AN-531 are used in blind applications for the temporary attachment of sheet metal for riveting and the permanent assembly of nonstructural assemblies. AN-535 is a plain head self-tapping screw used in the attachment of nameplates or in sealing drain holes in corrosion-proofing tubular structures and is not intended to be removed after installation. Self-tapping screws should never be used to replace standard screws, nuts, bolts, or rivets in the original structure.

(c) Pins. The three types of pins used in aircraft structures are: The taper pin, the flathead pin, and the cotter pin. Pins are used in shear applications and for safetying.

(1) Taper pins (AN-385 and AN-386) plain and threaded, are used in joints which carry shear loads and where 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 (AN-975) and shear nut (safetied with cotter pin) or self-locking nut.

(2) The flathead pin (AN-392 through AN-406) commonly called a clevis pin, is usually used in conjunction with tie rod terminals and in secondary controls which are not subject to continuous operation. The pin should be safetied with a cotter pin and is customarily installed with the head up so that if the cotter pin fails or works out, the pin will remain in place.

(3) The AN-380 cotter pin is used for safetying bolts, screws, nuts, other pins, and in various applications where such safetying is necessary. The AN-381 cotter pin is used in locations where nonmagnetic material is required, or in locations where resistance to corrosion is desired.

(d) Nuts-(1) Self-locking nuts. Self-locking nuts are acceptable for use on certificated aircraft subject to the restrictions on the pertinent "Manufacturers' Recommended Practice Sheets." Self-locking nuts are used on aircraft to provide tight connections which will not shake loose under severe vibration. Two types of self-locking nuts are currently in use, the all-metal type and the fibre or nylon lock type. Self-locking nuts should not be used at joints which subject either the nut or bolt to rotation. They may be used with antifriction bearings and control pulleys provided the inner race of the bearing is clamped

to the supporting structure by the nut and bolt. Nuts which are attached to the structure should be attached in a positive manner to eliminate rotation or misalinement when tightening the bolts or screws.

(i) All-metal lock nuts are constructed with either the threads in the locking insert out-of-phase with the load-carrying section or with a saw-cut insert with a pinched-in thread in the locking section. The locking action of the all-metal nut depends upon the resiliency of the metal when the locking section and loadcarrying section are engaged by screw threads.

(ii) Fiber or nylon lock nuts are constructed with an unthreaded fiber locking insert held securely in place. The fiber or nylon has a smaller diameter than the nut, and when a bolt or screw is entered, it taps into the insert, producing a locking action. After the nut has been tightened, round or chamfered end bolts, studs or screws should extend at least the full round or chamfer through the nut. Flat end bolts, studs or screws should extend at least 32 inch through the nut. When fiber-type self-locking nuts are reused, care should be exercised that the fiber has not lost its locking friction or become brittle. They should not be reused if they can be run up finger tight. Bolts five-sixteenth-inch diameter and over with cotter-pin holes may be used with self-locking nuts but only if free from burrs around the holes. Bolts with damaged threads and rough ends should never be used. Do not tap the fiberlocking insert.

(iii) (a) Self-locking nut bases are made in a number of forms and materials for riveting and welding to aircraft structure or parts.

(b) Certain applications require the installation of self-locking nuts in channels, an arrangement which permits the attachment of many nuts with only a few rivets. These channels are tracklike bases with regularly spaced nuts which are either removable or nonremovable. The removable type carries a floating nut which can be snapped in or out of the channel thus making possible the ready removal of damaged nuts. Nuts such as the clinch-type and splinetype which depend on friction for their anchorage are not acceptable for use in aircraft structures.

(iv) Self-locking nuts may be used on aircraft engines and accessories when

heir use is specified by the engine manuacturer in his bulletins or manuals. Reer to § 18.30-14 for detailed installation nstructions.

((2) Aircraft castle nut (AN-310). The castle nut is used with drilled-shank AN hex-head bolts, clevis bolts, eye bolts, drilled-head bolts or studs, and is designed to accommodate a cotter pin or lock wire as a means of safetying.

(1)

(3) Miscellaneous aircraft nuts. The plain nut (AN-315 and AN-335) has limited use on aircraft structures and requires an auxiliary locking device such as a check nut or lock washer.

(ii) Light hex nuts (AN-340 and AN345) are used in miscellaneous applications and must be locked by an auxiliary device.

(iii) The check nut AN-316 is used as a locking device for plain nuts, screws, threaded rod ends and other devices.

(iv) The castellated shear nut AN320 is designed for use with clevis bolts and threaded taper pins, which are normally subjected to shearing stress only.

(v) Wing nuts AN-350 are intended for use on hose clamps and battery connections, etc., where the desired tightness is ordinarily obtained by the use of the fingers or hand tools.

(vi) Sheet spring nuts, such as speed nuts, are used with standard and sheet metal self-tapping screws in nonstructural locations. They find various uses in supporting line clamps, conduit clamps, electrical equipment, doors, and the like, and are available in several types.

access

(vii) Two commercial types of highstrength internal or external wrenching nuts are available, the internal and external wrenching Elastic Stop Nut and the Unbrako internal and external wrenching nut. Both are of the selflocking type, are heat-treated, and are capable of carrying the high-strength bolt tension load.

(e) Washers. The types of washers used in aircraft structure are: Plain washers, lock washers, and special washers.

(1) Plain washers AN-960 and AN-970 are widely used under hex nuts to provide a smooth bearing surface, to act as a shim and to adjust the position of castellated nuts with respect to drilled cotter-pin holes in bolts. Plain washers should be used under lock washers to

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prevent damage to surfaces. Cadmiumplated steel washers should be used under bolt heads or nuts on aluminum alloy or magnesium structures where corrosion if it occurs will then be between the washer and the steel. The AN-970 steel washer provides a greater bearing area than the plain type and is used in wooden structures under both bolt heads and nuts to prevent local crushing of the surface.

(2) Lock washers AN-935 and AN-936 may be used with machine screws or bolts whenever the self-locking or castellated type of nut is not applicable. They should not be used as fastenings to primary or secondary structures or where subject to frequent removal or corrosive conditions.

(3) Ball-socket and seat-washers AN950 and AN-955 are used in special applications where the bolt is installed at an angle to the surface, or where perfect alinement with the surface is required at all times. These washers are used together.

(4) Taper-pin washers AN-975 are used with the threaded taper pin.

(5) NAS-143 washers for internal wrenching nuts and bolts are used with NAS internal wrenching bolts. Type "C" is countersunk to seat the bolthead shank radius and a plain-type washer is used under the nut. Both of these washers are heat-treated from 125,000 to 145,000 p. s. i.

(f) Rivets (1) Standard solid-shank rivets. (i) The universal-head rivets AN-470 are used in aircraft construction in both interior and exterior locations.

(ii) Roundhead rivets AN-430 and AN-435 are used in the interior of aircraft except where clearance is required for adjacent members.

(iii) Flathead rivets AN-441 and AN442 are used in the interior of the aircraft where interference of adjacent members does not permit the use of roundhead rivets.

(iv) Brazier-head rivets AN-455 and AN-456 are used on the exterior surfaces of aircraft where flush riveting is not essential.

(v) All protruding-head rivets may be replaced by AN-470 universal-head rivets. This rivet has been adopted as the standard for protruding-head rivets in this country.

(vi) Countersunk-head rivets AN-426 (100°), AN-425 (78°), and AN-420 (90°)

are used on the exterior surfaces of aircraft to provide a smooth aero-dynamic surface, and in other applications where a smooth finish is desired. The 100° countersunk-head has been adopted as the standard in this country.

(vii) Material applications:

(a) A-17S-T3 is the most commonly used rivet material utilized in aluminum alloy structures. Its main advantage lies in the fact that it may be used in the condition received without any further treatment.

(b) The 17S-T3 and 17S-T31 and 24S-T4 rivets are used in aluminum alloy structures where strength higher than that of the A17S-T3 rivet is needed. (See ANC-5 for differences between the 2 types of 17ST rivets specified here.)

(c) The 2S rivet of pure aluminum is used for riveting nonstructural parts fabricated from the softer aluminum alloys, such as 2S, 3S, and 52S.

(d) When riveting magnesium alloy structures, 56S rivets are used exclusively due to their corrosion-resistant qualities in combination with the magnesium alloys.

(e) Mild steel rivets are used primarily in riveting steel parts. Galvanized rivets should not be used on steel parts subjected to high heat.

(f) Corrosion-resistant steel rivets are used primarily in riveting corrosion-resistant steel parts, such as firewalls, exhaust stack bracket attachments and similar structures.

(g) Monel rivets are used in special cases for riveting high nickel steel alloys and nickel alloys. They may be used interchangeably with stainless steel rivets as they are more easily driven. However, it is preferable to use stainless steel rivets in stainless steel parts.

(h) Copper rivets are used for riveting copper alloys, leather, and other nonmetallic materials. This rivet has only limited usage in aircraft.

(2) Blind rivets. Blind rivets MS20600, MS-20601, MS-20602, MS-20603 may be substituted for the normally required solid rivets in accordance with the blind rivet manufacturer's recommendations. They should not be used where the failure of a few rivets will seriously impair the airworthiness of the aircraft. Design allowables for blind rivets are specified in ANC-5 "Strength of Metal Aircraft Elements."

(3) Hi-shear rivets. (1) Hi-shear rivets are sometimes used in connections where the shearing loads are the primary design consideration. Their use is restricted to such connections. It should be noted that hi-shear rivet patterns are not to be used for the installation of control surface hinges and hinge brackets. Also, they should not be painted prior to assembly even where dissimilar metals are being joined, but each end should be touched up with zinc chromate primer to allow the later application of the general airplane finish schedule.

(ii) Hi-shear rivets should be replaced only by the same type rivet. The installation and inspection should be in accordance with procedures recommended by the manufacturer.

(g) Fasteners (cowl and fairing). A number of patented fasteners are in use on aircraft. A variety of these fasteners are commercially available and the manufacturer's recommendations concerning the proper use of these types of fasteners should always be considered in other than replacement application.

(h) Unconventional attachments. Unconventional or new attachment devices should not be used in the primary structure unless approved by a representative of the Civil Aeronautics Administration. [Supp. 1, 18 F. R. 7391, Nov. 21, 1953, as amended by Supp. 2, 19 F. R. 1950, Apr. 7, 1954]

§ 18.30-7 Corrosion protection, cleaners, and paint removers (FAA policies which apply to § 18.30).

(a) Corrosion protection. Almost all metals used in aircraft are subject to corrosion. Materials such as steel will rust, and aluminum and magnesium will form corrosion products, unless properly protected. Stainless steel, brasses, and copper alloys normally form a surface film which prevents further surface corrosion; however, under certain conditions, particularly when in contact with dissimilar metals, even these alloys must be protected. Corrosion is promoted by contact of metals with materials that absorb water. For example: wood, sponge rubber, felt, etc., may be sources of serious corrosion unless proper protection is used. Specific aspects of the more common types of corrosion are covered in subparagraphs (1) through (5) of this paragraph.

(1) Dissimilar metals corrosion. (i) When two dissimilar metals are in con

tact and are connected by an electrolyte (water), accelerated corrosion of one of the metals may occur. For this reason metals have been divided into certain groups, based on their susceptibility to this form of corrosion. Unprotected contact between metals of different groups may result in dissimilar metals corrosion; therefore, contact between metals of dissimilar groups should be prevented or the contact surface should be adequately protected.

(ii) Similar metal groups (refers to surface of metal).

Group 1-Magnesium alloys.

Group 2-Zinc, cadmium, lead, tin, steel. Group 3-Copper and its alloys, nickel and its alloys, chromium and stainless steels. Group 4-All aluminum alloys.

(iii) Aluminum alloys (group 4) may I be further subdivided into the following subgroups:

Subgroup A-2S, 3S, 52S, 61S, 220, 355, 356, all clad alloys, such as Alclad and Pureclad. Subgroup B-14S, 178, 24S, 75S, 195.

(iv) Under severe corrosive conditions, the above subgroups should be considered as dissimilar metal groups insofar as corrosion protection is concerned. This is particularly true where a relatively large area of an alloy classified in subgroup B is in contact with a relatively small area of subgroup A, in which case serve corrosion of the subgroup A alloy may be expected.

(2) Intergranular corrosion. Intergranular corrosion occurs in certain aluminum alloys which are improperly heattreated. For example, 24S alloys should be quenched quickly after heat-treatment in order to prevent intergranular corrosion. Since 24S alloy contains metals other than aluminum, particularly copper, severe corrosion may result if this alloy is quenched slowly, and a reduction in strength may result in improperly quenched 24S alloys when subjected to corrosive conditions. This type of corrosion is difficult to detect in its original stage except by microscopic examinations. When well advanced it is characterized by scaling and blistering. Surface protection of slowly quenched 24S alloy will retard intergranular corrosion, but diffusion of the base metal into the surface coating will eventually destroy its effectiveness. The only protection which is regarded as being sufficiently adequate for air-quenched 24S is cladding of the aluminum alloy with pure aluminum. It should be noted that

in some cases even clad alloys may be susceptible to intergranular corrosion. Other surface protection, such as anodizing, and subsequent coatings, such as zinc chromate primer, heavy greases, etc., may also prevent intergranular corrosion in cases where susceptibility of the alloy to intergranular corrosion is not too great.

(3) Stress corrosion. This type of corrosion occurs when certain metals, mostly aluminum and magnesium alloys, are exposed to high stress and corrosive conditions. Stress corrosion has occurred in aluminum when steel bushings were pressed into the aluminum parts with too tight a fit, and were exposed to corrosive conditions. Stress corrosion can also occur in cold-worked metals which are not properly stress relieved.

(4) Chemical. (i) Corrosion protection against chemicals used in dusting and spraying operations is covered in detail in Civil Aeronautics Manual 8, Aircraft Airworthiness, Restricted Category (Part 8 of this chapter). Reference should be made to that document for detailed information.

(ii) In general, corrosion protection measures against chemicals involves cleaning and/or surface protection and specific rules can be laid down only for the particular chemicals used. Operators are warned against the use of mercury compounds as their corrosive effects are particularly rapid. Under certain conditions some of the mercury compounds may cause structural failure within an hour.

(5) Fretting. Fretting corrosion is a surface phenomenon which may occur when repeated relative motion of small amplitude is allowed to take place between closely fitting components. It is characterized by surface stains, corrosion, pitting and the generation of oxides. Certain aircraft parts have been known to fail by fretting corrosion, as for instance, antifriction bearings, connecting rods, knuckle pins, splined shafts, and clamped or bolted flanges; and close periodic inspections should be made of such parts. Where evidence of fretting corrosion is found, the affected parts should be replaced.

(b) Corrosion protection measures. Surfaces which are completely dry cannot corrode. If a metal can be protected from moisture due to rain, condensation, or other causes, corrosion need not be feared. Dirt, surface film, etc., -on

metal surfaces tend to retain moisture and hence promote corrosion. Waterabsorbing materials, such as certain cleaners and calcium chloride, which may occasionally be used as a snow remover on runways are especially dangerous in this regard.

(1) Anodizing and related processes. In anodizing, aluminum alloys are placed in an electrolytic bath, causing a thin film of aluminum oxide to form on the surface of the aluminum. This film is resistant to corrosion and affords a good paint base. Other processes which do not provide as good a corrosive protection as anodizing are, however, good paint bases. These processes are:

(i) Alkaline cleaning followed by chromic acid dip.

(ii) Alcoholic phosphoric acid cleaner. (iii) Alkaline dichromate treatment. (2) Plating. Steels are commonly plated with other metals to prevent corrosion. Plating is accomplished by placing the article in an electrolytic bath and metal from the plating solution is deposited on it. Various metals used in plating vary in the corrosion protection they afford steel. For instance, cadmium and zinc corrode before the steel does, hence slight breaks or cracks through the plating of these metals will not result in rusting of the exposed steel, since the surface metal is corroded and protects the steel. Chromium does not protect steel by this method, as steel will corrode before the chromium does, and thus depends for its protection on the tightness of the plate.

(3) Parkerizing and bonderizing. These processes do not appear to be equal in corrosion protection to plating and are not generally acceptable as a substitute for plating; however, both are good paint bases.

(4) Dichromate treatment for magnesium. The dichromate treatment consists of boiling magnesium parts in a solution of sodium dichromate, resulting in a coating with little resistance to corrosion but which is a good paint base.

(5) Chromium pickle treatment for magnesium. In this process the magnesium parts are placed in a solution of nitric acid and sodium dichromate. This will protect the magnesium during storage and acts as a bond for subsequent organic finishes.

(6) Galvanic anodizing treatment for magnesium. This is an electrolytic proc

ess used to provide a paint base and corrosion preventive film on magnesium alloys containing manganese.

(7) Cladding. Aluminum alloys which are susceptible to corrosion are frequently aluminum. clad with pure Slight pits, scratches, or other defects through the cladding material will not result in corrosion of the core, since the pure aluminum on the edges of the defect will be preferentially corroded, protecting the core.

(8) Metal spraying. In this process metallic wire such as aluminum or zinc The is fed into a special spray gun. metal is melted and sprayed on the object to be protected, which must be thoroughly clean to prevent peeling of the sprayed coat. A "metallized" surface has very good resistance to corrosion if properly applied and of sufficient thickness.

(9) Organic coatings. Zinc chromate primer, enamels, chlorinated rubber compounds, etc., are organic coatings commonly used on metals to protect them. The finishes should be applied according to the instructions of the manufacturer.

When do ped

(10) Dope-proofing. fabrics are applied over an organic finished metal structure, the dope will have a tendency to loosen the finish on the metal. For this reason, organic coatings on the metal are usually covered with a dope-proof paint, with metal foil, or with cellulose tape to prevent the dope from striking through.

(11) Tube interiors. The interiors of structural steel and aluminum tubing should be protected against corrosion. A small amount of water entrapped in a tube can corrode entirely through the tube thickness in a short period of time. For this reason, most structural tubing is flushed with hot linseed oil, paralketone, or other corrosion inhibitor. Hot flushing results in a good coating. The flushing liquid is usually introduced through small holes drilled in the tubing. These holes should be plugged with a screw or by other means to prevent entry of moisture. Air and watertight sealing of the tubing will also give adequate protection against corrosion if the tubing is internally dry before being sealed.

(c) Corrosion-proofing of landplanes and seaplanes. In the repair or alteration of aircraft, corrosion-proofing the same or equivalent to that originally applied should be used unless the repair