EASA Chapter 05

CHAPTER 5

AIRCRAFT HARDWARE

INTRODUCTION

The importance of aircraft hardware is oftenoverlooked because of the small size of most items.However, the safe and efficient operation of any aircraftdepends upon the correct selection and use of aircrafthardware. This chapter discusses the various types ofthreaded fasteners, quick-release fasteners, rivets,electrical hardware, and other miscellaneous hardware.You must make sure that items of aircraft hardwareremain tightly secured in the aircraft. Therefore, wewill discuss proper safetying methods in this chapter.

Aircraft hardware is identified for use by itsspecification number or trade name. Threaded fastenersand rivets are identified by Air Force-Navy (AN),National Aircraft Standard (NAS), and MilitaryStandard (MS) numbers. Quick-release fasteners areidentified by factory trade names and size designations.

When aircraft hardware is ordered from supply, thespecification numbers and the factory part numbers arechanged into stock numbers (SN). This change isidentified by using a part-number cross-referenceindex.

Q5-1. How is aircraft hardware identified for use?

THREADED FASTENERS

LEARNING OBJECTIVE: Identify com-mon types of threaded fasteners and themethods used to properly install and safetythem.

In modern aircraft construction, thousands of rivetsare used, but many parts require frequent dismantlingor replacement. It is more practical for you to use someform of threaded fastener. Some joints require greaterstrength and rigidity than can be provided by riveting.We use various types of bolts, screws, and nuts to solvethis problem.

Bolts and screws are similar in that both have ahead at one end and a screw thread at the other.

However, there are several differences between them.The threaded end of a bolt is always relatively blunt. Ascrew may be either blunt or pointed. The threaded endof a bolt must be screwed into a nut. The threaded endof the screw may fit into a nut or directly into thematerial being secured. A bolt has a fairly shortthreaded section and a comparatively long grip length(the unthreaded part). A screw may have a longerthreaded section and no clearly defined grip length. Abolt assembly is generally tightened by turning a nut.The bolt head may or may not be designed to be turned.A screw is always designed to be turned by its head.Another minor difference between a screw and a bolt isthat a screw is usually made of lower strength materials.

Threads on aircraft bolts and screws are of theAmerican National Aircraft Standard type. Thisstandard contains two series of threads—nationalcoarse (NC) and national fine (NF). Most aircraftthreads are of the NF series.

Bolts and screws may have right- or left-handthreads. A right-hand thread advances into engagementwhen turned clockwise. A left-hand thread advancesinto engagement when turned counterclockwise.

AIRCRAFT BOLTS

Many types of bolts are used in modern aircraft,and each type is used to fasten something in place.Before discussing some of these types, it might behelpful if we list and explain some commonly used boltterms. You should know the names of bolt parts and beaware of the bolt dimensions that must be considered inselecting a bolt.

The three principal parts of a bolt are the head,grip, and threads, as shown in figure 5-1. Two of theseparts might be well known to you, but perhaps grip is anunfamiliar term. The grip is the unthreaded part of thebolt shaft. It extends from the threads to the bottom ofthe bolt head. The head is the larger diameter of the boltand may be one of many shapes or designs.

5-1

To choose the correct replacement for anunserviceable bolt, you must consider the length of thebolt. As shown in figure 5-1, the bolt length is thedistance from the tip of the threaded end to the head ofthe bolt. Correct length selection is indicated when thebolt extends through the nut at least two full threads.See figure 5-2. If the bolt is too short, it will not extendout of the bolt hole far enough for the nut to be securelyfastened. If it is too long, it may extend so far that itinterferes with the movement of nearby parts.

In addition, if a bolt is too long or too short, its gripwill usually be the wrong length. As shown in figure5-2, the grip length should be approximately the sameas the thickness of the material to be fastened. If thegrip is too short, the threads of the bolt will extend intothe bolt hole. The bolt may act like a reamer when thematerial is vibrating. To prevent this, make certain thatno more than two threads extend into the bolt hole.Also, make certain that any threads that enter the bolthole extend only into the thicker member that is beingfastened. If the grip is too long, the nut will run out ofthreads before it can be tightened. In this event, a boltwith a shorter grip should be used. If the bolt gripextends only a short distance through the hole, a washermay be used.

A second bolt dimension that must be considered isdiameter. As shown in figure 5-1, the diameter of thebolt is the thickness of its shaft.

The results of using a wrong diameter bolt shouldbe obvious. If the bolt is too big, it cannot enter the bolthole. If the diameter is too small, the bolt has too muchplay in the bolt hole.

The third and fourth bolt dimensions that should beconsidered when you choose a bolt replacement arehead thickness and width. If the head is too thin or toonarrow, it might not be strong enough to bear the loadimposed on it. If the head is too thick or too wide, it

might extend so far that it interferes with the movementof adjacent parts.

AN bolts come in three head styles—hex head,clevis, and eyebolt. NAS bolts are available in

5-2

ANF0502

BOLT GRIP LENGTH CORRECT

BOLT GRIP LENGTH TOO SHORT

BOLT GRIP LENGTH TOO LONGFigure 5-2.—Correct and incorrect grip lengths.

ANF0501

DIAMETER

LENGTH

THREADS GRIP

HEADWIDTH

HEADTHICKNESS

Figure 5-1.—Bolt terms and dimensions.

countersunk, internal wrenching, and hex head styles.MS bolts come in internal wrenching and hex headstyles. Head markings indicate the material of whichstandard bolts are made. Head markings may indicate ifthe bolt is classified as a close-tolerance bolt. See figure5-3. Additional information, such as bolt diameter, boltlength, and grip length, may be obtained from the boltpart number.

SCREWS

The most common threaded fastener used inaircraft construction is the screw. The three most usedtypes are the machine screw, structural screw, and theself-tapping screw, as shown in figure 5-4. Figure 5-4also shows the three head slots—straight, Phillips, andReed and Prince.

Structural Screws

Structural screws are used for assembly ofstructural parts, as are structural bolts. They are madeof alloy steel and are properly heat-treated. Structuralscrews have a definite grip length and the same shearand tensile strengths as the equivalent size bolt. Theydiffer from structural bolts only in the type of head.

5-3

ANF0503

COUNTERSUNK HEAD BOLT

INTERNAL WRENCHING BOLTDRILLED HEX HEAD BOLT

CLEVIS BOLT

EYEBOLT

CLOSETOLERANCE(STEEL ORALUMINUM

ALLOY)

ALUMINUMALLOY

(62,000 P.S.I.)

CORROSIONRESISTANT

STEEL(125,000 P.S.I.)

STEEL125,000 P.S.I)

STEEL(150,000 P.S.I.)

HEAD MARKINGS

Figure 5-3.—Types of bolts and bolt head markings.

ANF0504

MACHINE SCREWSTRUCTURAL SCREW SELF-TAPPING

SCREW

PHILLIPS HEAD REED ANDPRICE HEAD

Figure 5-4.—Screws.

These screws are available in countersunk head, roundhead, and brazier head types. See figure 5-5.

Machine Screws

The commonly used machine screws are the roundhead, flat head, fillister head, pan head, truss head, andsocket head types.

Self-Tapping Screws

A self-tapping screw is one that cuts its owninternal threads as it is turned into the hole.Self-tapping screws may be used only in comparativelysoft metals and materials. Self-tapping screws may befurther divided into two classes or groups—machineself-tapping screws and sheet metal self-tappingscrews.

Machine self-tapping screws are usually used forattaching removable parts, such as nameplates, tocastings. The threads of the screw cut mating threads in

the casting after a hole has been predrilled undersize.Sheet metal self-tapping screws are used for suchpurposes as temporarily attaching sheet metal in placefor riveting. Sheet metal self-tapping screws may beused to permanently assemble nonstructural unitswhere it is necessary to insert screws in difficult to getto areas.

CAUTION

Self-tapping screws should never be used toreplace standard screws, nuts, or rivets originallyused in the structure.

Setscrews

Setscrews are used to position and hold com-ponents in place, such as gears on a shaft. Setscrews areavailable with many different point styles. They areclassified as hexagon-socket and fluted-socket headlesssetscrews.

NUTS

Aircraft nuts may be divided into two generalgroups—nonself-locking and self-locking nuts.Nonself-locking nuts are those that must be safetied byexternal locking devices, such as cotter pins, safetywire, or locknuts. The locking feature is an integral partof self-locking nuts.

Nonself-locking Nuts

The most common of the nonself-locking nuts arethe castle nut, the plain hex nut, the castellated shearnut, and the wing nut. Figure 5-6 shows thesenonself-locking nuts.

5-4

ANF0505

COUNTERSUNKHEAD GRIP

GRIP

GRIP

LENGTH

LENGTH

LENGTH

ROUNDHEAD

BRAZIERHEAD

Figure 5-5.—Structural screws.ANF0506

CASTLE NUT PLAIN HEX NUT

CASTELLATEDSHEAR NUT

WING NUT

Figure 5-6.—Nonself-locking nuts.

Castle nuts are used with drilled-shank ANhex-head bolts, clevis bolts, or studs. They are designedto accept a cotter pin or lockwire for safetying.

Castellated shear nuts are used on such parts asdrilled clevis bolts and threaded taper pins. They arenormally subjected to shearing stress only. They mustnot be used in installations where tension stresses areencountered.

Plain hex nuts have limited use on aircraftstructures. They require an auxiliary locking devicesuch as a check nut or a lock washer.

Wing nuts are used where the desired tightness canbe obtained by the fingers and where the assembly isfrequently removed. Wing nuts are commonly used onbattery connections.

Self-Locking Nuts

Self-locking nuts provide tight connections thatwill not loosen under vibrations. Self-locking nuts

approved for use on aircraft meet critical specificationsas to strength, corrosion resistance, and heat-resistanttemperatures. New self-locking nuts must be used eachtime components are installed in critical areasthroughout the entire aircraft. Self-locking nuts arefound on all flight, engine, and fuel control linkage andattachments. There are two general types ofself-locking nuts. They are the all-metal nuts and themetal nuts with a nonmetallic insert to provide thelocking action. The Boots aircraft nut and the Flexlocnut are examples of the all-metal type. See figure 5-7.The elastic stop and the nonmetallic insert lock nut areexamples of the nonmetallic insert type. All-metalself-locking nuts are constructed either of two ways.The threads in the load-carrying portion of the nut thatis out of phase with the threads in the locking portion isone way. The second way is with a saw-cut top portionwith a pinched-in thread. The locking action of thesetypes depends upon the resiliency of the metal.

5-5

ANF0507

BOOTS AIRCRAFT NUT FLEXLOC NUT

NONMETALIC-INSERT

LOCK NUT

ELASTICSTOP NUT

ELASTIC TWO-LUGANCHOR NUT

BOOTS AIRCRAFT CHANNEL ASSEMBLY

ELASTIC STOP NUT CHANNEL ASSEMBLY

Figure 5-7.—Self-locking nuts.

The elastic stop nut is constructed with anonmetallic (nylon) insert, which is designed to lockthe nut in place. The insert is unthreaded and has asmaller diameter than the inside diameter of the nut.

Self-locking nuts are generally suitable for reuse innoncritical applications provided the threads have notbeen damaged. If the locking material has not beendamaged or permanently distorted, it can be reused.

NOTE: If any doubt exists about the condition of anut, use a new one!

When you anchor lightweight parts, the sheetspring nut may be used. See figure 5-8. Applicationsinclude supporting line clamps, electrical equipment,and small access doors. It is made of sheet spring steeland is cut so as to have two flaps. The ends of theseflaps are notched to form a hole that is somewhatsmaller in diameter than the screw used. The sheetspring nut has a definite arch that tends to flatten out asthe screw pulls the flaps in toward the threads. Thisflattening action forces the flaps of the nut tightly intothe threads of the screw. The springiness of the sheetspring nut pushes upward on the screw threads, bindingthem and locking the screw in place. With the sheetspring nut, either a standard or a sheet metalself-tapping screw is used.

INSTALLATION OF NUTS AND BOLTS

You must be certain that each bolt is made of thecorrect material. Examine the markings on the head todetermine whether a bolt is steel or aluminum alloy.

It is of extreme importance to use like bolts inreplacement. In every case, refer to the applicablemaintenance instruction manual and illustrated partsbreakdown.

Be sure that washers are used under the heads ofboth bolts and nuts unless their omission is specified. Awasher guards against mechanical damage to thematerial being bolted and prevents corrosion of thestructural members. An aluminum alloy washer may beused under the head and nut of a steel bolt securingaluminum alloy or magnesium alloy members.Corrosion will attack the washer rather than themembers. Steel washers should be used when joiningsteel members with steel bolts.

Whenever possible, the bolt should be placed withthe head on top or in the forward position. Thispositioning helps prevent the bolt from slipping out ifthe nut is accidentally lost.

Make sure that the bolt grip length is correct.Generally speaking, the grip length should equal thethickness of the material being bolted together. Notmore than one thread should bear on the material, andthe shank should not protrude too far through the nut.Figure 5-2 shows examples of correct and incorrect griplength.

Application of Torque

Torque is the amount of twisting force appliedwhen you are tightening a nut. If torque values arespecified in the appropriate manual, a torque wrenchmust be used. Regardless of whether torque values arespecified or not, all nuts in a particular installation mustbe tightened a like amount. This permits each bolt in agroup to carry its share of the load. It is a good practiceto use a torque wrench in all applications.

Safetying of Nuts and Bolts

It is very important that all nuts except theself-locking type be safetied after installation. Thisprevents nuts from loosening in flight because ofvibration. Methods of safetying are discussed later inthis chapter.

Q5-2. What are the three principal parts of a bolt?

Q5-3. What are the three most commonly usedscrews in aircraft construction?

Q5-4. What general group of aircraft nuts requirean external locking device, such as cotterpins, safety wire, or locknuts?

5-6

ANF0508

TOP VIEW SIDE VIEW

STARTING POSITION DOUBLE-LOCKEDPOSITION

INWARDTHREAD

LOCK

A B

C D

Figure 5-8.—Sheet spring nut.

Q5-5. What is the purpose of placing a washerunder the head of a bolt?

TURNLOCK FASTENERS

LEARNING OBJECTIVE: Recognize thethree common types of turnlock fasteners(quick-action panel fasteners) and how theyoperate.

Turnlock fasteners are used to secure plates, doors,and panels that require frequent removal for inspectionand servicing. Turnlock fasteners are also referred to asquick-action panel fasteners. These fasteners areavailable in several different styles and are usuallyreferred to by the manufacturer's trade name. Some ofthe most common are the Camloc, Airloc, and Dzus.

CAMLOC FASTENERS

The Camloc 4002 series fastener consists of fourprincipal parts—receptacle, grommet, retaining ring,and stud assembly. See figure 5-9. The receptacleconsists of an aluminum alloy forging mounted in astamped sheet metal base. The receptacle assembly isriveted to the access door frame, which is attached tothe structure of the aircraft. The grommet is a sheetmetal ring held in the access panel by the retaining ring.Grommets are available in two types—the flush typeand the protruding type. In addition to serving as thegrommet for the hole in the access panel, it also holdsthe stud assembly. The stud assembly consists of a stud,cross pin, spring, and spring cup. The assembly isdesigned so that it can be quickly inserted into thegrommet by compression of the spring. Once installedin the grommet, the stud assembly cannot be removedunless the spring is again compressed.

5-7

OUTER MEMBER

STUD ASSEMBLY

GROMMET

GROMMETRETAINING

RING

RIVET

INNERMEMBER

RECEPTACLE STUDRETAINING

RING

OUTER MEMBER

STUD ASSEMBLY

GROMMET

GROMMETRETAINING

RING

INNERMEMBER

RECTACLE

STUD ASSEMBLY

FLUSH ORPROTRUDING

GROMMET

GROMMETRETAINING

RING

STUD RETAININGRING (USED ON

SOME FASTENERS)

RECEPTACLE

PROTRUDING TYPE INSTALLATION

FLUSH TYPE INSTALLATION

RIVET

ANF0509

Figure 5-9.—Camloc 4002 series fastener.

The Camloc high-stress panel fastener, shown infigure 5-10, is a high-strength, quick-release,rotary-type fastener. It may be used on flat or curved,inside or outside panels. The fastener may have either aflush or protruding stud. The studs are held in the panelwith flat or cone-shaped washers. The latter being usedwith flush fasteners in dimpled holes. This fastener maybe distinguished from screws by the deep No. 2 Phillipsrecess in the stud head and by the bushing in which thestud is installed.

AIRLOCK FASTENERS

Figure 5-11 shows the parts that make up an Airlocfastener. Similar to the Camloc fastener, the Airlocfastener consists of a receptacle, stud, and cross pin.The stud is attached to the access panel and is held inplace by the cross pin. The receptacle is riveted to theaccess panel frame.

Two types of Airloc receptacles are available—thefixed type (view A) and the floating type (view B). Thefloating type makes for easier alignment of the stud in

the receptacle. Several types of studs are also available.In each instance the stud and cross pin come as separateunits so that the stud may be easily installed in theaccess panel.

DZUS FASTENERS

Dzus fasteners are available in two types. One is thelight-duty type, used on box covers, access hole covers,and lightweight fairing. The second is the heavy-dutytype, which is used on cowling and heavy fairing. Themain difference between the two types of Dzusfasteners is a grommet, used with the heavy-dutyfasteners. Otherwise their construction features areabout the same.

Figure 5-12 shows the parts making up a light-dutyDzus fastener. Notice that they include a spring and astud. The spring is made of cadmium-plated steel musicwire and is usually riveted to an aircraft structuralmember. The stud comes in a number of designs (asshown in views A, B, and C) and mounts in a dimpledhole in the cover assembly.

5-8

ANF0510

1

2

3

4

5

10

6

2 6 7

8

9

5

10

1. TENSION SPRING2. STUD ASSEMBLY3. RETAINING RING4. RETAINING RING

5. RECEPTACLE ASSEMBLY6. RECEPTACLE ATTACHING RIVETS7. OUTER SKIN8. INNER SKIN

9. INSERT10. COVER

Figure 5-10.—Camloc high-stress panel fastener.

5-9

ANF0511

FIXED TYPE

(A) FLOATING TYPE

(B)

CROSSPIN

RECEPTACLE

PANEL STUD

Figure 5-11.—Airloc fastener.

ANF0512

FLUSHTYPE

OVALTYPE

WINGTYPE

STUD

DIMPLEDHOLE

COVERASSEMBLY

SPRING

A

B

C

Figure 5-12.—Dzus fastener.

Position the panel or plate on the aircraft beforesecuring it in place. The spring riveted to the structuralmember enters the hollow center of the stud, which isretained in the plate or panel. Then, when the stud isturned about one-fourth turn, the curved jaws of thestud slip over the spring and compress it. The resultingtension locks the stud in place, thereby securing thepanel or plate.

Q5-6. What are the three most common types ofturnlock fasteners?

RIVETS

LEARNING OBJECTIVE: Identify thesolid rivets, blind rivets, and rivnuts commonlyused in aircraft construction.

There are hundreds of thousands of rivets in theairframe of a modern aircraft. This is an indication ofhow important rivets are in the construction of aircraft.A glance at any aircraft will disclose the thousands ofrivets in the outer skin alone. In addition to being usedin the skin, rivets are used in joining spar and ribsections. They are also used for securing fittings tovarious parts of the aircraft, and for fastening bracingmembers and other parts together. Rivets that aresatisfactory for one part of the aircraft are oftenunsatisfactory for another part.

Two of the major types of rivets used in aircraftconstruction are the solid rivet and the blind rivet. Thesolid rivet must be driven with a bucking bar. The blindrivet is installed when a bucking bar cannot be used.

SOLID RIVETS

Solid rivets are classified by their head shape, size,and the material from which they are manufactured.Rivet head shapes and their identifying code numbersare shown in figure 5-13. The prefix MS identifieshardware under the control of the Department ofDefense and that the item conforms to militarystandards. The prefix AN identifies specifications that

are developed and issued under joint authority of theAir Force and the Navy. Solid rivets have five differenthead shapes. They are the round head, flat head,countersunk head, brazier head, and universal headrivets.

Round Head Rivets

Round head rivets are used on internal structureswhere strength is the major factor and streamlining isnot important.

Flat Head Rivets

Flat head rivets, like round head rivets, are used inthe assembly of internal structures where maximumstrength is required. They are used where interferenceof nearby members does not permit the use of roundhead rivets.

Countersunk Head Rivets

Countersunk head rivets, often referred to as flushrivets, are used where streamlining is important. Oncombat aircraft practically all external surfaces areflush riveted. Countersunk head rivets are obtainablewith heads having an inclined angle of 78 and 100degrees. The 100-degree angle rivet is the mostcommonly used type.

Brazier Head Rivets

Brazier head rivets offer only slight resistance tothe airflow and are used frequently on external surfaces,especially on noncombat-type aircraft.

Universal Head Rivets

Universal head rivets are similar to brazier headrivets. They should be used in place of all otherprotruding-head rivets when existing stocks aredepleted.

BLIND RIVETS

There are many places on an aircraft where accessto both sides of a riveted structural part is impossible.When attaching many nonstructural parts, the fullstrength of solid-shank rivets is not necessary and theiruse adds extra weight. For use in such places, rivetshave been designed that can be formed from theoutside. They are lighter than solid-shank rivets but areamply strong. Such rivets are referred to as blind rivets

5-10

ANF0513

MS20470UNIVERSAL

AN 430ROUND

AN 456BRAZIER

MS20426COUNTER-

SUNK

AN 442FLAT

Figure 5-13.—Rivet head shapes and code numbers.

or self-plugging because of the self-heading feature.Figure 5-14 shows a general type of blind rivet.

RIVNUTS

The rivnut is a hollow aluminum rivet that iscounterbored and threaded on the inside. The rivet isinstalled with the aid of a special tool. Rivnuts are usedprimarily as a nut plate. They may be used as rivets insecondary structures such as instruments, brackets, andsoundproofing materials. After rivnuts are installed,accessories can be fastened in place with screws.

Rivnuts are manufactured in two head styles,countersunk and flat, and in two shank designs, openand closed ends. See figure 5-15.

Open-end rivnuts are the most widely used. Theyare preferred in place of the closed-end type. However,in sealed flotation or pressurized compartments, theclosed-end rivnut must be used.

Further information concerning rivets may befound in the Structural Hardware Manual, NAVAIR01-1A-8.

Q5-7. What are the two major types of rivets used inaircraft construction?

Q5-8. What type of rivets are used wherestreamlining is important?

MISCELLANEOUS FASTENERS

LEARNING OBJECTIVE: Recognize themiscellaneous fastener used to fasten specialpurpose units.

Some fasteners cannot be classified as rivets,turnlocks, or threaded fasteners. Included in thiscategory are snap rings, turnbuckles, taper pins, flathead pins, and flexible connector/clamps.

SNAP RINGS

A snap ring is a ring of metal, either round or flat incross section, that is tempered to have springlike action.This springlike action holds the snap ring firmly seatedin a groove. The external types are designed to fit in agroove around the outside of a shaft or cylinder. Theinternal types fit in a groove inside a cylinder. A specialtype of pliers is made to install each type of snap ring.Snap rings may be reused as long as they retain theirshape and springlike action.

TURNBUCKLES

A turnbuckle is a mechanical screw deviceconsisting of two threaded terminals and a threadedbarrel. Figure 5-16 shows a typical turnbuckleassembly.

5-11

ANF0514

LOCKINGCOLLAR

NOTESHEET

GAP

LOCKINGCOLLAR

INSERTED

(A)INSTALLED

(B)

Figure 5-14.—Self-plugging rivet (mechanical lock).

ANF0515

COUNTERSUNKFLAT HEAD

OPENEND CLOSED

ENDCLOSED

END

OPENEND

Figure 5-15.—Sectional view of rivnut showing head and enddesigns.

ANF0516

L (THREADS FLUSH WITH ENDS OF BARREL)

BARRELSWAGING TERMINAL

PINEYE

Figure 5-16.—Typical turnbuckle assembly.

Turnbuckles are fitted in the cable assembly for thepurpose of making minor adjustments in cable lengthand for adjusting cable tension. One of the terminalshas right-hand threads and the other has left-handthreads. The barrel has matching right- and left-handinternal threads. The end of the barrel with the left-handthreads can usually be identified by a groove or knurlaround that end.

When installing a turnbuckle in a control system, itis necessary to screw both of the terminals an equalnumber of turns into the barrel. It is also essential thatyou screw both turnbuckle terminals into the barreluntil not more than three threads are exposed.

After you adjust a turnbuckle properly, it must besafetied. We will discuss the methods of safetyingturnbuckles later in this chapter.

TAPER PINS

Taper pins are used in joints that carry shear loadsand where the absence of clearance is essential. Seefigure 5-17. The threaded taper pin is used with a taperpin washer and a shear nut if the taper pin is drilled. Usea self-locking nut if the taper pin is undrilled. When ashear nut is used with the threaded taper pin andwasher, the nut is secured with a cotter pin.

FLAT HEAD PINS

The flat head pin is used with tie-rod terminals orsecondary controls, which do not operate continuously.The flat head pin should be secured with a cotter pin.The pin is normally installed with the head up. Seefigure 5-17, view C. This precaution is taken tomaintain the flat head pin in the installed position incase of cotter pin failure.

FLEXIBLE CONNECTORS/CLAMPS

Some of the most commonly used clamps areshown in figure 5-18. When installing a hose betweentwo duct sections, the gap between the duct ends shouldbe one-eighth inch minimum to three-fourths inchmaximum. When you install the clamps on theconnection, the clamp should be one-fourth inchminimum from the end of the connector. Misalignmentbetween the ducting ends should not exceed one-eighthinch maximum.

Marman type clamps, commonly used in ductingsystems, should be tightened to the torque valueindicated on the coupling. Use the torque value asspecified on the clamp or in the applicable maintenanceinstruction manual.

Q5-9. What are five fasteners that are included inthe category of miscellaneous fasteners?

AIRCRAFT ELECTRICAL SYSTEMHARDWARE

LEARNING OBJECTIVE: Identify thespecial hardware found in an aircraft'selectrical system.

An important part of aircraft electrical maintenanceis determining the correct type of electrical hardwarefor a given job. You must become familiar with wireand cable, connectors, terminals, and bonding.

5-12

ANF0517

STATIONARYMEMBER

STATIONARYMEMBER

PLAINTAPER

PIN

MOVABLEMEMBER

MOVABLEMEMBER

COTTERPIN

COTTERPIN

CASTELLATEDNUT

TAPERPIN

WASHER

THREADTAPER

PIN

FLATHEADPIN

WASHER

A. PLAIN TAPER PIN INSTALLED

B. THREADED TAPER PIN INSTALLED

C. FLAT HEAD PIN INSTALL

Figure 5-17.—Types of aircraft pins.

WIRE AND CABLE

For purposes of electrical installations, a wire isdescribed as a stranded conductor covered with aninsulating material. The term cable, as used in aircraftelectrical installations, includes the following:

� Two or more insulated conductors contained inthe same jacket (multiconductor cable)

� Two or more insulated conductors twistedtogether (twisted pair)

� One or more insulated conductors covered witha metallic braided shield (shielded cable)

� A single insulated conductor with a metallicbraided outer conductor (RF cable)

For wire replacement work, the aircraftmaintenance instruction manual should be consultedfirst. The manual normally lists the wire used in a givenaircraft.

CONNECTORS

Connectors are devices attached to the ends ofcables and sets of wires to make them easier to connectand disconnect. Each connector consists of a plugassembly and a receptacle assembly. The two

5-13

ANF0518

MARMAN BAND CLAMP

MARMAN BAND CLAMPS TOBE USED IN TIGHT AREAS

AN737 BAND CLAMP FORSTANDARD INSTALLATIONS

AN737 BAND CLAMP

1/8 - INCH MAXIMUM MISALIGNMENT

1/8 INCH MINIMUM3/4 INCH MAXIMUM

1/4 INCH MINIMUM

Figure 5-18.—Flexible line connectors.

assemblies are coupled by means of a coupling nut.Each consists of an aluminum shell containing aninsulating insert that holds the current-carryingcontacts. The plug is usually attached to the cable end,and is the part of the connector on which the couplingnut is mounted. The receptacle is the half of theconnector to which the plug is connected. It is usuallymounted on a part of the equipment. One type ofconnector commonly used in aircraft electrical systemsis shown in figure 5-19.

TERMINALS

Since most aircraft wires are stranded, it isnecessary to use terminal lugs to hold the strandstogether. This allows a means of fastening the wires toterminal studs. The terminals used in electrical wiringare either of the soldered or crimped type. Terminalsused in repair work must be of the size and typespecified in the applicable maintenance instructionmanual. The crimped-type terminals are generallyrecommended for use on naval aircraft. Soldered-typeterminals are usually used in emergencies only.

The basic types of solderless terminals are shownin figure 5-20. They are the straight, right angle, flag,and splice types. There are variations of these types.

BONDING

When you connect all the metal parts of an aircraftto complete an electrical unit, it is called bonding.Bonding connections are made of screws, nuts,washers, clamps, and bonding jumpers. Figure 5-21shows a typical bonding link installation.

An aircraft can become highly charged with staticelectricity while in flight. If the aircraft is improperlybonded, all metal parts do not have the same amount ofstatic charge. A difference of potential exists betweenthe various metal surfaces. If the resistance betweeninsulated metal surfaces is great enough, charges canaccumulate. The potential difference could becomehigh enough to cause a spark. This constitutes a firehazard and also causes radio interference. If lightingstrikes an aircraft, a good conducting path for heavycurrent is necessary to minimize severe arcing andsparks.

Bonding also provides the necessary low-resistance return path for single-wire electricalsystems. This low-resistance path provides a means ofbringing the entire aircraft to the earth's potential whenit is grounded.

5-14

ANF0519

RECEPTACLEASSEMBLY

COUPLINGNUT

PLUG SOCKETASSEMBLY

Figure 5-19.—Connector assembly.

ANF0520

TOUGUE

BARREL

TERMINALHOLE

STRAIGHTRIGHT ANGLE

FLAGSPLICE

Figure 5-20.—Basic types of solderless terminals.

ANF0521

Figure 5-21.—Typical bonding link installation.

When you perform an inspection, both bondingconnections and safetying devices must be inspectedwith great care.

Q5-10. What manual should you consult to findcorrect replacement wires for a givenaircraft?

SAFETY METHODS

LEARNING OBJECTIVE: Recognize theprocedures for the safetying of fasteners andelectrical system hardware.

Safetying is a process of securing all aircraft bolts,nuts, capscrews, studs, and other fasteners. Safetyingprevents the fasteners from working loose due tovibration. Loose bolts, nuts, or screws can ruin enginesor cause parts of the aircraft to drop off. To carry out aninspection on an aircraft, you must be familiar with thevarious methods of safetying. Careless safetying is asure road to disaster. Always use the proper method forsafetying. Always safety a part you have just unsafetiedbefore going on to the next item of inspection. You

should always inspect for proper safetying throughoutthe area in which you are working.

There are various methods of safetying aircraftparts. The most widely used methods are safety wire,cotter pins, lock washers, snap rings, and special nuts.Some of these nuts and washers have been describedpreviously in this chapter.

SAFETY WIRING

Safety wiring is the most positive and satisfactorymethod of safetying. It is a method of wiring togethertwo or more units. Any tendency of one unit to loosen iscounteracted by the tightening of the wire.

Nuts, Bolts, and Screws

Nuts, bolts, and screws are safety wired by thesingle-wire double-twist method. This method is themost common method of safety wiring. A single-wiremay be used on small screws in close spaces, closedelectrical systems, and in places difficult to reach.

Figure 5-22 illustrates the following steps requiredto install a standard double-twist safety wire for twobolts with right-hand threads.

5-15

ANF0522

STEP 1 STEP 2STEP 3 STEP 4

STEP 5 STEP 6

STEP 7

STEP 8

STEP 9STEP 10

Figure 5-22.—Standard double-twist safety wire installation procedures.

Step 1. Assemble the unit. Torque the bolts andcarefully align the safety wire holes.

Step 2. Insert the proper size wire through thehole in the first bolt.

Step 3. Bend the left end of the wire clockwisearound the bolt head and under the other end of thewire.

Step 4. Pull the loop tight against the bolt head.Grasp both ends of the wire. Twist them in a clockwisedirection until the end of the braid is just short of thesecond bolt.

Step 5. Check to ensure that the loop is stilltightly in place around the first bolt head. Grasp thewire with pliers just beyond the end of the braid. Whileholding it taut, twist it in a clockwise direction until thebraid is stiff.

NOTE: The braid must be tight enough to resistfriction or vibration wear, but should not beovertightened.

Step 6. Insert the upper end of the safety wirethrough the hole in the second bolt. Pull the braid untilit is taut.

Step 7. Bring the other end of the wire counter-clockwise around the bolt head and under theprotruding wire end.

Step 8. Tighten the loop and braid the wire ends ina counterclockwise direction. Grasp the wire with thepliers just beyond the end of the braid and twist in acounterclockwise direction until the braid is stiff. Makesure you keep the wire under tension.

Step 9. With a final twisting motion, bend thebraid to the right and against the head of the bolt.

Step 10. Cut the braid, being careful that betweenthree and six full twists still remain. Avoid sharpprojecting ends.

Figure 5-23 shows various methods commonlyused in safety wiring nuts, bolts, and screws. Examples1, 2, and 5 of figure 5-23 show the proper method ofsafety wiring bolts, screws, square head plugs, andsimilar parts when wired in pairs. Examples 6 and 7show a single-threaded component wired to a housingor lug. Example 3 shows several components wired inseries. Example 4 shows the proper method of wiringcastellated nuts and studs. Note that there is no looparound the nut. Example 8 shows several componentsin a closely spaced, closed geometrical pattern, usingthe single-wire method.

When drilled-head bolts, screws, or other parts aregrouped together, they are more conveniently safetywired to each other in a series rather than individually.The number of nuts, bolts, or screws that may be safetywired together depends on the application. Forinstance, when you are safety wiring widely spacedbolts by the double-twist method, a group of threeshould be the maximum number in a series.

When you are safety wiring closely spaced bolts,the number that can be safety wired by a 24-inch lengthof wire is the maximum in a series. The wire is arrangedin such a manner that if the bolt or screw begins toloosen, the force applied to the wire is in the tighteningdirection.

5-16

ANF0523

1 2

3

4 5

6 7

8Figure 5-23.—Safety wiring methods.

Torque all parts to the recommended values, andalign holes before you attempt to proceed with thesafetying operation. Never overtorque or loosen atorqued nut to align safety wire holes.

Oil Caps, Drain Cocks, andValves

These units are safety wired as shown in figure5-24. In the case of the oil cap, the wire is anchored toan adjacent fillister head screw. This system applies toany other unit that must be safety wired individually.Ordinarily, anchorage lips are conveniently locatednear these individual parts. When this provision is notmade, the safety wire is fastened to some adjacent partof the assembly.

Electrical Connectors

Under conditions of severe vibration, the couplingnut of a connector may vibrate loose. With sufficientvibration, the connector could come apart. When thisoccurs, the circuit carried by the cable opens. Theproper protective measure to prevent this occurrence isby safety wiring, as shown in figure 5-25. The safetywire should be as short as practicable. It must beinstalled in such a manner that the pull on the wire is inthe direction that tightens the nut on the plug.

Turnbuckles

After you adjust a turnbuckle properly, safety it.There are several methods of safetying turnbuckles.Only two of these methods have been adopted by themilitary services. These methods are shown in views

5-17

ANF0523

OIL CAPSDRAIN COCKS

VALVES

NOTE:THE SAFETY WIRE ISSHOWN INSTALLEDFOR RIGHT-HANDTHREADS. THE SAFETYWIRE IS ROUTED IN THEOPPOSITE DIRECTIONFOR LEFT-HAND THREADS.

Figure 5-24.—Safety wiring oil caps, drain cocks, and valves.

ANF0525

RECEPTACLE

BULKHEADOR

PLATESTANDARD FILLISTER

HEAD SCREW(DRILLED HEAD)

PLUG

ADAPTER

Figure 5-25.—Safety wiring attachment for plug connectors.

(A) and (B) of figure 5-26. The clip-locking method isused only on the most modern aircraft. An example ofan aircraft using this method is the EA-6B. Theseaircraft use a turnbuckle that is designed for use withthe wire clip. The older type of aircraft still use theturnbuckles that require the wire-wrapping method.

Detailed instructions for using both the clip-locking and the wire-wrapping methods of safetying

turnbuckles can be found in Aviation StructuralMechanic (AM), NAVEDTRA 14315.

GENERAL SAFETY WIRINGRULES

When you use the safety wire method of safetying,follow these general rules:

5-18

ANF0526

STRAIGHTENDHOOK

SHOULDER

HOOKLIP

LOOPENDHOOK

LOOP

HOOKEND

LOCK CLIP, NAS 651

DIRECTION OF PULL FOR INSPECTION

LOCKWIRETURNBUCKLE FORK

AN161 OR AN162

TURNBUCKLEBARREL AN155

4 TURNSWRAP CABLE

4 TURNSWRAP

TURNBUCKLEEYE AN170 THIMBLE

AN100

SWAGED TERMINALAN666 OR AN669

4 TURNSWRAP

LOCKWIRE

(A)

(B)

Figure 5-26.—Safetying turnbuckles. (A) Clip-locking method; (B) wire-wrapping method.

1. A pigtail of one-fourth to one-half inch (threeto six twists) should be made at the end of the wiring.This pigtail must be bent back or under to prevent itfrom becoming a snag.

2. The safety wire must be new upon eachapplication.

3. When you secure castellated nuts with safetywire, tighten the nut to the low side of the selectedtorque range, unless otherwise specified. If necessary,continue tightening until a slot aligns with the hole.

4. All safety wires must be tight after installation,but not under such tension that normal handling orvibration will break the wire.

5. Apply the wire so that all pull exerted by thewire tends to tighten the nut.

6. Twists should be tight and even, and the wirebetween the nuts should be as taut as possible withoutbeing overtwisted.

COTTER PINS

Use cotter pins to secure bolts, screws, nuts, andpins. Some cotter pins are made of low-carbon steel,while others consist of stainless steel, and thus are moreresistant to corrosion. Use stainless steel cotter pins inlocations where nonmagnetic material is required.Regardless of shape or material, use all cotter pins forthe same general purpose—safetying. Figure 5-27shows three types of cotter pins and how their size isdetermined.

NOTE: Whenever uneven-prong cotter pins areused, the length measurement is to the end of theshorter prong.

Cotter pin installation is shown in figure 5-28. Usecastellated nuts with bolts that have been drilled forcotter pins. Use stainless steel cotter pins. The cotterpin should fit neatly into the hole, with very littlesideplay. The following general rules apply to cotter pinsafetying:

� Do not bend the prong over the bolt end beyondthe bolt diameter. (Cut it off if necessary.)

� Do not bend the prong down against the surfaceof the washer. (Again, cut it off if necessary.)

� Do not extend the prongs outward from thesides of the nut if you use the optionalwraparound method.

� Bend all prongs over a reasonable radius.Sharp angled bends invite breakage. Tap theprongs lightly with a mallet to bend them.

Q5-11. What is the purpose of safetying aircrafthardware?

Q5-12. What is the most common method of safetywiring?

Q5-13. What are the two methods of safetyingturnbuckles used by the military services?

Q5-14. What type of cotter pin should you use whennonmagnetic material is required?

WASHERS

LEARNING OBJECTIVE: Recognize thetwo primary functions of washers as used inaircraft/engine construction.

Washers used in aircraft structures may be groupedinto three general classes—plain, lock washers, and

5-19

ANF0527

UNEVENPRONG

OPTIONAL

DIAMETER

LENGTH

Figure 5-27.—Types of cotter pins.

ANF0528PREFERREDOPTIONAL

Figure 5-28.—Cotter pin installations.

special washers. Figure 5-29 shows some of the mostcommonly used types.

PLAIN WASHERS

Plain washers are widely used under AN hex nutsto provide a smooth bearing surface. They act as a shimin obtaining the correct relationship between thethreads of a bolt and the nut. They also aid in adjustingthe position of castellated nuts with respect to drilledcotter pin holes in bolts. Plain washers are also usedunder lock washers to prevent damage to surfaces ofsoft material.

LOCK WASHERS

Lock washers are used whenever the self-lockingor castellated type nut is not used. Sufficient friction isprovided by the spring action of the washer to preventloosening of the nut because of vibration. Lock washers

must not be used as part of a fastener for primary orsecondary structures.

Star Lock Washers

The star lock or shakeproof washer is a roundwasher made of hardened and tempered carbon steel,stainless steel, or Monel. This washer can have eitherinternal or external teeth. Each tooth is twisted, oneedge up and one edge down. The top edge bites into thenut or bolt and the bottom edge bites into the workingsurface. It depends on spring action for its lockingfeature. This washer can be used only once because theteeth become somewhat compressed after being used.

Tab Lock Washers

Tab lock washers are round washers designed withtabs or lips that are bent across the sides of a hex nut orbolt to lock the nut in place. There are various methodsof securing the tab lock washer to prevent it fromturning, such as an external tab bent downward 90degrees into a small hole in the face of the unit, anexternal tab that fits a keyed bolt, or two or more tablock washers connected by a bar. Tab lock washers canwithstand higher heat than other methods of safetying,and can be used safely under high vibration conditions.Tab lock washers should be used only once because thetab tends to crystallize when bent a second time.

SPECIAL WASHERS

Special washers such as ball seat and socketwashers and taper pin washers are designed for specialapplications.

Q5-15. Washers used in aircraft structures aregrouped into what three general classes?

SUMMARY

In this chapter you have been introduced to thevarious types of aircraft hardware used in naval aircraftand the procedures for maintaining their security. It isessential that the correct hardware be used at all timesfor the safe and efficient operation of naval aircraft.

5-20

ANF0529

PLAN BALL SEAT& SOCKET

TAPER PIN

SPECIAL WASHERS

TAB LOCK WASHER STAR LOCK WASHER

Figure 5-29.—Various types of washers.