FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician is examining a wing box beam spar assembly. Which structural member is designed primarily to resist the tension and compression bending loads at the top and bottom of the spar?
- ASpar web
- BStringer
- CSpar cap (flange)
- DRib cap strip
Show answer & explanationAnswer & explanation
Correct answer: C. Spar cap (flange)
The spar cap (or flange) forms the top and bottom edges of the spar and is designed to carry the tension and compression bending loads, while the spar web (the vertical member between the caps) carries shear loads.
Why the other options are wrong
- A. The spar web resists shear loads, not the primary bending tension/compression loads.
- B. Stringers support fuselage or wing skin against buckling, not the primary bending member of a spar.
- D. Rib cap strips help maintain rib airfoil shape, not primary spar bending loads.
Spar Cap (Flange)
The spar cap is the top and bottom member of a wing spar that carries tension and compression bending loads, while the spar web between the caps carries shear loads.
- Located at top and bottom of the spar
- Carries bending (tension/compression) loads
- Spar web carries shear between the caps
- Often thicker/heavier material than the web
Memory trick: Cap on top and bottom takes the bend; web in the middle takes the shear.