Part-66 Module 6 common mistakes
Part-66 Module 6 · Understand the traps and how to avoid them.
Classifying 304 stainless steel as magnetic and hardenable by heat treatment.
Candidates assume every steel is magnetic and can be hardened, so they transfer carbon-steel behaviour onto austenitic stainless.
What to do instead
Learn the family rule: 300-series is austenitic, non-magnetic and hardened only by work hardening; 400-series ferritic/martensitic is magnetic and martensitic grades respond to heat treatment.
Choosing 7075-T6 as the correct material for a welded aircraft structure.
A 'highest strength equals best choice' heuristic overrides knowledge of weldability limits.
What to do instead
Memorise the weldability split: 6061 is the common weldable heat-treatable alloy, while 2024 and especially 7075 are generally not fusion welded; match alloy to joining method, not just strength.
Selecting dye penetrant inspection to find delamination inside a CFRP laminate.
Candidates carry metal NDT habits across to composites without checking whether the defect reaches the surface.
What to do instead
Use methods that see internal structure: tap testing, ultrasonic inspection or radiography for delamination and disbonds; penetrant only reveals surface-breaking defects in non-porous materials.
Stating that the steel member corrodes when aluminium and steel are joined in a wet environment.
Candidates remember that aluminium corrodes easily but do not apply the galvanic series to decide which metal is anodic.
What to do instead
Apply the rule that the less noble (anodic) metal is consumed: aluminium is anodic to steel, so the aluminium side corrodes; quote the series order before choosing the victim.
Selecting a bolt whose threads fall inside the shear plane of the joint.
Candidates size the bolt by overall length instead of by grip length relative to material thickness.
What to do instead
Use grip length equal to the total thickness of the parts being joined, keep threads out of the bearing/shear plane, and allow no more than about three threads exposed beyond the nut.
Identifying 37 degrees as the automotive flare angle and 45 degrees as the aircraft angle.
The two common flare standards are memorised as a pair but swapped under exam pressure.
What to do instead
Anchor it with the fittings: AN/JIC aircraft flares are 37 degrees, SAE automotive flares are 45 degrees, and the two must never be intermixed.
Claiming a compression spring may be stretched to restore or increase its free length.
Candidates treat springs as ductile wire that can be re-formed without consequence.
What to do instead
Recognise that stretching beyond the elastic limit produces permanent set and changes rate and load; reject or replace springs that are overstressed, and check free length and rate rather than reforming them.
Choosing a cylindrical roller bearing to carry a pure axial thrust load.
The simplification that roller bearings carry heavy loads is applied without checking the direction of load.
What to do instead
Match bearing type to load direction: cylindrical rollers are radial-only, while thrust ball, tapered roller or spherical roller bearings are used where axial loads must be carried.
Approving a knot or a U-bolt clamp as a repair for a broken aircraft control cable.
Everyday hardware intuition suggests clamping or knotting is an acceptable field fix.
What to do instead
State that control cables are joined only by properly swaged terminals or splices; knots, cable clamps and soldered joints are prohibited, and inspection looks for broken wires, corrosion and correct tension.
Stating that a larger AWG number indicates a thicker electrical wire.
The gauge scale runs opposite to intuitive size numbering, so candidates invert it.
What to do instead
Remember that increasing AWG number means decreasing conductor cross-section; use the wire gauge table and current rating rather than judging size from the number alone.