Airframe Structures and Airframe Systems (Nig. CARs Part 7 / NCAA-AC-PEL 018)
What this domain tests
This domain examines the airframe both as a load-carrying structure and as a set of interacting systems. Within the Nig. CARs Part 7 airworthiness and maintenance framework and the NCAA-AC-PEL 018 AME syllabus, you must recognise primary and secondary structure, trace how loads pass from skin into stringers, frames and spars, and apply approved data when inspecting, repairing or rigging airframe systems.
Structure: load paths and repair rules
- Primary structure carries flight, pressurisation and ground loads; cracks or corrosion there are airworthiness matters, not cosmetic defects.
- Monocoque skin carries most of the load; semimonocoque construction adds stringers, frames and bulkheads to stabilise the skin and redistribute load; a truss fuselage uses diagonal members in tension and compression.
- Fail-safe design provides alternate load paths and crack stoppers; safe-life items are retired at a declared life.
- Repairs must restore load path and stiffness: approved rivet size, pitch of about 3D–4D, minimum edge distance of 2D, and correct grip length.
- Stop-drilling a crack is a controlled temporary action only where approved data permits it.
Worked example
A semimonocoque fuselage skin repair is proposed using 1/8 in (0.125 in) AD rivets. The measured edge distance from hole centre to the doubler edge is 0.22 in. Minimum edge distance = 2 × 0.125 = 0.25 in. Because 0.22 in is less than 0.25 in, the rivet row is non-conforming and may tear out under pressurisation load. Re-lay out the doubler so edge distance is at least 0.25 in, then recheck pitch and fastener count. Edge distance controls tear-out; pitch controls net-section strength and sheet buckling between rivets.
Traps that cost marks
- Treating skin as non-structural on a semimonocoque aircraft.
- Mixing fail-safe and safe-life terminology.
- Overlooking galvanic corrosion where aluminium meets carbon fibre or stainless steel.
- Confusing pressure with flow when fault-finding hydraulics.
- Rigging flight controls without checking cable tension at the stated temperature and confirming turnbuckle safety.