FIGURE 7-17. CABLE WEAR PATTERNS INDIVIDUAL OUTER WIRES WORN MORE THAN 50% WINT INDIVIDUAL OUTER WIRES WORN 40-50% (NOTE BLENDING OF WORN AREAS) INDIVIDUAL OUTER WIRES WORN LESS THAN 40% (WORN AREAS INDIVIDUALLY DISTINGUISHABLE) Par 7-149 Page 7-41
Par 7-149 Page 7-42 FIGURE 7-18. WORN CABLE (REPLACEMENT NECESSARY)
h. A s 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 7-19. INTERNAL END VIEW OF CABLE WEAR
Par 7-149 Page 7-43
i. Areas especially conducive to cable corrosion are battery compartments, lavatories, wheel wells, etc. where a concentration of corrosive fumes, vapors, and liquids can accumulate.
Carefully 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 external rust and corrosion with a clean, dry, coarse-weave rag, or fiber brush. Do not use metallic 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.
j. Examine cable runs for incorrect routing, fraying, twisting, or wear at fair-leads, pulleys, anti-abrasion strips, and guards. Look for interference with adjacent structure, equipment, wiring, plumbing, and other controls. Inspect cable systems for binding, full travel, and security of attaching hardware. 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.
k. 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 that 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.
l. 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 cable. Maintain pulley alignment to prevent the cable from riding on the flanges and chafing against guards, covers, or adjacent structure. Check all pulley brackets and guards for damage, alignment, and security.
m. 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 7-44 FIGURE 7-20. PULLEY WEAR PATTERNS
n. In spect fair-leads for wear, breakage, alignment, cleanliness, and security. Examine cable routing at fair-leads to ensure that defection angles are no greater than 3 degrees maximum.
Determine that all guides and anti-abrasion strips are secure and in good condition.
o. 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.
7-150. CORROSION AND RUST PREVENTION. To ensure a satisfactory service life for aircraft control cables, use a cable lubricant to reduce internal friction and prevent corrosion.
a. 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 corrosion-resistant steel cable does not require this treatment for rust prevention.
b. Lubrication and corrosion preventive treatment of carbon steel cables may be affected 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 7-45 FIGURE 7-21. CORROSION 7-
151. 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 7-
152. CABLE MAINTENANCE. Frequent inspections and preservation measures, such as rust-prevention 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 corrosion-prevention compound. Provide corrosion protection for these cable sections by lubricating with a light coat of grease or general-purpose, low-temperature oil.
7-153. CABLE TENSION ADJUSTMENT. 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
Par 7-153 Page 7-46 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 7-47 SECTION 9. TURNBUCKLES
7-165. 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 clip-locking 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.
7-166. 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 7-23, item A.) On initial installation, the turnbuckle terminals should not be screwed inside the turnbuckle barrel more than four threads.
(See Figure 7-23, item B.)
FIGURE 7-23. TURNBUCKLE THREAD TOLERANCE
Par 7-167 Page 7-48
7-167. 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 7-49 SECTION 10. SAFETY METHODS FOR TURNBUCKLES
7-179. 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, Aircraft Turnbuckle Assemblies and/or Turnbuckle Safetying Devices. 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.
7-180. 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, image (A).
FIGURE 7-24. SAFETYING TURNBUCKLES
Par 7-180 Page 7-50
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.
b. 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.
c. 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 binding the laid wires in place before cutting the wrapped wire off.
d. 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.
e. 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, image (B).
7-181. SINGLE-WRAP METHOD. The single-wrap methods described in the following paragraphs and as illustrated in Figure 7-24, images (C) and (D) are acceptable, but are not the equal of the double-wrap methods.
a. 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, image (C).
b. For the method shown in Figure 7-24, image (D), 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.
7-182. 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
Par 7-182 Page 7-51 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.
FIGURE 7-25. SECURING TURNBUCKLES
a.
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, nickel-chromium iron alloy (Inconel), nickel-copper alloy (Monel), or aluminum alloy. For safety cable diameter of safety wire size and material, see Table 7-8.
b. 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 Double 0.020 Stainless steel, Monel, and “K” Monel.1 1/16 Single 0.040 Copper, brass.1 3/32 Single 0.040 Copper, brass.1 1/8 Single 0.040 Stainless steel, Monel, and “K” Monel.
1/8 Double 0.040 Copper, brass.1 1/8 Single 0.057 min. Copper, brass.1 5/32 and greater Double 0.040 Stainless steel, Monel, and “K” Monel.1
Par 7-183 Page 7-52 Cable Size Type of Wrap Diameter of Safety Wire Material (Annealed Condition) 5/32 and greater Single 0.057 min. Stainless steel, Monel, or “K” Monel.1 5/32 and greater Double 0.0512 Copper, brass.1 1 Galvanized or tinned steel, or soft iron wires are also acceptable.
7-183. SPECIAL LOCKING DEVICES. Several turnbuckle locking devices are available for securing turnbuckle barrels such as wire-locking 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.
FIGURE 7-26. CLIP-TYPE LOCKING DEVICE 7 -184. 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.
a. Engage threads of turnbuckle barrel with threads of cable terminal and turn barrel until proper cable tension is reached.
b. Align slot in barrel with slot in cable terminal.
c. Hold lock clip between thumb and forefinger at loop end and insert straight end of clip into opening formed by aligned slots.
d. Bring hook end of lock clip over hole in center of turnbuckle barrel and seat hook loop into hole.
e. 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. However, do not reverse wire locking clips.