harness loading of seat back. Attachment of the upper end of a shoulder harness system to the back of
• forward facing seat, or to the floor benind the seat, presents a special problem in ensuring seat integrity for accident conditions. (In this case, the upper end refers to the end of a shoulder harness that extends over the shoulder and to the rear of the occupant.) In an accident environment, occupant restraint forces in the shoulder belts introduce longitudinal forces to the seat back which are transmitted as bending stresses in the seat back structure and then as additional tension stresses in the rear seat legs and additional compressive and buckling stresses in the front seat legs. All of these seat stresses are considerably higher in magnitude than those generated through a safety belt alone, or when the upper end of the shoulder harness is attached to the airframe, assuming the same accident conditions. A seat not designed for the additional forces applied through the shoulder harness is likely to exhibit weakness in one or more oi these reatures, which will deieat the purpose of the shoulder harness installation. Therefore, careful determination of the bending strength of the seat back, transfer of this load through the seat back fold over and latch mechanism, and then the tension and compressive or buckling strength of the seat legs and seat attachments to the aircraft, including floor track strength, is necessary for ensuring proper occupant restraint.
Non-folding Lseat backs. Rigid, non-folding seat backs designed for shoulder harness attachments present hazards to occupants seated behind the seats. Consequently.
seating positions behind such a seat should also be equipped with shoulder harnesses.
6. RESTRAINT LOADS.
safety beit installations have changed through the Yable i The form of the specifications have also changed. Strength criteria. Minimum strength criteria for highlights the evolution of minimum strength specifications for small airplanes, beginning with the first Department of Commerce "Aeronautics Bulletin" (Bulletin 7-A) to specify minimum safety belt strength, to the civil Aeronautics Board "Civil Air Regulations" (CAR), through the Federal Aviation Administration "Federal Aviation Regulations" (FAR). The minimum strength criteria for any particular airplane in service depends on the certification basis of the airplane.
Chap 2 Par 5 29
TABLE 1 Solution of Safety Belt Minimum Strength Criteria for Saall Airplanes.
REGULATION EFFECTIYE DATE MINTON STRENGTH SPECIFIED Bulletin 7-4 July 1, 1929 1000 1b. for 170 Ib. occupant.
20 fitting frotor.
Bulletin 7-4 January 1, 1931 1,20 fitting factor, occupant. 1000 16 Bulletin 7-A October 1, 1934 1000 lb. directed 45 deg. forwara CAR CAR Part 04 Part 04 361, 1, 1944 31 1938 and ..20 "fitting factor. • occupant CAR Part 04a November 1, 1947 CAR Part 03 December 15, 1946 1i0 initing faotor. occupant, CAR Part 03 November 1 1949 CAR Part 3 May 15, 1956 Side- Up - 3.0g (acrobatio) • 1¡03 attachment fitting factor.
Ib, occupant, CAR Part 4a Apri1 7, 1950 Syd - 0.08 (normal, acrobatic).
170 filen factor.
FAR Part 23 June 6, 1964
1.33 attachment 170 1в, оссирЕдЕ.
b. General practice. A determination of design loads for combined shoulder harness and safety belt installations in civil aircraft has been a long standing issue. Part of the issue has been resolved by the issuance of TSO-Cll4.
systems in civil aircraft. there was no aviation standard for shoulder harness consequently, commercial production shoulder harness systems usually have the same rated strength as the minimum specified for the safety belt.
Shoulder harness attachment points designed into aircraft by manufacturers generally follow the same practice.
Chap 2 30 Par 6