Advisory U.S. Department Circular of Transportation Federal Aviation Administration Subject: Date: 6/4/93 AC No: 21-34 Initiated by: ATR-120 Change:
SHOULDER HARNESS-
SAFETY BELT INSTALLATIONS PURPOSE. This advisory circular (AC) provides information and guidance pertinent to an acceptable means, but not the only means, for installation of shoulder harness and safety belt restraint systems at all seat locations on all previously type certificated aircraft.
RELATED FEDERAL AVIATION REGULATIONS (FAR) SECTIONS.
Sections 23.561, 23.785, and 23.1413 of FAR 23; Sections
27.561, 27.785, and 27.1413 of FAR 27; Sections 29.561,
29.785, and 29.1413 of FAR 29; and Sections 91.107 and 91.203 of FAR 91.
3. BACKGROUND.
Basic principles. This AC focuses on basic principles regarding design and installation of combined shoulder harness and safety belt restraint systems which evolve to satisfy the applicable requirements of the FAR. In particular, the following subjects are discussed:
(1) common shoulder harness - safety belt designs;
(2) types of hardware available;
(3) ramifications of some hardware combinations;
(4) effects of the installation geometry:
(5) strength characteristics needed to accommodate an accident loading environment; and
(6) structural attachment concepts.
Emphasis. Emphasis is placed on performance of a combined shoulder harness and safety belt installation in an accident environment. The accident environment produces dynamic action and reaction forces which peak to some value and decline to zero or reverse direction in milliseconds, and most accidents will involve more than one force application cycle. Interaction between the occupant and the seat should be considered, as well as the interaction between the occupant and the shoulder harness-safety belt installation.
Designs to consider. A number of combined shoulder harness and safety belt designs evolved to satisfy the occupant protection requirements of the FAR. some designs developed for automotive and military aircraft applications may also be applicable in civil aircraft. In conjunction with the shoulder harness and safety belt development, a variety of integral hardware components has evolved to satisfy the coupling, the adjustment, the crewmember movement, and the stowage requirements.
d. Experience. Experience has shown that indiscriminate combinations and assembly of hardware components may result in an inadequate shoulder harnesssafety belt assembly. The same experience has shown that installation geometry and attachment techniques influence the restraint effectiveness of shoulder harness and safety belt installations. In addition, experience has shown that the cabin area surrounding each seat affects the shoulder harness-safety belt combination that is selected.
Rationale. Since certain ramifications can occur with the variety of available shoulder harness-safety belt systems and associated hardware, this AC presents information for hardware and system selection, as well as installation concepts, which produce an effective response and a functional installation in the accident environment.
Approval. An aircraft certification office should assist in evaluating data necessary for the addition of a shoulder harness assembly to an installation previously approved with a safety belt only.
John K. McGrath Manager, Aircraft Engineering Division Aircraft Certification Service ii
CONTENTS Page CONTENTS 111 LIST OF FIGURES. vi CHAPTER 1. GENERAL INFORMATION
1. JUSTIFICATION FOR INSTALLING SHOULDER HARNESSES
a. Experience. . 1
b. Objections. 1
c. Benefits. 1
SHOULDER HARNESS CONFIGURATIONS
a. Categories. 1
b. General aspects.. 1
c. Example configurations. 2
d. Civil aircraft shoulder harness. 2 CHAPTER 2. SHOULDER HARNESS INSTALLATION CONSIDERATIONS
3. SHOULDER HARNESS ASSEMBLY DETAILS
a. Webbing. 5
(1) Webbing width and hardware match.
(2) Webbing thickness and hardware match.
(3) Webbing weave and hardware match
(4) Webbing elasticity and occupant displacement.
b. Cable..
(1) Use flexible cable 6
(2) Avoid sharp bends.
(3) Select clevises carefully.
c. Energy absorbing devices.
d. Buckles.........
(1) Buckle release.
(2) Buckle types.
e. Adjustments hardware
(1) Three-Bar slide adjuster
(2) Tilt lock adjuster 10
(3) Can lock adjuster 13
f. Webbing retractors. 13
(1) Emergency locking retractor. 13
(2) Automatic locking retractor. 14
(3) General retractor precautions. 14
g. Attachment end fittings. 16
(1) Minimize bending 16
(2) Self alignment. 17
(3) Keeper damage. 17
4. INSTALLATION GEOMETRY Page
a. Safety belt... 17
(1) Shallow angle. 1 8
(2) Steep angle18
(3) Webbing effects. 18
b. Dual shoulder harness. 20
(1) Optimum harness geometry. 20
(2) The negative-G strap. 21
(3) Alternative dual shoulder belts. 21
(4) Webbing guides. 22
c. Single diagonal shoulder harness. . 23
(1) Proper installation. 24
(2) Improper lower attachment 24
(3) Upper end attachment. 24
(4) Occupant height. 25
d. Spinal compression. 25
e. Added structural loads. 27
(1) Excessive angle - excessive loads. 27
(2) Excessive angle - excessive motion - 27
(3) Webbing guides. 27 Side facing seats.. 27
g. Installation compliance. 28
5. SHOULDER HARNESS EFFECTS ON SEAT INTEGRITY
a. Need for seat integrity. - 28
b. Lower shoulder harness attachment to seat. • 28
(1) Existing belt attachments on seat. . 28
(2) Rear seat leg attachments. 28 Shoulder harness loading of seat back. • 29
d. Non-folding seat backs. 29
6. RESTRAINT LOADS
a. Strength criteria..
D. General practice...
C. Restraint load distribution.
d. Qualification of installation.
STRUCTURAL ATTACHMENTS
a. Preferred method..
b_ Scope and intent.
(1) Concept 1..
(2) Concept 2.
(3) Concept 3.
iv
Details of attachments. 32
d. Bulkhead attachments. .. 32
e. wing carry-through and beltframe attachments. 34 £. Stringer attachments. . 35 Floor attachments. 35
(1) Reinforcement. 36
(2) Retractor mounting. 36
h. Attachment to welded tube structure. , 36 İ. Attachment to wooden structure. 37
(1) Nuts and bolts. 37
(2) Bolt head backup. .37
(3) Wood splitting. 38
(1) Avoid glued Joint. . 38 CHAPTER 3. CONCLUSIONS
8. SUMMARY..... • 39
a. Intent of AC.... . 39
b. Shoulder harness-Safety belt checklist..39
C_ Special consideration for engineering. • 10
LIST OF FIGURES Page
1. Typical Aircraft Shoulder Harness Systems. :. 2
2. Retractors and Manual Length Adjusters. .. 3
3. Hardware Application Concepts _ 4
4. Lift Lever and Rotary Buckles. •. 8
5. Three-Bar Slide Webbing Loop. • 9
6. Tilt Lock Adjuster Webbing Loop . 11 Cam Lock Adjuster . 12
8. Retractor Webbing Entry - Exit Angle at Guide. • 15
9. End Fittings. . 17
10. Safety Belt Installation Angle 19 1l. Occupant Reactions with Dual Shoulder Harness. . 20
12. Continuous Loop Safety Belt-Shoulder Harness. . 21
13. Single Diagonal Shoulder Harness Geometry. . 23
14. Shoulder Harness Attachment Elevation. • 26
15. Possible Bulkhead Attachment and Reinforcement. 33
16. Possible Wing Carry-Through or Beltframe Attachment and Reinforcement...... .. 34
17. Dual Shoulder Harness Attachment to Floor.. •. 35
18. Attachment and Reinforcement of Welded Tube Structure. 37 vi
CHAPTER 1. GENERAL INFORMATION
JUSTIEICATION FOR INSTALLING SHOULDER HARNESSES Experience. Accident experience has provided substantial evidence that use of a shoulder harness in conjunction with a safety belt can reduce serious injuries to the head, neck, and upper torso of aircraft occupants and has the potential to reduce fatalities of occupants involved in an otherwise survivable accident.
b. Objections. Most aircraft occupants readily accept use of a safety belt for security during turbulence, acrobatic maneuvers, or agricultural flying. A shoulder harness, however, is generally associated with the relatively rare occurrence of an accident. It is often heard that a shoulder harness is cumbersome, unwieldy, hot, and uncomfortable to use. Such objections for not installing and using a shoulder harness should be dispelled in view of the benefits gained from a correctly designed, installed, and used shoulder harness-safety belt system.
Benefits. Shoulder harness-safety belt systems prevent serious head, neck, and upper torso injuries in what may be relatively minor accidents in terms of aircraft damage, and they can prevent irreversible or fatal injuries in more severe accidents. Therefore, the major benefits of shoulder harnesses occur in an accident environment, but they can be of no benefit if they are not available for use in an accident.
2. SHOULDER HARNESS CONFIGURATIONS.
a. Categories. Shoulder harness assemblies are categorized as single shoulder belt and dual shoulder belt.
The single shoulder belt configuration is normally arranged diagonally across the occupant's upper torso and is often referred to as a 3-point system. The dual shoulder belt is a symnetrical arrangement of the two belts with one belt passing over each shoulder of the occupant, and is frequency referred to as a 1-point system, or a 5-point system if a negative-G strap is used. These assembly configurations, as well as others, may be acceptable with proper design and evaluation.
General aspects. The aspects to consider in the selection and installation of a shoulder harness-safety belt assembly are the security of the hardware, the containment and strength of the assembly relative to the installation geometry, the ease and extent of belt length adjustment, the means for rapid release, and the comfort of the wearer.
Chap 1 1 Par 1
Example configurations. Figures 1, 2, and 3 illustrate various shoulder harness-safety belt configurations applied in aircraft. Figure l illustrates generalized configurations. Figures 2 and 3 11lustrate more details of hardware applications for the purpose of security, length adjustment, and release of the assemblies.
d. Civil aircraft shoulder harness. Shoulder harness systems should be designed to meet the static strength requirements specified by the ultimate inertial forces an occupant experiences under emergency landing conditions as prescribed in the FAR or the torso restraint systems Technical Standard Order (TSO) C114. Figure 1 11lustrates four shoulder harness system configurations frequently used at front seat positions of civil aircraft.
(a)
(0) (d) Chap 1 Par 2
1 00 m no
(a) (b)
ITEM IDENTITY
1. Inertial reel
2. Shoulder strap
3. Lap belt anchor
4. Buckle with shoulder strap connection
5. Lap belt
6. Adjuster/Fitting Figure 2. Retractors and Manual Length Adjusters I deup 3
AC 21-34 6/4/93
ITEM IDENTITY Bucklo abdombly Singlo - poini release bucklo в. Negattre - 6 Strap Tladonn anchor Lop balt to strap shoulder
o. Adjusta
ITEM DENTIT soor strai sepbly
(a) INVERTED "Y", NEGATIVE-G
STRAP APPLICATION
(b) DUAL SHOULDER HARNESS
ADVANCE CONCEPT Figure 3. Hardware Application Concepts 4 Par
CHAPTER 2. SHOULDER HARNESS INSTALLATION CONSIDERATIONS SHOULDER HARNESS ASSEMBLY DETAILS.
Webbing. Webbing is one element common to all shoulder harness-safety belt assemblies. Webbing should be made of synthetic materials to avoid deterioration by mildew and water exposure. Other webbing characteristics to consider are the width, thickness, weave, and elasticity.
(1) Webbing width and hardware match. Hardware, such as length adjusters and end fittings, should be appropriate for the webbing width to preclude accelerated webbing wear and cutting of the webbing under restraint Technical Standard Order (TSO) C114, Torso Restraint Systems, permits a minimum webbing width of 1.8 inches.
majority of shoulder harness-safety belt systems designed for civil aircraft use a nominal 2.0 inch wide webbing. Webbing widths of 2.25 to 3.0 inches and associated hardware are also available for special purpose applications. For the reasons previously stated, it is poor practice to adapt hardware designed for one width to another width of webbing.
(2) Webbing thickness and hardware match.
important to match hardware to the webbing thickness.
Nominal webbing thicknesses of 0.04 and 0.06 inches are common in civil aircraft shoulder harness-safety belt assemblies. Thicker webbing is available for special purpose applications. Thickness contributes to maintaining more webbing contact with occupants under loads. Retractors and manual length adjusters are sensitive to webbing thickness.
Webbing weave and hardware match. Webbing weave and the design of length adjustment hardware should be matched for proper functioning of the adjuster. A herringbone weave is used for most new webbing. Adjustment hardware designed for the coarse perpendicular warp and fill type webbing may permit excessive slippage of the herringbone weave webbing under loads.
(4) Webbing elasticity and occupant displacement.
Webbing commonly used in shoulder harness-safety belt designs is an elastic material. Nylon is the most common material, and with the herringbone weave the elasticity (stretch) of the 2.0-inch wide webbing is generally between 17 and 20 percent under a tensile force of 2500 pounds. Dacron webbing is also available. The 2.0-inch wide dacron webbing with the herringbone weave exhibits an elasticity of about 8 percent under a tensile force of 2500 pounds. Consequently, the length of webbing in shoulder belts should be evaluated relative to the webbing elasticity and the permitted Chap 2 Par 3 5
displacement of an occupant under loads. If limited space is available in the aircraft for displacement of an occupant under loads, substantial elongation of the webbing should not be possible.
Cable. The use of steel aircraft cable offers a means of reducing the amount of occupant displacement resulting from webbing stretch. Cable also offers a means for extending belts to a suitable attachment point.
Precautions regarding the use of a steel cable are:
(l) Use flexible cable. Flexible cable is needed to prevent fatigue failure of the cable strands through flexing during normal use of the shoulder harness or safety Cable intended for guy wire purposes is normally unsuitable.
(2) Avoid sharp bends. Avoid situations where the cable bends over sharp edges of structure during normal use.
Running the cable through structural members without a grommet or pulley guide accelerates local work hardening failure of the cable strands. A bend radius of at least 4 times the cable diameter should be provided if bending is unavoidable.
(3) Select clevises carefully. selection of any cable clevis needs careful attention. Control cable clevises normally have insufficient strength. Cable characteristics and cable terminal techniques are provided in AC 43.13-1A, Acceptable Methods, Techniques, and Practices - Aircraft Inspections and Repairs.
Energy absorbing devices. The use of energy absorbing devices in the webbing of the shoulder harness or safety belt is not recommended primarily because of the increased potential for secondary occupant impacts. Energy absorbing devices, also known as load limiting devices, are best incorporated in the airframe structure or aircraft seat.
Secondary impact and insufficient torso retention should be expected when energy absorbing devices are used in webbing, and webbing elongation should be limited.
Buckles. Buckles are the basic means for securing the various segments of a shoulder harness-safety belt system around an occupant while also providing the means of release of the system. The equipment requirements for safety belts and shoulder harnesses in the FAR for a metal-to-metal coupling provide improved security and reliability over any method of coupling which relies on clamping of the webbing.
Although the load bearing components of the buckle latch and Chap 2 6 Par 3