Materials and Hardware
What you must understand
Know aircraft structural materials: aluminium alloys (with heat treatment designations), titanium, steel, magnesium and composite materials including fibre-reinforced plastics and honeycomb. Understand ferrous and non-ferrous metal properties, corrosion types (galvanic, stress, crevice, exfoliation) and the reason for surface protection such as anodising, plating and paints. Study fasteners: solid rivets and their types, blind rivets, bolts (AN, MS, NAS identification), nuts, washers, locking devices, screws and inserts. Understand thread terminology, grip length, torque and safetying methods (lockwire, cotter pins, self-locking nuts). Know bearings, seals, springs, pipes, unions and control cables, and the meaning of standard parts identification.
Traps in questions
- Dissimilar metal contact: galvanic corrosion needs an electrolyte; the more noble metal survives, the less noble corrodes.
- Rivet identification: head marking codes indicate alloy and temper; do not guess from appearance alone.
- Grip length: bolt grip must match the material thickness; too short or too long is a defect.
- Lockwire direction: must oppose the loosening direction of the fastener.
- Composite versus metal: composites do not yield like metals and can hide damage such as delamination; inspection methods differ.
Worked example
A joint uses two dissimilar metals, aluminium and stainless steel, in a damp environment. Aluminium is less noble, so it becomes the anode and corrodes. Mitigation: isolate with sealant or a barrier, or select compatible materials.
Rule of thumb
For any fastener question, check four things: correct type, correct grip, correct torque and correct locking.