Aircraft Materials, Hardware and Maintenance Practices
This domain tests aircraft alloys and their heat treatment, corrosion and non-destructive testing, standard hardware and locking, and the maintenance controls that keep an aircraft airworthy. Expect questions on identifying a material, selecting a fastener and understanding mandatory versus advisory documents.
Alloys and heat treatment
2024 is an aluminium-copper alloy with good fatigue strength but lower corrosion resistance; it is often clad with pure aluminium as Alclad. 7075 is an aluminium-zinc alloy with high strength and lower corrosion resistance. 6061 is a general-purpose aluminium-magnesium-silicon alloy with good corrosion resistance and weldability. The temper designation matters: T3 means solution heat treated, cold worked and naturally aged; T6 means solution heat treated and artificially aged.
Corrosion and NDT
Corrosion types include surface, galvanic, pitting, intergranular and stress corrosion. Galvanic corrosion needs dissimilar metals and an electrolyte; isolation and protective coatings reduce it. Dye penetrant finds surface-breaking defects. Magnetic particle inspection applies only to ferromagnetic materials. Eddy current detects surface and near-surface defects in conductive materials. Ultrasonic and radiographic methods detect internal defects.
Hardware and maintenance
AN bolts are identified by head markings and grip length; grip must cover the material stack without bottoming the threads in the hole. Rivet edge distance is commonly at least two diameters. Lockwire must be installed so that it tends to tighten the fastener. Airworthiness Directives are mandatory; Service Bulletins are normally advisory unless made mandatory by an AD or the operator’s approved maintenance programme.
Worked example
Two sheets are 1.2 mm and 1.5 mm thick, giving a stack of 2.7 mm. For a 4 mm rivet, minimum edge distance is about 2D = 8 mm and pitch is about 3D to 4D = 12–16 mm. Select a rivet whose grip length covers the 2.7 mm stack; a grip shorter than the stack cannot form a proper clinch, while an excessively long grip produces a weak joint.
Traps
Do not use magnetic particle inspection on aluminium. Do not assume a higher-strength alloy has better corrosion resistance. A Service Bulletin is not automatically mandatory. Do not judge bolt length by shank length alone; grip and thread length both matter.