Airframe Mechanic Certification Aircraft Structures 5 — Questions and Answers
Question 1: What is the term for the ratio of the ultimate load that causes structural failure to the design limit load an aircraft is expected to experience in service?
- Load factor
- Safety factor (Correct answer)
- Reserve factor
- Margin of safety
Correct answer: Safety factor
The safety factor (typically 1.5 for FAR Part 25 transport aircraft) is the ratio of ultimate load to limit load, providing a margin for uncertainties.
Question 2: Which inspection method is most appropriate for detecting subsurface fatigue cracks in aluminum wing skins without removing the skin?
- Dye penetrant inspection
- Eddy current inspection (Correct answer)
- Magnetic particle inspection
- Visual inspection with a borescope
Correct answer: Eddy current inspection
Eddy current inspection can detect subsurface cracks in non-ferrous metals like aluminum without requiring surface preparation or skin removal.
Question 3: A doubter (doubler) plate installed over a cracked fuselage skin repair serves primarily to:
- Restore the original aerodynamic contour
- Restore original strength and stop-drill the crack (Correct answer)
- Seal the pressurized fuselage against air leaks
- Provide a base for repainting the affected area
Correct answer: Restore original strength and stop-drill the crack
A doubler plate bridges the cracked area, restores load-carrying capacity, and distributes stress away from the crack tip after stop-drilling.
Question 4: What structural phenomenon occurs when thin aircraft skin panels buckle under shear loading but still carry additional load through diagonal tension?
- Column Euler buckling
- Wagner field or diagonal tension (Correct answer)
- Crippling failure
- Inter-rivet buckling
Correct answer: Wagner field or diagonal tension
Wagner field (diagonal tension) describes post-buckling behavior where shear-buckled skin carries additional load through diagonal tensile stress fields.
Question 5: When determining the minimum edge distance for a rivet installed in an aircraft skin repair, what is the generally accepted minimum distance from rivet centerline to the sheet edge?
- 1.5 times the rivet diameter
- 2 times the rivet diameter (Correct answer)
- 3 times the rivet diameter
- 0.5 inch regardless of rivet size
Correct answer: 2 times the rivet diameter
FAA AC 43.13-1B specifies a minimum edge distance of 2 times the rivet diameter to prevent shear-out failure of the sheet material.
Question 6: In a damage-tolerant aircraft structure, what is the purpose of fracture mechanics-based inspection intervals?
- To eliminate cracks before they can form
- To ensure cracks are detected and repaired before reaching critical size (Correct answer)
- To identify structures that need immediate replacement
- To certify that no fatigue damage exists in the fleet
Correct answer: To ensure cracks are detected and repaired before reaching critical size
Fracture mechanics sets inspection intervals so that any crack growing from the assumed initial flaw size will be found and repaired before it reaches the critical size that causes failure.
Question 7: Which condition must be verified before performing a flush rivet repair on a pressurized fuselage skin to ensure it meets original design intent?
- The repair uses only solid shank rivets, never blind rivets
- The repair restores original skin thickness, strength, and pressure-tightness (Correct answer)
- The doubler is installed on the exterior surface only
- The repair is painted before pressure testing
Correct answer: The repair restores original skin thickness, strength, and pressure-tightness
A pressurized fuselage repair must restore original material thickness and strength while also maintaining airtight integrity against the pressure differential in flight.
