Aircraft hardware and fasteners are subjected to constant vibration, aerodynamic loads, and operational stresses during flight. To prevent nuts, bolts, screws, turnbuckles, and other components from loosening, various safetying methods are used throughout the aircraft. Proper installation and inspection of safetying devices are essential to maintain structural integrity, system reliability, and flight safety.
Safetying Methods
Safetying is the process of securing aircraft bolts, nuts, screws, pins, and other fasteners so that they do not loosen due to vibration or operational loads. Knowledge of safetying methods and equipment is essential for performing aircraft maintenance and inspections.
There are various methods of safetying aircraft parts. The most widely used methods are safety wire, cotter pins, lock washers, snap rings, and special nuts, such as self-locking nuts, pal nuts, and jam nuts. Some of these nuts and washers have been described in the Aircraft Materials, Hardware, and Processes series.
Pins
The three main types of pins used in aircraft structures are the taper pin, flathead pin, and cotter pin. Pins are used in shear-load applications and for safetying purposes. Roll pins are finding increasing use in aircraft construction.
Taper Pins
Plain and threaded taper pins (AN385 and AN386) are used in joints that carry shear loads and where the absence of play is essential. The plain taper pin is drilled and usually safetied with wire. The threaded taper pin is used with a taper pin washer (AN975) and shear nut (safetied with a cotter pin or safety clip) or self-locking nut.
Flathead Pin
Commonly called a Clevis pin, the flathead pin (MS20392) is used with tie rod terminals and in secondary controls, which are not subject to continuous operation. The pin is customarily installed with the head up so that if the cotter pin fails or works out, the pin remains in place.
Cotter Pins
The AN380 cadmium-plated, low-carbon steel cotter pin is used for safetying bolts, screws, nuts, other pins, and in various applications where such safetying is necessary. The AN381 corrosion-resistant steel cotter pin is used in locations where nonmagnetic material is required or in locations where resistance to corrosion is desired.
Roll Pins
The roll pin is a pressed fit pin with chamfered ends. It is tubular in shape and is slotted the full length of the tube. The pin is inserted with hand tools and is compressed as it is driven into place. Pressure exerted by the roll pin against the hole walls keeps it in place until deliberately removed with a drift punch or pin punch.
Safety Wiring
Safety wiring is the most positive and satisfactory method of safetying cap screws, studs, nuts, bolt heads, and turnbuckle barrels, which cannot be secured by any other practical means. It is a method of wiring together two or more units in such a manner that any tendency of one to loosen is counteracted by the tightening of the wire.
Nuts, Bolts, & Screws
Nuts, bolts, and screws are safety wired by the single wire or double twist method. The double twist method is the most common method of safety wiring. The single wire method may be used on small screws in a closely spaced closed geometrical pattern, on parts in electrical systems, and in places that are extremely difficult to reach. Safety wiring should always be performed using conventional methods or as required by the manufacturer, especially for Light Sport Aircraft (LSA).
Figure 1 is an illustration of various methods that are commonly used in safety wiring nuts, bolts, and screws.
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| Figure 1. Safety wiring methods |
Careful study of Figure 1 shows that:
- Examples 1, 2, and 5 illustrate the proper method of safety wiring bolts, screws, square-head plugs, and similar parts when wired in pairs.
- Example 3 illustrates several components wired in series.
- Example 4 illustrates the proper method of wiring castellated nuts and studs. (Note that there is no loop around the nut.)
- Examples 6 and 7 illustrate a single-threaded component wired to a housing or lug.
- Example 8 illustrates several components in a closely spaced closed geometrical pattern using a single wire method.
When drilled head bolts, screws, or other parts are grouped together, they are more conveniently safety wired to each other in a series rather than individually. The number of nuts, bolts, or screws that may be safety wired together is dependent on the application. For instance, when safety wiring widely spaced bolts by the double twist method, a group of three should be the maximum number in a series.
When safety wiring closely spaced bolts, the number that can be safety wired by a 24-inch length of wire is the maximum in a series. The wire is arranged so that if the bolt or screw begins to loosen, the force applied to the wire is in the tightening direction.
Parts being safety wired should be torqued to recommended values and the holes aligned before attempting the safetying operation. Never over torque or loosen a torqued nut to align safety wire holes.
Oil Caps, Drain Cocks, & Valves
These units are safety wired as shown in Figure 2. In the case of the oil cap, the wire is anchored to an adjacent fillister head screw.
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| Figure 2. Safety wiring attachment for plug connectors |
This method applies to any other unit that must be safety wired individually. Ordinarily, anchorage lips are conveniently located near these individual parts. When such provision is not made, the safety wire is fastened to some adjacent part of the assembly.
Electrical Connectors
Under conditions of severe vibration, the coupling nut of a connector may vibrate loose and, with sufficient vibration, the connector may come apart. When this occurs, the circuit carried by the cable opens. The proper protective measure to prevent this occurrence is safety wiring as shown in Figure 3.
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| Figure 3. Safety wiring attachment for plug connectors |
The safety wire should be as short as practicable and must be installed in such a manner that the pull on the wire is in the direction that tightens the nut on the plug.
Turnbuckles
After a turnbuckle has been properly adjusted, it must be safetied. There are several methods of safetying turnbuckles; however, only two methods are discussed in this section. These methods are illustrated in Figure 4.
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| Figure 4. Safetying turnbuckles: (A) clip-locking method and (B) wire-wrapping method |
The clip locking method is used only on the most modern aircraft. The older type aircraft still use the type turnbuckles that require the wire wrapping method.
Double Wrap Method
Of the methods using safety wire for safetying turnbuckles, the double wrap method is preferred, although the single wrap methods described are satisfactory. The method of double wrap safetying is shown in Figure 4.
Use two separate lengths of the proper wire as shown in Figure 5.
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| Figure 5. Turnbuckle safetying guide |
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. Then 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. Then pass the wires at each end of the turnbuckle in opposite directions through the holes in the turnbuckle eyes or between the jaws of the turnbuckle fork, as applicable.
Bend the laid wires into place before cutting off the wrapped wire. 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, if possible, through the hole provided in the terminal for this purpose and wrap both ends around the shank as described above.
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 described.
Single Wrap Method
The single wrap safetying methods described in the following paragraphs are acceptable but are not the equal of the double wrap 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 that the wires cross each other twice.
Thread both wire ends through the hole in the middle of the barrel so that the third crossing of the wire ends is in the hole. Again, spiral the two wire ends in opposite directions around the remaining half of the turnbuckle, with the wires crossing each other twice.
Then, pass one wire end through the cable eye or fork, or through the hole in the swaged terminal. In the manner described above, wrap both wire ends around the shank for at least four turns each, cutting off the excess wire.
An alternate to the above method is to 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 end around the shank for at least four turns, cutting off the excess wire. After safetying, no more than three threads of the turnbuckle threaded terminal should be exposed.
General Safety Wiring Rules
When using the safety wire method of safetying, the following general rules should be followed:
- A pigtail of 1⁄4 to 1⁄2 inch (three to six twists) should be made at the end of the wiring. This pigtail must be bent back or under to prevent it from becoming a snag.
- The safety wire must be new upon each application.
- 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 aligns with the hole.
- All safety wires must be tight after installation, but not under such tension that normal handling or vibration breaks the wire.
- The wire must be applied so that all pull exerted by the wire tends to tighten the nut.
- Twists should be tight and even, and the wire between the nuts as taut as possible without overtwisting.
- The safety wire should always be installed and twisted so that the loop around the head stays down and does not tend to come up over the bolt head, causing a slack loop.
Cotter Pin Safetying
Cotter pin installation is shown in Figure 6. Castellated nuts are used with bolts that have been drilled for cotter pins. The cotter pin should fit neatly into the hole with very little side play.
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| Figure 6. Cotter pin installation |
The following general rules apply to cotter pin safetying:
- The prong bent over the bolt end should not extend beyond the bolt diameter. (Cut it off if necessary.)
- The prong bent down should not rest against the surface of the washer. (Again, cut it off if necessary.)
- If the optional wraparound method is used, the prongs should not extend outward from the sides of the nut.
- All prongs should be bent over a reasonable radius. Sharp angled bends invite breakage. Tapping lightly with a mallet is the best method of bending the prongs.
Snap Rings
A snap ring is a ring of metal, either round or flat in cross section, which is tempered to provide spring-like action. This spring-like action holds the snap ring firmly seated in a groove. The external types are designed to fit in a groove around the outside of a shaft or cylinder and may be safety wired. Safety wiring of an external type snap ring is shown in Figure 7.
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| Figure 7. External type snap ring with safety wire installation |
The internal types fit in a groove inside a cylinder and are never safetied. A special type of pliers is designed to install each type of snap ring. Snap rings can be reused as long as they retain their shape and spring-like action.
What is the "double twist" method of safety wiring?
How should a cotter pin be properly installed in a castellated nut?
What is the rule for aligning safety wire holes during torquing?
When is the "double wrap" method used for turnbuckles?
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