5.4 Anchor Nuts, Self-Tapping Screws & Dowels
Key Takeaways
- An anchor nut consists of a self-locking nut body, a lugged base plate and attaching rivets, and is available in fixed, floating, right-angle, gang-channel and blind forms.
- All-metal self-locking anchor nuts are rated to about 230 degrees Celsius while fibre or nylon-insert types are limited to about 121 degrees Celsius.
- Self-tapping screws must never replace standard screws, nuts, bolts or rivets in aircraft structure; they are for temporary sheet attachment during riveting and permanent non-structural assembly.
- Type A self-tapping screws have a sharp point and coarse thread while Type B have a blunt point and finer thread; AN504 and AN530 are the common aircraft standards.
- Dowels locate mating parts and carry shear while bolts clamp; taper pins run in matched reamed tapers, and clevis pins carry shear only and are retained by a washer and cotter pin.
Closing Out Sub-Module 6.5.2
Section 5.2 covered bolt types and their specifications, machine screws and studs. Sub-module 6.5.2 Bolts, studs, and screws (level 2 for A, B1, B3 and B2) also lists three items that behave quite differently from a bolt-and-nut joint and carry their own airworthiness rules:
- "Nuts: self-locking, anchor, standard types" — the anchor nut (plate nut) family
- "Self-tapping screws, dowels"
These are the fasteners you meet on every access panel, fairing, fillet and split casing, and they generate a disproportionate number of exam questions precisely because their limitations are strict.
Anchor Nuts (Nut Plates)
An anchor nut is a nut permanently attached to the back of a structure so that a screw can be installed and removed from one side only. Without them, every removable panel on an aircraft would need a second person holding a spanner behind an inaccessible skin.
Construction
| Element | Function |
|---|---|
| Nut body | Usually an all-metal self-locking element (an out-of-round or deformed top thread) so the screw cannot back out under vibration |
| Base plate with lugs | Carries the rivet holes that attach the nut to the structure |
| Attaching rivets | Small solid rivets — typically reduced-head or countersunk rivets so the plate sits flush and the screw head still lands on the panel |
The families you must recognise
| Type | Description | Typical use |
|---|---|---|
| Fixed, one-lug | A single lug and one attaching rivet; used where an adjacent structural member leaves room for only one rivet | Panel edges, close to a flange |
| Fixed, two-lug | Two lugs and two rivets, symmetrical — the standard general-purpose anchor nut | Access panels, fairings, cowlings |
| Floating | The nut is captive in a cage and can move a small amount laterally | Where hole-position tolerance stack-up means the screw would otherwise not line up |
| Right-angle / corner | Lugs offset to clear an adjacent radius or flange | Confined corners |
| Gang channel | A strip of floating nuts in a common channel | Long removable panels and fairing runs, where fitting individual plates would take hours |
| Self-clinching / blind ("rivnut" type) | Installed from one side into a prepared hole; no attaching rivets | Blind locations and thin sheet where riveting behind is impossible |
The common all-metal self-locking families are the MS21047 and MS21051 fixed plate nuts (one- and two-lug forms) and the MS21059 two-lug floating plate nut. Fibre- or nylon-insert anchor nuts also exist but inherit the fibre nut's temperature restriction (see below).
Installation and inspection rules
- Temperature. All-metal self-locking anchor nuts are rated to roughly 230 °C (450 °F); fibre or nylon-insert types are limited to about 121 °C (250 °F). Never fit a fibre-insert plate nut around an engine, an exhaust or a bleed-air duct.
- Locking torque. The self-locking feature is checked by the prevailing (drag) torque required to run the screw down before it seats. If the screw can be turned by hand with no drag, the locking element is worn out and the plate nut is replaced. Remember from Section 5.3 that this drag torque is added to the specified clamping torque.
- Never use a self-locking nut of any kind on a joint where the bolt or screw rotates in service — the locking element wears out in minutes.
- Attaching-rivet condition. A loose attaching rivet lets the plate nut spin, and a spinning plate nut cannot be removed without drilling. Inspect for smoking or working rivets around plate nuts on frequently opened panels.
- Anchor nuts are not a structural joint. They anchor a screw; the panel's load path is the screw and the surrounding structure, not the plate.
Self-Tapping Screws
A self-tapping screw forms or cuts its own thread as it is driven into a plain drilled hole. Two forms are used:
| Type | Point and thread | Behaviour |
|---|---|---|
| Type A | Sharp gimlet point, coarse widely-spaced thread | Thread-forming; for thin sheet and soft materials |
| Type B | Blunt point, finer and more closely spaced thread with a full unthreaded lead | Thread-forming in thicker sheet; the blunt point resists over-penetration |
The common aircraft standards are AN504 (round-head self-tapping screw) and AN530 (round-head sheet-metal screw, type B). Thread-cutting variants remove material and leave a chip, and are used in castings and plastics.
The airworthiness rule
Self-tapping screws must never be used as a replacement for standard screws, nuts, bolts or rivets in any aircraft structure.
Their only two legitimate uses are:
- Temporary attachment of sheet metal for riveting — the classic use is skin-pinning during a repair, where sheet-metal screws hold the panel in position while the rivet pattern is drilled and driven; the screws are then removed and replaced by rivets.
- Permanent assembly of non-structural items — nameplates, data placards, trim, insulation blankets, small non-load-bearing fairings.
Common failure modes are a stripped hole (the screw spins and no longer clamps — the hole must be repaired, not simply refilled with a bigger screw at the technician's discretion), corrosion in the formed thread, and over-penetration into wiring or plumbing behind a panel.
Dowels and Pins
Dowels are unthreaded cylindrical or tapered pins whose job is location and shear, not clamping. Where two machined parts must be assembled repeatedly in exactly the same relative position — the two halves of an accessory gearbox, an engine crankcase split line, a bearing housing and its cap — the bolts provide the clamping force and the dowels carry the shear and set the alignment.
| Pin type | Construction | Application |
|---|---|---|
| Solid dowel pin | Ground parallel pin, interference fit in one part, close sliding fit in the other | Gearbox and crankcase split lines, bushing anti-rotation |
| Spring (roll) pin | Rolled sleeve of spring steel that compresses when driven, gripping by its own radial spring force | Retaining levers and collars, anti-rotation of light components; tolerant of a less precise hole |
| Taper pin — plain | Standard taper, drilled at the small end and secured with safety wire | Shear joints where absence of play is essential — control system connections |
| Taper pin — threaded | Taper body with a threaded extension, used with a taper-pin washer and a shear nut secured by a cotter pin, or a self-locking nut if undrilled | Same, where a positive mechanical retention is required |
| Flathead (clevis) pin | Plain shank with a head, retained by a cotter pin and washer | Control-rod ends and turnbuckle clevises, where the load is in shear only |
| Shear pin | Deliberately weak pin sized to fail at a defined torque | Torque-limiting drives, propeller governor and pump drives |
Aircraft taper pins and their reamed holes are matched: the hole is finished with a taper reamer to the same taper as the pin, so that the pin makes full-contact bearing over its whole length. A taper pin driven into a straight hole bears on a line, not a surface, and will pound the hole oval.
Clevis pins carry shear only — they must never be used where the load has a tension component. In a clevis-pin joint the head goes uppermost wherever possible so that if the cotter pin is lost the pin cannot fall out under gravity.
Practical Scenarios
Scenario 1 — the "just use a bigger screw" trap. A technician finds a stripped self-tapping screw hole in a wing-root fairing and fits the next size up. Correct in principle for a non-structural fairing if the maintenance data allows it; wrong if the hole is in structure, and wrong if the larger screw reduces the remaining edge distance below limits. If the item is structural, the correct action is a plate nut and a machine screw, or a repair per the SRM — never a bigger self-tapper.
Scenario 2 — the spinning plate nut. An access panel screw will not come out; the plate nut is rotating. The attaching rivets have worked loose. The nut must be drilled out and a new plate nut riveted in place; simply drilling out the screw leaves an unusable hole.
Scenario 3 — the hot plate nut. A fibre-insert anchor nut is found on a cowling attachment adjacent to an exhaust stack. It is beyond its 121 °C limit; the correct replacement is an all-metal self-locking type rated to 230 °C.
Exam Traps
- Anchor nuts are not a way of turning a non-structural panel into a structural one — they only make a nut captive.
- Self-tapping screws never replace structural fasteners. Any answer that lets a self-tapper substitute for a rivet or a bolt in structure is wrong.
- Type A = sharp point, coarse thread; Type B = blunt point, finer thread. Do not invert them.
- A dowel locates and shears; a bolt clamps. A question asking what carries the shear across a gearbox split line wants the dowel, not the bolts.
- A clevis pin takes shear only and is retained by a cotter pin and washer; a taper pin removes play by matching a reamed taper.
- The 230 °C / 121 °C split for all-metal versus fibre locking elements applies to anchor nuts exactly as it does to free nuts.
What is the permitted use of a self-tapping screw such as an AN504 or AN530 on an aircraft?
An access panel screw cannot be removed because the anchor nut behind the skin is rotating. What has failed and what is the correct rectification?
Two halves of an accessory gearbox casing are assembled with through-bolts and two ground pins. What is the specific function of the pins?
Which statement about the correct fitting of taper pins and clevis pins is true?