6/1/79 *the limit load, then the maximum breaking strength of the towline assembly is established at the design load of 800 pounds. Thus, the towline will break well before structural damage will occur to the towplane.
Another approach can be applied if the limit load carrying capabilities of a tow hook and fuselage are known. In this case, the known load value can be divided by 1.5 to arrive at the design load capabilities if the tow hook and fuselage limit loads are known to be 1,800 pounds. By dividing by 1.5 (1800 ÷ 1.5 = 1,200) we arrive at a design load value of 1,200 pounds. Thus, the maximum breaking strength of the towline assembly is established at 1,200 pounds and provides protection for the towplane.
Thus, in considering tow hook installations, one may establish maximum towline breaking strength
(1) Dividing the known limit load capabilities of the fuselage and two hook installation by 1.5; or
(2) Knowing the design load needs of the towline assembly and multiplying by 1.5 to arrive at a limit load. Then by analysis or static testing, determine that the hook and fuselage are capable of withstanding that limit load.
support a limit load equal to at least two times the operating weight of the banner.
127. STRUCTURAL TESTING. Adequacy of the aircraft structure to withstand the required loads can be determined by either static test or structural analysis.
a. Static testing.When using static tests to verify structural strength, subject the tow hitch to the limit load (per paragraph 126 a or b) in a rearward direction within the appropriate cone of displacement per figure 8.2. Testing to be done in accordance with the procedures of Chapter 1, paragraph 3, of this handbook.
b. Structural analysis. If the local fuselage structure is not substantiated by static test for the proposed tow load, using a method that experience has shown to be reliable, subject the fuselage to engineering analysis to determine that the local structure is adequate. Use fitting factor of 1.15 or greater in the loads for this analysis.
128. ATTACHMENT POINTS. Tow-hitch mechanisms are characteristically attached to, or at, tiedown points or tailwheel brackets on the air- Chap 8 AC 43.13-2A, CHG 1 frame where the inherent load-bearing qualities can be adapted to towing loads. Keep the length of the hitch-assembly arm from the airframe attachment point to the tow hook to a minimum as the loads on the attachment bolts are multiplied by increases in the moment arm.
129. ANGLES OF TOW. Tests should be conducted on the system at various tow angles to insure that:
a. There is no interference with the tailwheel or adjacent structure.
b. The towline clears all fixed and movable surfaces at the maximum cone of displacement and full surface travel.
c. The mechanism does not significantly decrease the clearance from the tailwheel to the
d. The tow hitch does not swivel. Experience has shown swiveling could result in fouling both the release line and towline during operations by the towplane.
e. The opened jaw of the hitch does not strike any portion of the aircraft.
130. PLACARDS. A placard should be installed in a conspicuous place in the cockpit to notify the pilot of the structural design limits of the tow system.
The following are examples of placards to be inbreaking strength not to exceed a. For glider tow-"Glider towline assembly pounds."
b. For banner tow-Tow hitch limited to maximum weight of pounds."
* Value established per paragraph 126 a (1) pr (2).
** Banner hitch limitations are one-half the load applied per paragraph 126 b.
131. WEIGHT AND BALANCE. In most cases, the weight of the tow-hitch assembly will affect the fully loaded aft c.g. location. To assure that the possibility of an adverse effect caused by the installation has not been ignored, enter all pertinent computations in the aircraft weight & balance records. (In accordance with the provisions contained in FAR 43.5(a) (4).)
132. TOW RELEASE MECHANISM.
a. Release lever. A placard indicating the direction of operation should be installed to allay the possibility of confusion or inadvertant opera-* 59
AC 43.13-2A, CHG 1 6/1/79 * tion, and the design of the release lever should provide the following:
(1) Convenience in operation.
(2) Smooth and positive release operation.
(3) Positioned so as to permit the pilot to exert a straight pull on the release handle.
(4) Sufficient handle travel to allow for normal slack and stretch of the release cable.
(5) A sufficient handle/lever ration to assure adequate release force when the towline is under high loads. (See fig. 8.3)
(6) Protection of cables from hazards such as:
(a) Wear and abrasion during normal operation.
(b) Binding where cables pass through fairleads, pulleys, etc.
(c) Accidental release.
(d) Interference by other aircraft com-
(e) Freezing and moisture accumulation when fixed or flexible tubing guides are used.
b. Test of the release. A test of the release and hook for proper operation through all angles of critical loading should be made using the design load for the glider or banner.
c. Release cable. Representative size strength characteristics of steel release cable are as shown in figure 8.4; however, it is recommended that all internally installed release cables be ½6-inch or larger.
Nonflexible Flexible Carbon Steel Carbon Steel 7 x 7 and 7 x 19 1 x 7 and 1 x 19 (MIL-W-1511A and Diameter (MIL-W-6904B) MIL-C-5424A) inches Breaking strength (Los.) Pounds 100 f. Breaking strength (lbs.) Pounds 100 p.
1/32 185 .20 3/64 375 .55 1/16 500 .85 480 .75 5/64 800 1.40 - 3/32 1,200 2.00 920 1.60 FIGURE 8.3-Typical tow-hitch release handle. FIGURE 8.4.- Representative steel cable qualities.
133. [Deleted] Change 1.
FIGURE 8.5.- [Deleted] Change 1. * 60 Chap 8 U.S. GOVERNMENT PRINTING OFFICE: 1979 O--302-515
(DELETED) -- Change 2 Chap. 8 61
AC 43.13-2A Rev. 1977 FIGURE 8.6.—Tricycle gear aircraft.
FIGURE 8.7.-Conventional gear aircraft- leaf spring type tailwheel.
Chap. 8
Rev. 1977 AC 43.13-2A FIGURE 8.8.- Conventional gear aircraft- -shock strut type tailwheel.
FIGURE 8.9.-Conventional gear aircraft-tubular spring type tailwheel.
134.-145. [RESERVED] Chap. 8 63
Rev. 1977 AC 43.13-2A Chapter 9. SHOULDER HARNESS INSTALLATIONS Section 1. RESTRAINT SYSTEMS
146. GENERAL. primary objective in shoulder harness design is to prevent incapacitating and/or fatal injuries to personnel involved in a survivable crash condition in which the aircraft cabin structure remained reasonably intact.
Any harness configuration which achieves this objective is satisfactory from a safety viewpoint, regardless of the type of harness and mounting position used.
Basic requirements of the aircraft airworthiness rules are designed to provide an aircraft structure to give each occupant a reasonable chance of escaping serious injury in a crash land- These requirements adequately provide for conditions that can be expected to occur in various types of survivable accidents.
The human body has the inherent capability of withstanding decelerations of 20g's for time periods of up to 200 milliseconds (.2 second) without injury. Experience with aircraft used in agricultural and military operations shows that even in such unusual operations a high rate of survival in crashes is achieved when a restraint system is designed on the order of 20g to 25g deceleration loads.
In view of the foregoing, persons installing a shoulder harness may wish to use a restraint system designed to withstand 20g to 25g loads.
In addition, seat belts and seat belt anchorages designed to these load limits may be used.
147. TYPES OF RESTRAINT SYSTEMS. There are two generalized types of shoulder harnesses currently in use. They are the single diagonal type harness and the double over-the-shoulder type The over-the-shoulder harness may utilize either two independent attach points, or join in a "Y" configuration and attach at a single point. (See figs. 9.1 and 9.2) In all cases, however, the original safety belt or a combination harness utilizing a lap belt must be used in the installation.
148. ADVANTAGES OF DIFFERENT HARNESS TYPES. The single diagonal chest strap in combination with a lap belt is the simplest harness system and works effectively for longitudinal decelerations. However, during side decelerations, an occupant in this type harness has a tendency to slip out and away from the chest strap even when it fits snugly. The double over-the-shoulder type harness works well for both longitudinal and side decelerations.
149. MOUNTING CONFIGURATIONS. The type of shoulder restraint configuration acceptable for installation is dependent upon the attachments available in each individual aircraft. Basic harness mounting configurations are:
a. Seat mounted.
b. Airframe mounted.
(1) Side
(2) Ceiling
(3) Floor
(4) Directly rearward
150. STANDARDS. At the present time, there is a lack of standards for materials acceptable for use in shoulder harnesses. Until such time as a TSO is developed for shoulder harnesses, standards established in TSO-C22f pertaining to the materials and testing of safety belts may be accepted for this purpose.
151. MATERIALS.
a. Webbing. Synthetic materials, such as nylon and dacron, may be used for shoulder harness webbing. It is recommended that the webbing of the shoulder harness be the same as that of the lap belt to avoid problems in cleaning, staggered replacement of harness components due to wear or age, etc.
Chap 9 65
AC 43.13-2A
b. Fittings. Use hardware that:
(1) Conforms to an acceptable standard such as AN, NAS, TSO, or MIL-SPEC.
(2) Meets the strength required by FAR
23.1413 or 25.1413, as appropriate.
(3) Will not loosen in service due to vibration or rotational loads.
C. Inertia Reels. The function of the inertia reel is to lock and restrain the occupant in a crash yet provide the ability for normal movement without restrictions. In addition, automatic rewinding of any slack assures that the harness is always snug, which results in a more comfortable restraint system.
Self-contained inertia reel units may be mounted at readily accessible locations and will generally operate effectively in any attitude.
Their use in a body restraint system is satisfactory if mounted in accordance with acceptable methods, techniques, and practices, and will meet FIGURE 9.1.-Single diagonal type harness.
66 Rev. 1977 static strength requirements equal to those outlined in FAR 23.1413 Check the reel itself for the following operational hazards:
(1) Inadvertent Lockup. If the inertia mechanism is set at a low "g" setting, unwanted lockup, or binding, of the system may occur.
reel lockup range between .9 and 2.5 "g" is acceptable.
(2) Improper Webbing Length. Install adequate webbing on the takeup reel to allow the occupant to reach all necessary switches and controls in the cockpit. Any additional webbing will result in decreasing the reel spring "takeup" tension exerted on the shoulder.
(3) Incorrect Belt Opening Alignment. Position the reel so that the belt opening is aligned in the direction of loading. This will prevent the belt from rubbing and fraying due to normal usage.
FIGURE 9.2.-Double over-the-shoulder type harness.
152.-155. [RESERVED] Chap 9
Rev. 1977 AC 43.13-2A Section 2. EFFECTIVE RESTRAINT ANGLES
156. RELATIONSHIP OF THE HARNESS ASSEMBLY TO THE OCCUPANT. Most restraint systems are designed so that each belt section maintains a certain relationship to the body. The attachment end of a restraint belt must maintain a relative angle and spacing to the head and neck surfaces as it passes over the shoulder and away from the body. This angle must provide sufficient freedom to assure normal body movements of the seated occupant without neck contact or interfering with vision.
157. ATTACHMENT AREAS FOR SHOULDER HAR- NESS. Effective attachment areas for the various types of shoulder harnesses are defined as angles formed by the attachment ends. Assure that when installing a harness for one seat that in a crash, a passenger to the rear would not sustain head impact injuries on the harness or its attach point.
a. Single Diagonal Type Harness. The optimum rearward attachment area for this type of harness is within an angle of 30 degrees above the horizontal measured from the midpoint on the occupant's shoulder as shown in figures 9.3 and 9.4.
Belt attachments should be located to the rear and outboard of the shoulder. This mounting area is shown in figure 9.5.
(I) Attachment points inboard of this area would permit the harness to impinge on the neck and could result in neck injury during crash impact. In addition, the constant rubbing of the strap on the neck would be uncomfortable and, as a result, act as a distraction to the safe operation of the aircraft. Attachment points forward of this area would reduce the effectiveness of the harness, due to a lack of contact between the harness and the upper torso of the occupant. In addition, a shoulder strap attached forward of FIGURE 9.3.-Side mounted-single diagonal type harness.
Chap 9
AC 43.13-2A Rev. 1977 30 FIGURE 9.4.- Ceiling mounted inertia reel-single diagonal type harness.
the shoulder midpoint could obstruct vision and create a potential safety hazard.
(2) The harness should be kept snug as any decrease in the distance between the occupant's head and the forward cabin structure increases the opportunities for head impact injuries. Also, the chances for twisting out of the harness are significantly increased.
b. Double Over-the-Shoulder Type Harness. If this type of harness is intended to be mounted either directly rearward or to the ceiling, mount it within the 30-degree vertical angle as shown in figure 9.6.
Because of the limited number of rearward shoulder harness attachment points in many aircraft, a 5-degree angle below the horizontal is also considered acceptable.
68 Shoulder harness attach areas as viewed from above are shown in figure 9.7. These mounting areas may be used for either the independent or the "Y" type belts. The outboard limit is established to prevent the belt section from slipping off the shoulder, and the maximum inboard angle is limited to a point which will prevent impingement on the neck surface.
158. 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 as the optimum and not be interpreted as being mandatory. 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 a harness installation in an aircraft which otherwise could not be accom- Chap 9
Rev. 1977 plished. It is probable that other compromises may be necessary when adapting a specific restraint system to an aircraft in order to fit a body AC 43.13-2A of average dimensions. These compromises, however, should be permitted only when they are compatible with proper restraint functions.
FIGURE 9.5.-Acceptable mounting area-single diagonal type harness.
30° UP HORIZ. REF 5° DOWN FIGURE 9.6.--Ceiling mounted inertia reel-double over-the-shoulder type harness.
Chap 9 69
Rev. 1977 AC 43.13-2A FIGURE 9.7.-Acceptable mounting areas-double over-the-shoulder type harness.
159.-160. [RESERVED] 70 Chap 9
Rev. 1977 AC 43.13-2A Section 3. ATTACHMENT METHODS
161. STRUCTURAL ATTACHMENTS. For best results, the restraint system should be anchored to the primary aircraft structure. Design the structural attachment to spread the suddenly applied impact loads over as large an area of the structure as possible. The shoulder harness may be attached to selected secondary members which will deform slowly or collapse at a limited rate.
This will assist in dissipating the high impact #g" loads to a level tolerable to the human body.
However, the possibility of secondary members collapsing and making it difficult for an occupant to extract himself from the harness should not be overlooked.
162. FLOOR AND SEAT ATTACHMENTS. The double over-the-shoulder type harness shown in figure 9.8 may be used with either floor or seat mounting points, and typical installation methods are illustrated in figures 9.9 and 9.10. Two prerequisites necessary to ensure an effective restraint system are:
a. The seat structure and its anchorages should be capable of withstanding the additional "g" loads imposed by the restrained occupant during an abrupt deceleration. This capability may be determined by static testing in accordance with FAR 23.785, 25.785, 27.785, or 29.785, as appropriate; or, by securing a statement attesting this adequacy from the airframe manufacturer's engineering department.
b. The level of the seat back should at least be equal to the shoulder height of the seated occupant. This will reduce the inherent downward impact loads which would otherwise impart compressive forces. on the occupant's torso.
FIGURE 9.8.-Floor mounted inertia reel-double over-the-shoulder type harness.
Chap 9 251-957 0 - 77 - 4