Airframe Mechanic Certification Aircraft Structures 4 — Questions and Answers
Question 1: What is the primary advantage of a composite sandwich structure (face sheets with honeycomb core) over solid aluminum sheet?
- Lower manufacturing cost
- High stiffness-to-weight ratio with excellent bending resistance (Correct answer)
- Greater resistance to impact damage
- Simpler repair procedures
Correct answer: High stiffness-to-weight ratio with excellent bending resistance
Sandwich construction provides exceptional bending stiffness relative to weight by placing strong face sheets far from the neutral axis, separated by a lightweight core.
Question 2: During a pressurized aircraft's service life, the fuselage skin experiences repeated pressurization cycles. What fatigue mechanism is primarily responsible for eventual skin cracking?
- Creep at elevated temperature
- High-cycle vibration fatigue
- Low-cycle fatigue from pressurization/depressurization (Correct answer)
- Stress corrosion cracking
Correct answer: Low-cycle fatigue from pressurization/depressurization
Each flight cycle applies one pressurization/depressurization event, constituting low-cycle fatigue that drives crack initiation and propagation in fuselage skin.
Question 3: A wing rib's primary structural function is to:
- Carry spanwise bending loads to the fuselage
- Maintain the airfoil shape and transfer loads to the spar (Correct answer)
- Provide torsional rigidity to resist aileron reversal
- Act as a firewall between fuel bays
Correct answer: Maintain the airfoil shape and transfer loads to the spar
Ribs define and maintain the wing's airfoil cross-section and transfer chordwise aerodynamic loads to the wing spars.
Question 4: What is the term for an approved repair that deviates from the original design but restores the structure to an equivalent level of airworthiness?
- Alteration
- Modification
- Engineering Order (EO) repair
- Alternate means of compliance (AMOC) (Correct answer)
Correct answer: Alternate means of compliance (AMOC)
An Alternate Means of Compliance (AMOC) is FAA-approved authorization to use a method, standard, or practice different from that prescribed while still meeting airworthiness requirements.
Question 5: In a multi-spar wing, what is the primary function of the front spar compared to the rear spar?
- The front spar carries torsion; the rear spar carries bending
- The front spar carries most bending loads; the rear spar supports control surfaces and fuel (Correct answer)
- The front spar is structural; the rear spar is purely aerodynamic
- Both spars carry identical loads simultaneously
Correct answer: The front spar carries most bending loads; the rear spar supports control surfaces and fuel
In most designs the front spar carries the majority of bending and shear from lift, while the rear spar provides attachment for trailing-edge control surfaces and closes the torsion box.
Question 6: When a riveted lap joint is inspected and found to have 'working' rivets (fretting debris around rivet heads), this indicates:
- Proper rivet installation with normal wear
- The rivets are loose and the joint is transferring load inadequately (Correct answer)
- The material has been correctly dimpled
- The joint has exceeded its fatigue life only in theory
Correct answer: The rivets are loose and the joint is transferring load inadequately
Fretting debris (black powder or smearing) around rivet heads is evidence of rivet movement, indicating loose rivets that are no longer properly transferring shear loads.
Question 7: What type of corrosion preferentially attacks aluminum alloy grain boundaries and causes layers of metal to separate?
- Uniform surface corrosion
- Galvanic corrosion
- Intergranular corrosion (Correct answer)
- Crevice corrosion
Correct answer: Intergranular corrosion
Intergranular corrosion attacks along grain boundaries in susceptible alloys, eventually causing exfoliation where layers of material separate and flake off.
Airframe Mechanic Certification Aircraft Structures 4
What is the primary advantage of a composite sandwich structure (face sheets with honeycomb core) over solid aluminum sheet?