Par 7-149 Page 7-328 CABLE SYSTEM INSPECTION. Aircraft cable systems are subject to a variety of environmental conditions and deterioration.
Wire or strand breakage is easy to visually recognize. Other kinds of deterioration such as wear, corrosion, and/or distortion are not easily seen; therefore, control cables should be removed periodically for a more detailed inspection.
At each annual or 100 hour inspection, all control cables must be inspected for broken wires strands. Any cable assembly that has one broken wire strand located in a critical fatigue area must be replaced.
A critical fatigue area is defined as the working length of a cable where the cable runs over, under, or around a pulley, sleeve, or through a fair-lead; or any section where the cable is flexed, rubbed, or worked in any manner; or any point within 1 foot of a swaged-on fitting.
A swaged-on fitting can be an eye, fork, ball, ball and shank, ball and double shank, threaded stud, threaded stud and turnbuckle, compression sleeve, or any hardware used as a termination or end fitting on the cable. These fittings may be attached by various swaging methods such as rotary swaging, roll swaging, hydraulic pressing, and hand swaging tools. (See MIL-T-781.) The pressures exerted on the fittings during the swaging process sometimes pinch the small wires in the cable. This can cause premature failure of the pinched wires, resulting in broken wires.
Close inspection in these critical fatigue areas, must be made by passing a cloth over the area to snag on broken wires. This will clean the cable for a visual inspection, and detect broken wires if the cloth snags on the cable. Also, a very careful visual inspection must be made since a broken wire will not always protrude or stick out, but may lie in the strand and remain in the position of the helix as it was manufactured. Broken wires of this type may show up as a hairline crack in the wire. If a broken wire of this type is suspected, further inspection with a magnifying glass of 7 power or greater, is recommended.
Figure 7-16 shows a cable with broken wires that was not detected by wiping, but was found during a visual inspection. The damage b ecame readily apparent when the cable was removed and bent as shown in figure 7-16.
FIGURE 7-16. Cable inspection technique.
Kinking of wire cable can be avoided if properly handled and installed. Kinking is caused by the cable taking a spiral shape as the result of unnatural twist.
One of the most common causes for this twist is improper unreeling and uncoiling. In a kinked cable, strands and wires are out of position, which creates unequal tension and brings excessive wear at this part of the cable. Even though the kink may be straightened so that the damage appears to be slight, the relative adjustment between the strands has been disturbed so that the cable cannot give maximum service and should be replaced. Inspect cables for a popped core or loose strands. Replace any cable that has a popped core or loose strands regardless of wear or broken wires.
Par 7-149 Page 7-329 Nylon-jacketed cable with any cracks or necking down in the diameter of the jacket shall be replaced. Usable cable life is over when these conditions begin to appear in the nylon jacket.
External wear patterns will extend along the cable equal to the distance the cab le moves at that location and may occur on one side of the cable or on its entire circumference. Replace flex ible and nonflexible cables when the individual wires in each strand appear to blend together (outer wires worn 40 to 50 percent) as depicted in figure 7-17. Actual instances of cable wear beyond the recommended replacement point are shown in figure 7-18.
FIGURE 7-17. Cable wear patterns.
As wear is taking place on the exterior surface of a cable, the same condition is taking place internally, particularly in the sections of the cable which pass over pulleys and quadrants. This condition (shown in figure 7-19) is not easily detected unless the strands of the cable are separated. This type of wear is a result of the relative motion between inner wire surfaces. Under certain conditions, the rate of this type of wear can be greater than that occurring on the surface.
FIGURE 7-18. Worn cable (replacement necessary).
Areas especially conducive to cable corrosion are battery compartments, lavatories, wheel wells, etc.;
where a concentration of corrosive fumes, vapors, and liquids can accumulate. Ca refully examine any cable for corrosion, when it has a broken wire in a section that is not in contact with a wear-producing airframe component, such as a pulley, fair-lead, etc. If the surface of the cable is corroded, relieve cable tension and carefully force the cable open by reverse twisting and visually inspect the interior. Corrosion on the interior strands of the cable constitutes failure, and the cable must be replaced. If no internal corrosion is detected, remove loose extern al rust and corrosion with a clean, dry, coarse-weave rag, or fiber brush. Do not use metallic
Par 7-149 Page 7-330
FIGURE 7-19. Internal cable wear.
wool or solvents to clean installed cables. Use of metallic wool will embed dissimilar metal particles in the cables and create further corrosion problems. Solvents will remove internal cable lubricant allowing cable strands to abrade and further corrode. After thorough cleaning, sparingly apply specification MIL-C-16173, grade 4, corrosion-preventive compound to cable. Do not apply the material so thick that it will interfere with the operation of cables at fair-leads, pulleys, or grooved bellcrank areas.
Examine cable runs for incorrect routing, fraying, twisting, or wear at fair-leads, pulleys, antiabrasion strips, and guards. Look for interference with adjacent structure, equipment, wiring, plumbing, and other controls. Inspect cable syst ems for binding, full travel, and security of attaching ha rdware. Check for slack in the cable system by attempting to move the control column and/or pedals while the gust locks are installed on the control surfaces. With the gust locks removed, actuate the controls and check for friction or hard movement.
These are indications that excessive cable tension exists.
NOTE: If the control movement is stiff after maintenance is performed on control surfaces, check for parallel cables twisted around each other, or cables connected in reverse.
Check swaged terminal reference marks for an indication of cable slippage within the fitting. Inspect the fitting assembly for distortion and/or broken strands at the terminal. Ensure th at all bearings and swivel fittings (bolted or pinned) pivot freely to prevent binding and subsequent failure . Check turnbuckles for proper thread exposure and broken or missing safety wires/clips.
Inspect pulleys for roughness, sharp edges, and presence of foreign material embedded in the grooves.
Examine pulley bearings to ensure proper lubrication, smooth rotation; and freedom from flat spots, dirt, and paint spray. During the inspection, rotate the pulleys, which only turn through a small arc, to provide a new bearing surface for the cab le. Maintain pulley alignment to prevent the cable from riding on the flanges and chafing against guards, covers, or adjacent structure. Check all pu lley brackets and guards for damage, alignment, and security.
Various cable system malfunctions may be detected by analyzing pulley conditions. These include such discrepancies as too much tension, misalignment, pulley bearing problems, and size mismatches between cables and pulleys. Examples of these condition are shown in figure 7-20.
Par 7-149 Page 7-331
FIGURE 7-20. Pulley wear patterns.
Inspect fair-leads for wear, breakage, alignment, cleanliness, and security. Examine cable routing at fairleads to ensure that defecti on angles are no greater than 3° maximum. Determine that all guides and antiabrasion strips are secure and in good condition.
Examine pressure seals for wear and/or material deterioration. Seal guards should be positioned to prevent jamming of a pulley in case pressure seal fails and pieces slide along the cable.
CORROSION AND RUST PRE-VENTION. To ensure a satisfactory service life for aircraft control cables, use a cable lubricant to reduce internal friction and prevent corrosion.
If the cable is made from tinned steel, coat the cable with rust-preventive oil, and wipe off any excess. It should be noted that corrosionresistant steel cable does not require this treatment for rust prevention.
Lubrication and corrosion pre-ventive treatment of carbon steel cables may be effected simultaneously by application of compound MIL-C-16173, grade 4, or MIL-C-11796, Class I. MIL-C-16173 compound should be brushed, sprayed, or wiped on the cable to the extent it penetrates into the strands and adequately covers the cable surfaces. It will dry “tack free” in 24 hours at 77 °F. MIL-C-11796 compound is applied by dipping the cable for 1/2 minute into a tank of compound heated to 77 ° ± 5 °C (170 ° ± 9 °F) for 1/2 minute then removing it and wiping off the excess oil. (An example of cable corrosion, attributable to battery acid, is shown in figure 7-21.)
Par 7-150 Page 7-332 (and 7-40)
FIGURE 7-21. Corrosion.
WIRE SPLICES. Standard manufacturing splices have been mistaken for defects in the cable because individual wire end splices were visible after assembly of a finished cable length. In some instances, the process of twisting outer strands around the core strand may also slightly flatten individual outer wires, particularly in the area of a wire splice. This flattening is the result of die-sizing the cable, and does not affect the strength of the cable.
These conditions (as shown in figure 7-22) are normal, and are not a cause for cable rejection.
FIGURE 7-22. Manufacturer’s wire splice.
CABLE MAINTENANCE. Frequent inspections and preservation measures such as rustprevention treatments for bare carbon steel cable areas, will help to extend cable service life. Where cables pass through fair-leads, pressure seals, or over pulleys, remove accumulated heavy coatings of corrosionprevention compound. Provide corrosion protection for these cable sections by lubricating with a light coat of grease or general-purpose, low-temperature oil.
CABLE TENSION ADJUST-MENT. Carefully adjust, control cable tension in accordance with the airframe manufacturer’s recommendations. On large aircraft, take the temperature of the immediate area into consideration when using a tension meter. For long cable sections, use the average of two or three temperature readings to obtain accurate tension values. If necessary, compensate for extreme surface temperature variations that may be encountered if the aircraft is operated primarily in unusual geographic or climatic conditions such as arctic, arid, or tropic locations. Use rigging pins and gust locks, as necessary, to ensure satisfactory results. At the completion of rigging operations, check turnbuckle adjustment and safetying in accordance with section 10 of this chapter.
7-154.7-164. [RESERVED.]
Par 7-165 Page 7-333 (and 7-42)
SECTION 9. TURNBUCKLES
GENERAL. A turnbuckle is a device used in cable systems to provide a means of adjusting tension. Turnbuckles have barrel-shaped sleeves with internal left- and right-hand threads at opposite ends. The cables, with terminals attached, are made to such a length that, when the turnbuckle is adjusted to give the specified cable tension, a sufficient number of threads on the terminal ends are screwed into the barrel to hold the load. The cliplocking turnbuckle and its associated parts are identical to standard AN and MS parts except for a slot grooved on the interior of the barrel and the shanks of the forks, eyes, etc. The clip-locking turnbuckle parts have the following drawing numbers: MS21251, turnbuckle body;
MS21252, turnbuckle clevis end; MS21253, turnbuckle clevis end (for bearing); NAS649 and NAS651, turnbuckle clip; MS21254 and NAS648, turnbuckle eye (for pin); MS21255 and NAS647, turnbuckle eye end (for wire rope);
NAS645 and NAS646, turnbuckle fork;
MS21256, turnbuckle barrel locking clip;
AN130-170, turnbuckle assemblies; and, MS21259 and MS21260, terminal, wire rope, stud.
NOTE: Turnbuckles showing signs of thread distortion/bending should be replaced.
Turnbuckle ends are designed for providing the specified cable tension on a cable system, and a bent turnbuckle would place undesirable stress on the cable, impairing the function of the turnbuckle.
TURNBUCKLE INSTALLATION. (See figure 7-23.) When installing cable system turnbuckles, it is necessary to screw both threaded terminals into the turnbuckle barrel an equal amount. It is essential that turnbuckle terminals be screwed into the barrel so that not more than three threads on the terminal are exposed. (See figure 7-23A.) On initial installation, the turnbuckle terminals should not be screwed inside the turnbuckle barrel more than four threads. (See figure 7-23B.)
FIGURE 7-23. Turnbuckle thread tolerance.
WITNESS HOLE. Some manufacturers of turnbuckles incorporate a “witness hole,” in the turnbuckle barrel to ensure that the threaded cable terminals are screwed in far enough into the barrel. The “witness hole” can be inspected visually, or by using a piece of safety wire as a probe.
7-168.7-178. [RESERVED.]
Par 7-179 Page 7-334
SECTION 10. SAFETY METHODS FOR TURNBUCKLES
GENERAL. Safety all turnbuckles with safety wire using either the double or single-wrap method, or with any appropriately approved special safetying device complying with the requirements of FAA Technical Standard Order TSO-C21. The swaged and unswaged turnbuckle assemblies are covered by AN standard drawings. Do not reuse safety wire.
Adjust the turnbuckle to the correct cable tension so that no more than three cable threads are exposed on either side of the turnbuckle barrel.
DOUBLE-WRAP METHOD. Of the methods using safety wire for safetying turnbuckles, the method described here is preferred, although either of the other methods described is satisfactory. The method of double-wrap safetying is shown in figure 7-24(A).
Use two separate lengths of wire. Run one end of the wire through the hole in the barrel of the turnbuckle and bend the ends of the wire toward opposite ends of the turnbuckle.
Pass the second length of the wire into the hole in the barrel and bend the ends along the barrel on the side opposite the first. Spiral the two wires in opposite directions around the barrel to cross each other twice between the center hole and the ends.
Then pass the wires at the end of the turnbuckle in opposite directions through the hole in the turnbuckle eyes or between the jaws of the turnbuckle fork, as applicable, laying one wire along the barrel and wrapping the other at least four times around the shank of the turnbuckle and bindi ng the laid wires in place before cutting the wrapped wire off.
Wrap the remaining length of safety wire at least four turns around the shank and cut it off. Repeat the procedure at the opposite end of the turnbuckle.
When a swaged terminal is being safetied, pass the ends of both wires through the hole provided in the terminal for this purpose and wrap both ends around the shank as previously described. If the hole is not large enough to allow passage of both wires, pass the wire through the hole and loop it over the free end of the other wire, and then wrap both ends around the shank as previously described. Another satisfactory double-wrap method is similar to the previous method, except that the spiraling of the wires is omitted as shown in figure 7-24(B).
SINGLE-WRAP METHOD. The single-wrap methods described in the following paragraphs and as illustrated in figure 7-24(C) and (D) are acceptable, but are not the equal of the doublewrap methods.
Pass a single length of wire through the cable eye or fork, or through the hole in the swaged terminal at either end of the turnbuckle assembly. Spiral each of the wire ends in opposite directions around the first half of the turnbuckle barrel, so as to cross each other twice.
Thread both wire ends through the hole in the middle of the barrel so that the third crossing of wire ends is in the hole, again, spiral the two wire ends in opposite directions around the remaining half of the turnbuckle, crossing them twice. Then, pass one wire end through the cable eye or fork, or through the hole in the swaged terminals, in the manner previously described. Wrap both wire ends around the shank for at least four turns each, cutting off excess wire. This method is shown in figure 7-24(C).
Par 7-179 Page 7-335
F IGURE 7-24. Safetying turnbuckles.
For the method shown in figure 7-24D, pass one length of wire through the center hole of the turnbuckle and bend the wire ends toward opposite ends of the turnbuckle. Then pass each wire end through the cable eye or fork, or through the hole in the swaged terminal, and wrap each wire around the shank for at least four turns, cutting off excess wire. After safetying, no more than three threads of the turnbuckle threaded terminal should be exposed.
SAFETY-WIRE SECURED TURNBUCKLES. (See figure 7-25.) Before securing turnbuckles, threaded terminals should be screwed into the turnbuckle barrel until no more than three threads of either terminal are outside the barrel. After the turnbuckle has been adjusted for proper cable tension, two pieces of safety wire are inserted, half the wire length into the hole in the center of the turnbuckle barrel. The safety-wires are bent so that each wire extends half the length of the turnbuckle on top and half on bottom. The ends of the wires are passed through the hole in the turnbuckle eyes or between the jaws of the turnbuckle fork, as applicable. The wires are then bent toward the center of the turnbuckle and each wire is wrapped around the shank four times, binding the wrapping wires in place as shown in figure 7-25.
When a swaged terminal is being secured, one wire is passed through the hole in the terminal and is looped over the free end of the other wire and both ends wrapped around the shank. All lock wire used in the safetying of turnbuckles should be carbon steel, corrosion-resistant steel, ni ckel-chromium iron alloy (inconel), nickel-copper alloy (monel) or aluminum
Par 7-182 Page 7-336 alloy. For safety cable diam eter of safety wire size and material, refer to table 7-8.
Care should be exercised when safety wiring, particularly where corrosion will present a problem, because smaller wire sizes tend to crack when twisted.
TABLE 7-8. Turnbuckle safetying guide.
Cable Size Type of Wrap Diameter of Safety Wire Material (Annealed Condition) 1/16 Single 0.040 Copper, brass.
3/32 Single 0.040 Copper, brass.
1/8 Single 0.040 Stainless steel, Monel and “K” Monel.
1/8 Double 0.040 Copper, brass.
1/8 Single 0.057 min. Copper, brass.
5/32 and greater.
Double 0.040 Stainless steel, Monel and “K” Monel.
5/32 and greater Single 0.057 min. Stainless steel, Monel or “K” Monel.
5/32 and greater Double 0.0512 Copper, brass.
1 Galvanized or tinned steel, or soft iron wires are also acceptable.
SPECIAL LOCKING DEVICES. Several turnbuckle locking devices are available for securing turnbuckle barrels such as wirelocking clips. Persons intending to use a special device must ensure the turnbuckle assembly has been designed to accommodate such devices. A typical unit is shown in figure 7-26. When special locking devices are not readily available, the use of safety wire is acceptable.
ASSEMBLING AND SECURING CLIP-LOCKING TURNBUCKLES. (See table 7-9 and figure 7-27.) Wire clip-locking turnbuckles are assembled and secured in the following ways.
Engage threads of turnbuckle barrel with threads of cable terminal and turn barrel until proper cable tension is reached.
Align slot in barrel with slot in cable terminal.
Hold lock clip between thumb and forefinger at loop end and insert straight end of clip into opening formed by aligned slots.
Bring hook end of lock clip over hole in center of turnbuckle barrel and seat hook loop into hole.
Apply pressure to hook shoulder to engage hook lip in turnbuckle barrel and to complete safety locking of one end of turnbuckle.
NOTE: Repeat the above steps to safety lock the opposite end of turnbuckle. Both lock clips may be inserted in the same turnbuckle barrel hole or they may be inserted in opposite holes. Howe ver, do not reverse wire locking clips
- TURNBUCKLE FORK •TURNBUCKLE -WRAP, FOUR TURNS - CABLE
SAFETY WIRE BARREL
WRAP, FOUR TURNS TURNBUCKLE EYE -THIMBLE
SAFETY WIRE WRAP, FOUR TURNS титаний
- SWAGED TERMINAL FIGURE 7-25. Securing turnbuckles.
TURNBUCKLE BODY- LOCKING-CLIP MS21251 MS21256 FIGURE 7-26. Clip-type locking device.
Par 7-182 Page 7-337
Par 7-184 Page 7-338 (and 7-48) TABLE 7-9. Locking-clip application.
NOMINAL CABLE DIA.
THREAD UNF-3
LOCKING CLIP MS21256
TURNBUCKLE BODY MS21251 1/16 No. 6-40 -1 -2S 3/32 No. 10-32 -3S -2 -3L 1/8 -1 -4S 1/4-28 -2 -4L 5/32 -1 -5S -2 -5L 3/16 5/16-24 -1 -6S -6L 7/32 3/8-24 -2 -7L 1/4 -8L 9/32 7/16-20 -3 -9L 5/16 1/2-20 -10L
FIGURE 7-27. Assembling and securing clip-locking turnbuckles
7-185.7-195. [RESERVED.]
Par 7-196 Page 7-339
SECTION 11. HARDWARE IDENTIFICATION TABLES
TABLE 7-10. TABLE OF RIVETS.
Rivet Number Description AN427 Rivet, 100ºcsk. Head steel, monel, copper AN430 Rivet, round head al. Alloy AN441 Rivet, tinners Head, steel, ss, monel AN456 Rivet, brazier head, aluminum alloy AN123151-123750
Rivet, universal head & 100º steel, Inconel NAS 1200 Rivet, solid, 100º flush shear head NAS 1241 Rivet, solid, 100º flush head NAS 1242 Rivet, solid universal head NAS 1398 Rivet, blind, protruding head, locked spindle NAS 1399 Rivet, blind, 100º csk.
Head, locked spindle
TABLE 7-10. (CONTINUED) Rivet Number Description NAS 1738-1739 Rivet, blind, protruding & flush hd., mech.
locked spindle, bulbed NAS1806-1816 Rivet, hi-shear, flathead., ti. alloy NAS1906-1916 Rivet, hi-shear, 100° hd., ti. alloy AN124951-125550 Rivet, solid universal head & 100° csk. head, cres. steel, inconel AN125551-1255700 Rivet, solid universal head, steel AN426 MS20426 Rivet, solid, csk. 100° head al. alloy AN470 MS20470 Rivet, solid, universal head, al. & al. alloy MS9319 Rivet, solid univ. head, AMS 7233 MS9403 Rivet, solid, universal head, AMS 5737 MS16535 Rivet, tubular, oval head