the load path to the webbing are metal, a nonmetallic cover on the buckle for decoration, corrosion protection, etc., is an acceptable practice.
(l) Buckle release. For security, design of the release mechanism should minimize the possibility for inadvertent release by the occupant and premature release by inertial forces acting on the mechanism in an accident. For escape after an accident, buckle release characteristics should permit release with one finger.
(2) Buckle types. Buckles are typed, in a general sense, by the method for manually releasing the buckles.
'lift lever buckle" and the "rotary buckle" are two types commonly used in aircraft shoulder harness-safety belt systems. Examples of the lift lever and rotary buckles are illustrated in Figure 4. Rotary buckles permit separation of all segments of a 4 or 5-point shoulder harness system for unrestricted escape by the occupant. Rotary buckles also allow independent fastening of the shoulder harness straps or the negative-G strap. Lift lever buckles are generally used with systems composed of a safety belt and a single diagonal shoulder belt, but they have also been successfully used with dual shoulder belt systems. Lift lever buckles are used more often in aircraft safety belts.
Chap 2 Par 3 7
Figure 4. Lift Lever and Rotary Buckles
e. Adjustment hardware. A variety of manual length adjustment hardware is available, and all rely on friction between webbing and the hardware. All length adjusters are sensitive to how the webbing is threaded through the adjuster, and some are sensitive to positioning within the belt assembly. Some adjusters are sensitive to the type of webbing material and the weave. In particular, certain precautions are warranted for adjusters known, in a general sense, as the 3-bar slide adjuster, the tilt lock adjuster, and the cam lock adjuster.
(1) Three-Bar slide adjuster. A 3-bar slide adjuster is illustrated in Figure 5(a). The 3-bar slide adjuster was one of the earliest manual length adjusters used in aircraft to provide semi-permanent length adjustment of stock safety belts for installation with numerous seating and anchor point configurations.
Chap 2 8 Par 3
(1) Figures 5(b) and 5(c) 1llustrate the two methods most appropriate for threadiny webbing through a 3-bar slide adjuster; however, controlled tests, using low level cyclic loads, indicate that either method can permit minor i bang These slippage of the webbing through the adjuster.
tests also demonstrated that excessive slippage and overall belt extension will occur if the slots of the adjuster are too wide for the webbing thickness used, such as imm 3-Bor Side Adjuste webbing in an adjuster with (a)
0.25 inch slots.
(11) When using the 3-bar slide adjuster, thread only the adjuster. Additional thicknesses of webbing will increase the bending loads on the bars and promote premature failure of the adjuster. Therefore, it is poor practice to stow excess action A -A webbing in this type of adjuster by threading it back through the slots.
(111) It is good practice to locate the 3-bar slide adjuster as near the attachment fitting as possible (make end loop as small as extension by slippage of the possible) to minimize any belt Figure 5. Three-Bar Slide Webbing Loop tension end of the webbing through the adjuster.
(iv) A 3-bar slide adjuster may permit belt extension to occur to tightening and loosening cycles during normal use. Consequently, it is good practice to check the belt length periodically. For this reason, proper stitching of the webbing to the end fitting is preferred over the use of the 3-bar slide adjuster.
Chap 2 Par 3 9
(2) Tilt lock adjuster. Three types of tilt lock adjusters are illustrated in figure 6. Tilt lock adjusters rely on the mechanism of clamping the webbing between a locking bar and the frame of the adjuster, and the clamping force is produced by tension from the anchored end of the webbing. The locking bar may be smooth, but is more often roughened with knurling, serrations, or a sandpaper-like finish. The type of roughness on the cross bar is generally intended for one type of webbing material or webbing weave.
Some designs use a spring loaded cross bar to inhibit loosening of the shoulder harness or safety belt by vibration and normal occupant movement. Locking force and the amount of webbing wrapped around the locking bar is greatest when the frame plate is parallel to the webbing. Increasing the angle (tilt) between the frame plate and the tension end of the webbing decreases the clamping force on the webbing, which permits the webbing to be drawn through the adjuster in either direction. The primary precaution regarding the tilt lock adjuster is that proper occupant restraint is provided only if the webbing is properly looped around the lock bar.
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SPRING LOADED
TILT LOCK ADJUSTER
(a) ~ Lock Bor Lock Bar Spring Anchor Section A-A uckh (a1) PROPER WEBBING LOOP Nlockdor -Spring Free End! Section A-A obbin tisched Fol (a-2) IMPROPER WEBBING LOOP jucka nger1 Tit Lock Adjuat
(b) IN - LINE
TILT LOCK ADJUSTER (C)
• Knurled Lack Bar Free End Wobbing Away From Occupant To Anchor Section B - B Possiblo Froo End (8-1) Proper weBBing Loop Band Webbing To Anchor To Buckk Soctiom C-C Locked of Section 8/8 (C-1) PrOPER WEBBIG LOOP 30° or Mors (D-2) TILT LOCK ANGLE (Examolo) Figure 6. Tilt Lock Adjuster Webbing Loop Chap 2 Par 3 11
(1) The term "tilt lock" refers to the angle at which the webbing is locked. figure 6(b-2) illustrates this aspect. To maintain locking with the adjuster bearing on a curved surface, such as an occupant, a tilt lock adjuster should lock with a "tilt" angle of 30 degrees or more between the adjuster plate and webbing, with a webbing tension force of 20 pounds, and remain locked for all smaller angles.
(11) The adjuster 11lustrated in Figure 6(a) is sensitive to the manner in which the webbing is looped on the lock bar. As 11lustrated in Figure 6(a-1), the webbing should be looped around the lock bar in the manner which causes belt tension to draw on the flat side of the lock bar and applies shear loads to the thinner ears of the lock bar.
The shear reaction in the thinner ears is low. An improper webbing loop appears, and provides adjustment, about the same as a proper webbing loop on this type of adjuster, but the strength of the assembly is substantially reduced.
Therefore, shoulder harness and safety belt systems using this type of adjuster warrant close examination of the webbing loop on the lock bar of the adjuster.
(111) The tilt lock adjusters 11lustrated in figures 6(b) and 6(c) are similar in their method of providing security and adjustment with a knurled lock bar.
The two designs differ in the method of retaining the lock bars. Either of these adjusters may be integrated with a buckle insert or a buckle base plate. Also, the lock bar may be spring loaded for better resistance against belt extension through vibration and normal occupant movement. In some designs of the buckle insert adjuster, the plate is bent about 15 degrees to provide better conformity to an occupant and improved locking characteristics. A precaution is warranted regarding the bent buckle insert adjuster shown in figure 6(b-1), in that an improper webbing loop on the lock ar, or insertion into a buckle upside down, will defeat the mechanism for locking the webbing. Figure 6(b-1) 11lustrates the proper webbing loop around the lock bar for these types of adjusters. A decorative cover on these types of adjusters makes an improper webbing loop less obvious, and closer examination is needed to ensure integrity of the shoulder harness or safety belt.
Kauries for pring Loader oring Ca all Tob Com SECTION A-A Nose Figure 7. Cam Lock Adjuster Chap 2 12 Par 3
Cam lock adjuster. The cam lock webbing adjuster illustrated in figure 7 is common in military shoulder harness-safety belt systems. The cam is generally spring loaded for sufficient clamping of the webbing to prevent slippage of the webbing by vibration and normal occupant movement. Frequent adjustment with the cam lock adjuster may produce more rapid deterioration (fraying) of conventional webbing than some of the other types of adjusters. Other cam lock adjuster designs may be available.
The principal precaution is to ensure that the webbing is looped through the adjuster so that restraint loads tend to increase the clamping force of the cam.
Webbing retractors. Webbing retractors are frequently incorporated in shoulder harness and safety belt systems to satisfy the crewmember movement and/or webbing stowage requirements of the FAR. They can also enhance comfort and ease of length adjustment, which encourages use of the shoulder harness as well as the safety belt.
Retractors are categorized by the point in time when they lock to provide occupant restraint. These categories are the emergency locking retractor and the automatic locking retractor. It is difficult to distinguish between the two categories by a cursory visual examination.
Emergency locking retractor. Emergency locking retractors are frequently called "inertia reels" because their functional mechanism is characterized by the feature of providing positive restraint only when significant inertial forces are experienced. Three types of emergency locking retractors which may be encountered are as follows:
(1) One type of "inertia reel" appropriate for aircraft use is known as the webbing sensitive reel. It produces locking by a change in the rate (acceleration) of webbing withdrawal from the retractor, which is functional for occupant accelerations in any direction producing extension of the webbing. A locking acceleration of 0.75 to
1.5 G's is satisfactory.
(11) A second type of "inertia reel", which is common in automotive applications, is known as the vehicle sensitive reel. It produces locking by a physical acceleration of the retractor itself, or it may be locked by a remote sensor on the vehicle.
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(111) A third type of "inertia reel" has a dual locking mechanism which combines the favorable features of both the webbing sensitive reel and the vehicle sensitive reel. This type of reel is quite suitable for aircraft applications.
A precaution is that emergency locking retractors are for use on shoulder belts only. Their use on safety belts prevents proper tightening of the safety belt on the pelvis and promotes occupant submarining under the belt in a dynamic loading environment, because the design inherently permits a certain amount of webbing extension before it locks. Also, if turbulence occurs which does not produce accelerations large enough to provide locking, no restraint is provided.
Automatic locking retractor. Automatic locking retractors provide automatic retraction of webbing for length adjustment and stowage of webbing. Their functional mechanism is characterized by the feature of permitting free webbing extension for coupling of the safety belt, but the moment any webbing is automatically retracted, the locking mechanism locks to prevent further webbing extension. However, the designs usually prevent locking within about the first 25 percent (6 to 10 inches) of webbing extended from the retractor. Therefore, a precaution for use of the automatic locking retractors is to ensure that the mounting location for these retractors will actually produce locking (more than 25 percent webbing extension) when the occupant has the safety belt coupled around him. When automatic locking retractors are incorporated at both attachments of the safety belt, buckle positioning by the user becomes important for proper function of the retractors under emergency conditions.
(3) General retractor precautions. Experience has shown that certain features of retractors, in general, are worthy of mention to ensure proper function:
(1) A properly designed retractor incorporates a webbing guide to prevent folded webbing from wrapping onto the spool. The guide also serves to prevent webbing from wrapping onto an exposed locking ratchet. Some ratchets are sharp enough to penetrate and cut the webbing.
The guide may be an integral part of the retractor, or it may be provided by the retractor housing. Alternatively, straightening and alignment of the webbing to the retractor spool may be provided by a remote pull-through guide. Folded webbing wrapped on the spool may promote binding and disuse or malfunction of the shoulder harness or safety belt.
Chap 2 14 Par 3
Spool Locking Mechanism (Spring Housing
- One Type of Guide Entry- Exit Angle -Webbing 15
(ii) Proper mounting of retractors provides a straight line entry and exit of webbing through the integral or local webbing guide, as illustrated in Figure 8. The straight line entry and exit minimizes abrasive wear on the webbing, and more importantly, it minimizes frictional drag between the webbing and guide, which could inhibit functioning of the retractor. A remote webbing guide may be warranted when seat backs or other structures prevent straight line entry and exit of the webbing at the local webbing guide.
(111) Methods of attaching retractors to the airframe or seat are generally intended to load attachment fasteners in shear. Sufficient fastener holes are normally available in the retractor frame or base, and fasteners of sufficient strength should be used which fill the available holes. Attachment configurations resulting in bending or tension loads on the fasteners under accident conditions warrant scrutiny for strength of the fasteners and the frame or base of the retractor.
(iv) Retractors should be mounted to minimize exposure to dirt and abuse.
(v) Too much webbing on the retractor spool can prevent the locking device from engaging the ratchet.
For this reason, the locking function should be checked with the belts extended and coupled around a small occupant.
(vi) Check the ability of the retractor installation for sufficient retraction force to overcome webbing drag on the seat or through guides and other structures, to ensure proper retrieval of webbing slack during operation of the aircraft. The webbing retrieval feature may also improve occupant restraint in a multiple impact accident.
Attachment end fittings. A variety of end fittings may be found in service, but they are generally classed as "bolt-on fittings" or "quick disconnect fittings" as 1llustrated in figure 9.
(1) Minimize bending. The principal precaution for installing the fitting is to minimize bending stress in the fitting. The fittings of figure 9 are designed to load the attaching bolt in shear. When the application is likely to load the attaching bolt in bending or tension, a specially designed fitting is warranted, which is preformed to align the fitting with the principal forces from the belt.
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(2) is self alignme any ttach alion with fee. Self alignment. The quick disconnect fitting of figure 9 is intolerant of any substantial bending loads.
Therefore, the airframe or seat attachment needs to permit the quick disconnect end fitting to self align with tension forces applied through the belt webbing.
(3) Keeper damage. A second precaution in the installation of a quick disconnect fitting is that attachment at the floor subjects the spring-loaded keeper to possible damage from being stepped on, with subsequent possibility of unhooking in an accident environment. The keeper has a cotter pin hole and the use of a cotter pin attempts to preclude inadvertent unlatching.
Cotter Pin Bolt on End Fitting Quick Disconnect End Fitting Figure 9. End Fittings
4. INSTALLATION GEOMETRY.
a. Safety belt. A safety belt is often called a lap beit or a seat beit. It restrains the occupant at the pelvic region. Safety belts generally perform best when they act at an angle of about 45 degrees relative to the aircraft longitudinal axis, as in figure 10(a). Attachment of a safety belt to the floor for an adjustable position seat needs special attention in maintaining a proper belt angle, because the angle changes with seat positioning. Locating the attachment where the belt angle is between 45 and 55 degrees, as in figure 10(b), for all seat positions generally provides sufficient occupant restraint.
Chap 2 Par 4 17
(1) Shallow angle. If the safety belt is installed so that it acts along a shallow angle, as in figure 10(c), it is likely to slip off the pelvis of the occupant, and apply loads to the abdomen, with a likelihood of injury to internal organs. In addition, the shallow belt angle is prone to produce "anterior wedge fracture of the lumbar vertebra" as the upper torso flexes over the belt and is compressed under accident inertial forces. Muscular resistance to the upper torso flexure is unlikely for even the strongest individuals at decelerations above 3 or 4 G's.
(2) Steep angle. If the safety belt is installed at too steep an angle, as in figure 10(d), it will be ineffective in resisting forward movement of the occupant.
Since the belt can carry tension loads only, the occupant will move forward until the belt geometry is reoriented to an angle which generates enough tension in the belt to resist further forward movement. An extreme safety belt angle like that of figure 10(d) permits knee impact with the instrument panel resulting in knee or femur injury, or forward movement to the extent that the occupant slips off the front edge of the seat allowing the belt angle to become shallow with all the injury potential mentioned above.
(3) Webbing effects. Webbing elasticity, previously discussed in subparagraph 3a(1), permits additional occupant movement. Therefore, prudent selection of the safety belt attachment point will also consider the length of webbing involved and the relative effects of webbing elasticity.
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