Par 7-122 Page 7-312
FIGURE 7-3. Securing screws, nuts, bolts, and snaprings.
FIGURE 7-3a. Wire twisting by hand.
copper shear safety wire. Particular care should be exercised to ensure that the use of safety wire will not prevent emergency operation of the devices.
SAFETY-WIRING PROCEDURES. There are many combinations of safety wiring with certain basic rules common to all applications.
These rules are as follows.
When bolts, screws, or other parts are closely grouped, it is more convenient to safety wire them in series. The number of bolts, nuts, screws, etc., that may be wired together depends on the application.
Drilled boltheads and screws need not be safety wired if installed with self-locking nuts.
To prevent failure due to rubbing or vibration, safety wire must be tight after installation.
Safety wire must be installed in a manner that will prevent the tendency of the part to loosen.
Safety wire must never be over-stressed. Safety wire will break under vibrations if twisted too tightly. Safety wire must be pulled taut when being twisted, and
Par 7-124 Page 7-313 maintain a light tension when secured. (See figure 7-3a.)
Safety-wire ends must be bent under and inward toward the part to avoid sharp or projecting ends, which might present a safety hazard.
Safety wire inside a duct or tube must not cross over or obstruct a flow passage when an alternate routing can be used.
Check the units to be safety wired to make sure that they have been correctly torqued, and that the wiring holes are properly aligned to each other. When there are two or more units, it is desirable that the holes in the units be aligned to each other. Never overtorque or loosen to obtain proper alignment of the holes.
It should be possible to align the wiring holes when the bolts are torqued within the specified limits. Washers may be used (see paragraph 7-37) to establish proper alignment.
However, if it is impossible to obtain a proper alignment of the holes without undertorquing or overtorquing, try another bolt which will permit proper alignment within the specified torque limits.
To prevent mutilation of the twisted section of wire, when using pliers, grasp the wires at the ends. Safety wire must not be nicked, kinked, or mutilated. Never twist the wire ends off with pliers; and, when cutting off ends, leave at least four to six complete turns (1/2 to 5/8 inch long) after the loop. When removing safety wire, never twist the wire off with pliers. Cut the safety wire close to the hole, exercising caution.
Install safety wire where practicable with the wire positioned around the head of the bolt, screw, or nut, and twisted in such a manner that the loop of the wire fits closely to the contour of the unit being safety wired.
TWISTING WITH SPECIAL TOOLS. Twist the wire with a wire twister as follows. (See figure 7-4.)
CAUTION: When using wire twisters, and the wire extends 3 inches beyond the jaws of the twisters, loosely wrap the wire around the pliers to prevent whipping and possible personal injury. Excessive twisting of the wire will weaken the wire.
Grip the wire in the jaws of the wire twister and slide the outer sleeve down with your thumb to lock the handles or lock the spring-loaded pin.
Pull the knob, and the spiral rod spins and twists the wire.
Squeeze handles together to release wire.
SECURING OIL CAPS, DRAIN COCKS, AND VALVES. (See figure 7-4a.) When securing oil caps and drain cocks, the safety wire should be anchored to an adjacent fillister-head screw.
This method of safety wiring is applied to wingnuts, filler plugs, single-drilled head bolts, fillister-head screws, etc.; which are safety wired individually. When securing valve handles in the vertical position, the wire is looped around the threads of the pipe leading into one side of the valve,
Par 7-124 Page 7-314
FIGURE 7-4. Use of a typical wire twister.
double-twisted around the valve handle, and anchored around the threads of the pipe leading into the opposite side of the valve. When castellated nuts are to be secured with safety wire, tighten the nut to the low side of the selected torque range, unless otherwise specified;
and, if necessary, continue tightening until a slot lines with the hole. In blind tapped hole applications of bolts or castellated nuts on studs, the safety wiring should be in accordance with the genera l instructions of this chapter. Hollow-head bolts are safetied in the manner prescribed for regular bolts.
NOTE: Do not loosen or tighten properly tightened nuts to align safety-wire holes.
NOTE: Although there are numerous safety wiring techniques used to secure aircraft hardware, practically all are derived from the basic examples shown in figures 7-5 through 7-5b.
FIGURE 7-4a. Securing oil caps, drain cocks, and valves.
Par 7-126 Page 7-315
Examples 1, 2, 3, and 4 apply to all types of bolts, fillister-head screws, square-head plugs, and other similar parts which are wired so that the loos ening tendency of either part is counteracted by tightening of the other part. The direction of twist fr om the second to the third unit is counterclockwise in examples 1, 3, and 4 to keep the loop in positi on against the head of the bolt. The direction of twist from the second to the third unit in example 2 is clockwise to keep the wire in position around the second unit. The wire entering the hole in the thir d unit will be the lower wire, except example 2, and by making a counterclockwise twist after it leaves the hole, the loop will be secured in place around the head of that bolt.
Examples 5, 6, 7, & 8 show methods for wiri ng various standard items, NOTE: Wire may be wrapped over the unit rather than around it when wiring cast ellated nuts or on other items when there is a clearance problem.
Example 9 shows the method for wiring bolts in different planes. Note that wire should always be applied so that tension is in the tightening direction.
Hollow-head plugs shall be wired as shown with the tab bent inside the hole to avoid snags and possible injury to personnel working on the engine.
Correct application of single wire to closely spaced multiple group.
FIGURE 7-5. Safety-wiring procedures.
Par 7-127 Page 7-316
Examples 12 and 13 show methods for attaching lead seal to protect critical adjustments.
Example 14 shows bolt wired to a right-angle bracket with the wire wrapped around the bracket.
Example 15 shows correct method for wiring adjustable connecting rod.
Example 16 shows correct method for wiring the coupling nut on flexible line to the straight connector brazed on rigid tube.
Fittings incorporating wire lugs shall be wired as shown in Examples 17 and 18. Where no lock- wire lug is provided, wire should be applied as shown in examples 19 and 20 with caution being exerted to ensure that wi re is wrapped tightly around the fitting.
Small size coupling nuts shall be wired by wrapping the wire around the nut and inserting it through the holes as shown.
FIGURE 7-5a. Safety-wiring procedures.
Par 7-126 Page 7-317
Coupling nuts attached to straight connectors shall be wired as, shown, when hex is an integral part of the connector.
Coupling nuts on a tee shall be wired, as shown above, so that tension is always in the tightening direction.
Straight Connector (Bulkhead Type)
Examples 26, 27, and 28 show the proper met hod for wiring various standard fittings with checknut wired independently so that it need not be disturbed when removing the coupling nut.
FIGURE 7-5b. Safety-wiring procedures.
Par 7-140 Page 7-318 SECURING WITH COTTER PINS.
Cotter pins are used to secure such items as bolts, screws, pins, and shafts. Their use is favored because they can be removed and installed quickly. The diameter of the cotter pins selected for any application should be the largest size that will fit consistent with th e diameter of the cotter pin hole and/or the sl ots in the nut. Cotter pins should not be reused on aircraft.
To prevent injury during and after pin installation, the end of the cotter pin can be rolled and tucked.
NOTE: In using the method of cotter pin safetying, as shown in figures 7-6 and 7-7, ensure the prong, bent over the bolt, is seated firmly against the bolt sh ank, and does not exceed bo lt diameter. Also, when the prong is bent over the nut, ensure the bent prong is down and firmly flat against the nut and does not contact the surface of the washer.
Par 7-140 Page 7-319
FIGURE 7-6. Securing with cotter pins.
FIGURE 7-7. Alternate method for securing with cotter pins.
7-128.7-139. [RESERVED.]
Par 7-140 Page 7-320 SECTION 8. INSPECTION AND REPAIR OF CONTROL CABLES
AND TURNBUCKLES
GENERAL. Aircraft control cables are generally fabricated from carbon steel or corrosionresistant steel wire of either flexible or nonflexible-type construction.
CABLE DEFINITIONS. The following cable components are defined in accordance with Military Specifications MIL-W-83420, MIL-C-18375, and MIL-W-87161.
Wire Center. The center of all strands shall be an individual wire and shall be designated as a wire center.
Strand Center or Core. A strand center is a single, straight strand made of preformed wires, similar to the other strands comprising the cable, in arrangement and number of wires.
Independent Wire Rope Center (IWRC) 7 by 7. A 7 by 7 independent wire rope center as specified herein shall consist of a cable or wire rope of six strands of seven wires each, twisted or laid around a strand center or core consisting of seven wires.
FLEXIBLE CABLES. Flexible, preformed, carbon steel, Type I, composition A cables, MIL-W-83420, are manufactured from steel made by the acid-open-hearth, basic-open hearth, or electric-furnace process. The wire used is coated with pure tin or zinc. Flexible, preformed, corrosion-resistant, Type I, composition B cables, MIL-W-87161, MIL-W-83420, and MIL-C-18375 are manufactured from steel made by the electricfurnace process. (See table 7-3 and figure 7-8.)
These cables are of the 3 by 7, 7 by 7, 7 by 19, or 6 by 19 IWRC construction, according to the diameter as specified in table 7-3. The 3 by 7 cable consists of three strands of seven wires each. There is no core in this construction.
The 3 by 7 cable has a length of lay of not more than eight times or less than five times the nominal cable diameter. The 7 by 7 cable consists of six strands, of seven wires each, laid around a center strand of seven wires. The wires are laid so as to develop a cable which has the greatest bending and wearing properties. The 7 by 7 cable has a length of lay of not more than eight times or less than six times the cable diameter. The 7 by 19 cable consists of six strands laid around a center strand in a clockwise direction. The wires composing the seven individual strands are laid around a center wire in two layers. The center core strand consists of a lay of six wires laid around the central wire in a clockwise direction and a layer of 12 wires laid around this in a clockwise direction. The six outer strands of the cable consist of a layer of six wires laid around the center wire in a counterclockwise direction and a layer of 12 wires laid around this in a counterclockwise direction. The 6 by 19 cable consists of six strands of 19 wires each, laid around a 7 by 7.
MIL-C-18375 cable, although not as strong as MIL-W-83420, is equal in corrosion resistance and superior in non-magnetic and coefficient of thermal expansion properties.
NYLON-COATED CABLES.
Nylon-coated cable is made by extruding a flexible nylon coating over corrosion-resistant steel (CRES) cable. The bare CRES cable must conform and be qualified to MIL-W-83420. After coating, the jacketed cable must still conform to MIL-W-83420.
The service life of nylon-coated cable is much greater than the service life of the sa me cable when used bare.
Most cable wear occurs at pulleys where the cable bends. Wear
Par 7-140 Page 7-321 TABLE 7-3. Flexible cable construction and physical properties.
MINIMUM BREAKING STRENGTH (Pounds)
NOMINAL
DIAMETER
OF WIRE ROPE CABLE
CONSTRUCTION
TOLERANCE ON
DIAMETER (PLUS ONLY)
ALLOWABLE
INCREASE OF
DIAMETER AT
CUT END
MIL-W- 83420 COMP A
MIL-W-83420 COMP B (CRES)
MIL-C- 18375 (CRES)
INCHES INCHES INCHES LBS LBS LBS 1/32 3/64 1/16 1/16 3/32 3/32 1/8 5/32 3/16 7/32 1/4 9/32 5/16 11/32 3/8 7/16 1/2 9/16 5/8 3/4 7/8 1 1 - 1/8 1 - 1/4 1 - 3/8 1 - 1/2 3 x 7 7 x 7 7 x 7 7 x 19 7 x 7 7 x 19 7 x 19 7 x 19 7 x 19 7 x 19 7 x 19 7 x 19 7 x 19 7 x 19 7 x 19 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC 6 x 19 IWRC
0.006
0.008
0.010
0.010
0.012
0.012
0.014
0.016
0.018
0.018
0.018
0.020
0.022
0.024
0.026
0.030
0.033
0.036
0.039
0.045
0.048
0.050
0.054
0.057
0.060
0.062
0.006
0.008
0.009
0.009
0.010
0.010
0.011
0.017
0.019
0.020
0.021
0.023
0.024
0.025
0.027
0.030
0.033
0.036
0.039
0.045
0.048
0.050
0.054
0.057
0.060
0.062 110 270 480 480 920 1,000 2,000 2,800 4,200 5,600 7,000 8,000 9,800 12,500 14,400 17,600 22,800 28,500 35,000 49,600 66,500 85,400 106,400 129,400 153,600 180,500 110 270 480 480 920 920 1,760 2,400 3,700 5,000 6,400 7,800 9,000
12,000 16,300 22,800 28,500 35,000 49,600 66,500 85,400 106,400 129,400 153,600 180,500
1,300 2,000 2,900 3,800 4,900 6,100 7,600
11,000 14,900 19,300 24,300 30,100 42,900 58,000 75,200
is caused by friction between strands and between wires. In bare cable, this is aggravated by dirt and grit working its way into the cable; and the lubricant working its way out leaving dry, dirty wires rubbing against each other. In long, straight runs of cable, vibration workhardens the wires causing the brittle wires to fracture with eventual failure of the cable.
The nylon-jacket protects the cable in a threefold manner. It keeps the lubricant from oozing out and evaporating, it keeps dirt and grit out, and it dampens the vibrations, thereby, greatly reducing their effect on the cable.
NONFLEXIBLE CABLES. (Refer to table 7-4 and figure 7-9.) Nonflexible, preformed, carbon steel cables, MIL-W-87161, composition A, are manufactured by the same processes as MIL-W-83420, composition B, flexible corrosion-resistant steel cables. The nonflexible steel cables are of the 1 by 7 (Type I) or 1 by 19 (Type II) construction according to the diameter as specified in table 7-4. The 1 by 7 cable consists of six
Par 7-144 Page 7-322
FIGURE 7-8. Flexible cable cross section.
TABLE 7-4. Nonflexible cable construction and physical properties.
STRAND TYPE
NOMINAL
DIAMETER OF WIRE STRAND In.
TOLERANCE ON
DIAMETER (Plus Only) In.
ALLOWABLE
INCREASE IN
DIAMETER
AT THE END In.
CONSTRUCTION MIL-W-87161
MINIMUM BREAK
STRENGTH
COMP A & B Lbs.
I I II I II II II II II II II II II II II 1/32 3/64 3/64 1/16 1/16 5/64 3/32 7/64 1/8 5/32 3/16 7/32 1/4 5/16 3/8
0.003
0.005
0.005
0.006
0.006
0.008
0.009
0.009
0.013
0.013
0.013
0.015
0.018
0.022
0.026
0.006
0.008
0.008
0.009
0.009
0.009
0.010
0.010
0.011
0.016
0.019
0.020
0.021
0.024
0.027 1 x 7 1 x 7 1 x 19 1 x 7 1 x 19 1 x 19 1 x 19 1 x 19 1 x 19 1 x 19 1 x 19 1 x 19 1 x 19 1 x 19 1 x 19 185 375 375 500 500 800 1,200 1,600 2,100 3,300 4,700 6,300 8,200 12,500 17,500
FIGURE 7-9. Nonflexible cable cross section.
wires laid around a center wire in a counterclockwise direction. The 1 by 19 cable consists of a layer of six wires laid around a center wire in a clockwise direction plus twelve wires laid around the inner strand in a counterclockwise direction.
CABLE SPECIFICATIONS. Cable diameter and strength data are given in table 7-3 and table 7-4. These values are acceptable for repair and modification of civil aircraft.
CABLE PROOF LOADS. Cable terminals and splices should be tested for proper strength before installation. Gradually apply a test load equal to 60 percent of the cable-breaking strengths given in table 7-3 and
Par 7-144 Page 7-323 table 7-4, for a period of 3 minutes. Place a suitable guard over the cable during the test to prevent injury to personnel in the event of cable failure.
REPLACEMENT OF CABLES. Replace control cables when they become worn, distorted, corroded, or otherwise damaged. If spare cables are not available, prepare exact duplicates of the damaged cable. Use materials of the same size and quality as the original. Standard swaged cable terminals develop the full cable strength and may be substituted for the original terminals wherever practical. However, if facilities and supplies are limited and immediate corrective action is necessary, repairs may be made by using cable bushings, eye splices, and the proper combination of turnbuckles in place of the original installation. (See figure 7-12(c).)
Location of Splices. Locate splices so that no portion of the splice comes closer than 2 inches to any fair-lead or pulley. Locate connections at points where jamming cannot occur during any portion of the travel of either the loaded cable or the sl ack cable in the deflected position.
Cutting and Heating. Cut cables to length by mechanical means. The use of a torch, in any manner, is not permitted. Do not subject wires and cables to excessive temperature. Soldering the bonding braid to the control cable is not permitted.
Ball-and-Socket Type Terminals. Do not use balland-socket type terminals or other types for general replacement that do not positively prevent cable untwisting, except where they were utilized on the original installation by the aircraft manufacturer.
Substitution of Cable. Substitution of cable for hard or streamlined wires will not be acceptable unless specifically approved by a representative of the FAA.
MECHANICALLY-FABRI-CATED CABLE ASSEMBLIES.
Swage-Type Terminals. Swage-type terminals, manufactured in accordance w ith AN, are suitable for use in civil aircraft up to, and including, maximum cable loads. When swag ing tools are used, it is important that all the manufacturers’ instructions, including “go and no-go” dimensions, be followed in detail to avoid defective and inferior swaging.
Observance of all instructions should result in a terminal developing the full-rated strength of the cable.
Critical dimensions, both before and after swaging, are shown in table 7-5.
Terminals. When swaging terminals onto cable ends, observe the following procedures.
Cut the cable to the proper length allowing for growth during swaging. Apply a preservative compound to the cable ends before insertion into the terminal barrel.
NOTE: Never solder cable ends to prevent fraying, since the presence of the solder will greatly increase the tendency of the cable to pull out of the terminal.
Insert the cable into the terminal approximately 1 inch, and bend toward the terminal, then push the cable end entirely into the terminal barrel. The bending action puts a kink or bend in the cable end, and provides enough friction to hold the terminal in place until the swaging operation can be performed. Bending also tends to separate the strands inside the barrel, thereby reducing the strain on them.