Aircraft Nut Standards Quiz

Aircraft Nuts

• After tightening a nut, ensure the rounded or chamfered end of the bolt, stud, or screw extends at least the full length of the chamfer through the nut.
• A standard practice is to show 1 to 3 threads above the nut.
• Flat end bolts, studs, or screws need to extend at least 1/32 inch through the nut.
• Bolts with a 5/16-inch or greater diameter with cotter pin holes can be used for self-locking nuts but only if free from any burrs around the holes.
• Bolts with damaged threads and rough ends are not acceptable for use.

Aircraft Nuts - Nylon Locking Insert

• Do not tap the nylon locking insert.
• The self-locking action of an elastic stop nut involves the bolt threads impressing themselves into the untapped nylon.
• Avoid installing elastic stop nuts in locations where temperatures exceed 250°F (e.g., exhaust components). Temperatures exceeding this point reduce self-locking effectiveness.
• Self-locking nuts are acceptable for use on aircraft engines and accessories when the engine manufacturer specifies their use.

Aircraft Nuts - Self-Locking Nut Bases

• Self-locking nut bases use a variety of forms and materials for riveting and welding to aircraft structures or parts (Refer to figure 7-29).
• Certain applications require installing self-locking nuts in channels. This arrangement allows for attaching multiple nuts with minimal rivets.

Aircraft Nuts - Identification and Coding

• Part numbers identify the specific types of nuts (Refer to figure 7-25).
• Plain, AN315 (Fine) and AN335 (Coarse); Castle AN310; Plain check, AN316; Light hex, AN340 (Fine) and AN345 (Coarse); Castellated shear, AN320.
• Patented self-locking nuts use part numbers from MS20363 through MS20367. This group includes Boots, Flexloc, fiber locknut, elastic stop nut, and self-locking nut.
• Wing nuts use part number AN350.
• Additional code number information for various nuts is available in other figures.
• Letters and numbers that follow a part number signify material, size, thread type (e.g., right-hand or left-hand), and threads per inch.
• B signifies Brass; D signifies 2017-T aluminum alloy; DD signifies 2024-T aluminum alloy; C signifies stainless steel; and a dash (--) signifies a cadmium-plated carbon steel.

Aircraft Washers

• Aircraft washers used in the airframe, are available as plain, lock, or special-type washers.

• Plain washers AN960 and AN970 are used beneath hex nuts (Refer to Figure 7-30). These washers create a smooth bearing surface for the rotation of a bolt and are used as shims for adjusting bolt and nut assembly grip length..

• Plain washers are used to adjust the position of castellated nuts relative to drilled holes for cotter pins.

• Plain washers are used under lock washers to protect the surface material from damage.

• If used with aluminum or aluminum-alloy components and magnesium/aluminum alloy structures, utilize aluminum alloy washers to mitigate corrosion. However, cadmium plated steel washers are often more effective in preventing damage of nuts when bearing directly on a structure.

• Steel washers AN970 have a greater surface area. These are used on wooden structures

• Letter "L" in an AN960-416L designation indicates a thin or light-weight washer.

• Lockwashers, AN935 and AN936, are used with machine screws and bolts (Refer to figure 7-30). These washers support self-locking or castellated-type nuts where needed and provide enough friction to prevent loosening due to vibration.

• Lockwashers are not used with fasteners in primary or secondary structures, fasteners on any part where failure could harm personnel, joining a structure open to the airflow, or bolts subject to frequent removal.

• Avoid lockwashers if they are exposed to corrosion or beneath soft materials since they can damage surfaces.

• Shakeproof lockwashers, round in shape with tabs or lips bent upwards on the sides are used to secure hex nuts and bolts (Refer to figure 7-30). There are several ways to secure the lockwasher, and the washers can endure higher temperatures and high vibration. The tabs should only be bent once as subsequent bending can break them.

• Ball socket and seat washers AC950 and AC955 used for bolts installed at angles to a surface to provide precise alignment.

• NAS143 and MS20002 washers are used for internal wrenching bolts from the NAS144 through NAS158 series, and can be plain or countersunk (depressed).

• The countersunk washers are designated as NAS143C and MS20002C; designed to allow for clearance for the radius of the bolt head; plain washers are used beneath the nut.

• The NAS143C and MS20002C washers are used for internal wrenching bolts part 7-37

General Purpose Bolts

• The hex-head aircraft bolt (AN-3 through AN-20) is suitable for different applications with tension or shear. A 0.006-inch clearance is common for a 5/8 inch hole, with other sizes proportionately adjusted.
• Alloy steel bolts under AN No. 10-32 and aluminum alloy bolts under 1/4 inch in diameter should not be used on primary structures.
• Aluminium alloy nuts may be used with cadmium plated steel bolts. These components can be used for aircraft that aren't seaplanes to prevent corrosion.
• AN-73 drilled head bolts are comparable to standard hex bolts but have deeper drilled heads for safety-wire attachments.. AN-3 bolts are interchangeable with AN73 bolts
• Close tolerance bolts are machined precisely. They come with hex heads (AN-173 to AN-186) or countersunk heads (NAS-80 to NAS-86). These are used for applications requiring a precise fit, achieved using a 12- to 14-ounce hammer.

Internal Wrenching Bolts

• Internal wrenching bolts (MS-20004 to MS-20024 or NAS-495) are fabricated from high-strength steel and are suitable for both tension and shear applications.
• For steel components, countersink the bolt hole to accommodate the bolt's head's large corner radius. This feature contributes to the bolt's strength.
• In Dural (a high-strength aluminum alloy) materials, a specialized heat-treated washer ensures adequate bearing surface for the bolt head.
• The internal wrenching bolt's head is recessed for easy internal wrench insertion and removal.
• Bolts are used with special high-strength nuts.
• Replace internal wrenching bolts with others of the same type.
• Standard AN hex head bolts and washers are not suitable replacements due to their weaker strength.

Installation of Nuts, Washers, and Bolts

• Use slight clearances for bolt holes whenever bolts are subject to tension and do not involve reversal of load. This is common in pulley brackets, conduit boxes, lining trims, and miscellaneous brackets.

• Make bolt holes perpendicular to the surface to maximize bearing surface for the bolt's head and nut; avoid oversized or elongated holes.

• Bolts will transfer shear load only after the surrounding components have yielded to create bearing surface contact. Unlike rivets, bolts don't fill the hole by being swaged (permanently deformed).

• For overly sized or elongated holes on critical components, consult the aircraft/engine manufacturer for appropriate bolt selection.

• Factors such as edge distance, clearance, and load factor should be considered when selecting and installing bolts.

• Oversized or elongated holes in non-critical members can often be reamed to accommodate the next larger bolt size.

• The use of slightly larger sized drilled holes with a letter designation is acceptable, except in cases using AN hexagon bolts or NAS style close tolerance bolts.

• Light drive fits, requiring a light blow of a 12- to 14-ounce hammer to create movement, are acceptable. Heavy blows to move a bolt suggest too tight a fit.

• The term "light drive fit" defines movement with body weight pressure when holding the hammer-handle against the head of the bolt.

• Examine bolt markings to ensure proper material.

• Reuse similar type bolts in replacement.

• Follow Maintenance Instructions Manual and Illustrated Parts Breakdown.

• Install washers under both the bolt head and nut unless the instructions specify their omission.

• Use washers to prevent mechanical damage to the component and to help prevent corrosion.

• Aluminum alloy washers are typically used beneath steel bolts when attaching aluminum and magnesium members because corrosion tends to preferentially attack the washer rather than the members.

• Steel washers are used when connecting steel members with steel bolts.

• Position the bolt head up or forward whenever possible to prevent slipping in case the nut is unintentionally lost or detached.

• Ensure the bolt's grip length matches the material thickness being fastened.

• Slightly longer grip length bolts may be used by adding washers under the nut or bolt head..

• Use shims under plate nuts to ensure proper mounting.

• Safety bolts and nuts require safety wire after installation to prevent loosening from flight vibrations.

• All bolts and nuts (excluding self-locking types) should be safety-wired.

Test your knowledge on the specifications and characteristics of aircraft nuts, including self-locking types and maintenance practices. This quiz covers essential details such as proper use, identification codes, and performance factors to ensure safe and effective fastening in aviation.