📖 The lesson
Locking devices
Vibration and load reversals can loosen threaded fasteners. Every nut, bolt or screw in an aircraft is either self-locking or secured by a positive locking device.
Washers and locking plates
- Plain washers (AN960) spread load, protect surfaces and adjust grip length; they are not locking devices.
- Lock washers — split spring (AN935) and shakeproof (internal/external tooth, AN936) — rely on spring tension or bite. They are not used on primary or secondary structure, where they could cause corrosion or failure, or where the part is often removed.
- Tab washers have a tab bent into a hole or over an edge, and another tab bent up against a flat of the nut or bolt head. Reuse is normally not allowed.
- Locking plates fit over a nut and are screwed or riveted to the structure.
Pins, pal-nuts, circlips and keys
- Cotter (split) pins (AN380 cadmium steel, AN381 corrosion-resistant) pass through a castellated nut and a drilled bolt. The preferred method bends the upper prong over the bolt end and the lower prong down against a flat of the nut. Prongs must not be overlong, and a pin is never reused.
- Pal-nuts are thin pressed-steel lock nuts tightened against a plain nut.
- Circlips (internal/external retaining rings) locate parts in bores or on shafts; use the correct pliers and do not overstretch them. Wire circlips are similar.
- Keys (Woodruff, feather) lock gears or pulleys to shafts. Lock (locking) pins and taper pins lock collars and fittings.
Safety wiring
- Lockwire is corrosion-resistant steel, Monel, Inconel or (for electrical uses) copper. Copper is the breakable shear wire used on emergency handles and fire-bottle switches.
- Double-twist is the common method. Single-wire is used for small screws in a closely spaced pattern, for electrical systems and for emergency devices.
- The wire must be under slight tension, with no kinks or nicks, routed so the pull tightens each fastener, and finished with a pigtail of about ½ in bent back. Usually no more than three fasteners per wire run. Lockwire is never reused.
Quick-release fasteners
- Dzus (stud, grommet and spring), Camloc (stud, grommet and receptacle) and Airloc (stud and cross-pin) fasteners lock with a quarter turn; they secure cowlings and access panels.
- Safety clips (e.g. turnbuckle locking clips) and quick-release pins (ball-lock pins) secure parts that are often removed.
Aircraft rivets
Solid rivets
- Head types: universal (MS20470 — replaces AN430 round, AN442 flat, AN455/456 brazier and AN470), countersunk 100° (MS20426, flush for airflow), plus round and flat heads in older standards.
- Part number: MS20470AD4-6 = universal head, 2117 alloy, diameter 4/32 in (1/8), length 6/16 in.
- Icebox rivets (2017-D, 2024-DD) are too hard to drive in the T4 state. They are solution heat-treated and then kept refrigerated (about −18 °C or below) to delay age hardening. Once out of the cold they must be driven quickly: 2024 in about 10–20 minutes. They can be re-heat-treated.
- Rivet material should match the structure: 2117-AD for aluminium, 5056-B for magnesium, Monel for stainless steel and nickel alloys.
- Selection: rivet diameter is roughly three times the thickness of the thicker sheet. Rivet length = grip + 1.5 D.
- Layout: edge distance at least 2 D (2.5 D for countersunk rivets). Pitch (between rivets in a row) 3 to 4 D, never less than 3 D. Transverse pitch about 75 % of the rivet pitch.
- Driving: drill, deburr and cleco the sheets; drive with a rivet gun and bucking bar or squeeze. Countersink (thick sheet) or dimple (thin sheet) for flush rivets.
- Inspection: the shop head should be about 1.5 D wide and 0.5 D high, central and uncracked. The manufactured head must be tight and free from smiley marks. Sheets must be closed up without gaps.
- Removal: drill through the manufactured head with a drill one size smaller than the shank, to the depth of the head; pry the head off; then punch out the shank while supporting the sheet.
Blind rivets and special fasteners
- Used where only one side is accessible. Types: pull-through (hollow, non-structural); self-plugging friction-lock (stem held by friction — non-structural replacement only); mechanical-lock (CherryMAX, Olympic-Lok, Huck-Loc — a locking collar retains the stem, for structural use); and the Cherry rivnut (threaded blind insert).
- Blind rivets are installed with a hand or power puller. The stem breaks flush (or is trimmed and filed), and grip range must suit the material thickness.
- Hi-Lok, Hi-Tigue and lockbolts (pin + swaged or threaded collar) are high-strength alternatives in structural joints.