ATPL Airframe & Structures Questions

Duralumin, a traditional aircraft structural alloy based on aluminium with copper as its main additional constituent, is generally:

• Hard to weld by conventional fusion welding, but with good thermal conductivity
• Easy to weld, but with poor thermal conductivity
• Hard to weld, with poor thermal conductivity
• Easy to weld, with good thermal conductivity

Explanation: Duralumin is an aluminium/copper-based alloy with good strength and reasonably good thermal conductivity, but like many aluminium alloys it is comparatively hard to weld by conventional fusion welding - one reason riveted rather than welded construction has traditionally been favoured for aluminium airframes.

Airframe & Structures - Aircraft Alloys

In a stressed-skin wing structure, bending loads are primarily carried by:

• The spars together with the skin itself
• The stringers together with the spars
• The ribs together with the stringers
• The skin alone, independent of the spars

Explanation: In a stressed-skin design, the wing skin is a fully load-bearing structural member, not just an aerodynamic covering. Bending loads are shared between the spars, which run span-wise and are the primary bending members, and the skin itself, which is stressed to carry a meaningful share of the load.

Airframe & Structures - Stressed Skin Construction

In an older, non-stressed-skin wing structure, where the skin is not a primary load-bearing member, bending loads are primarily carried by:

• The spars alone
• The ribs alone
• The skin alone
• The spars together with the skin

Explanation: In a non-stressed-skin design, the skin contributes essentially nothing to carrying bending loads. That job falls entirely to the internal spar structure, with the skin serving mainly to maintain aerodynamic shape and, together with ribs, to transmit local aerodynamic pressure loads into the structure.

Airframe & Structures - Non-Stressed Skin Construction

In fuselage construction, one function of stringers, the longitudinal stiffening members running along the fuselage, is to:

• Support the skin and help carry some of the cabin pressurization loads as tensile stress
• Provide an attachment point for cabin insulation only
• Provide an alternate load path solely in the event a frame fails
• Primarily resist shear stress rather than tension

Explanation: Stringers run lengthwise between the fuselage's circumferential frames, stiffening and supporting the skin against buckling and helping the skin share the tensile stresses generated by cabin pressurization, in addition to carrying bending loads along the fuselage length.

Airframe & Structures - Fuselage Stringers

An aircraft designed and certified primarily for long-haul operations is generally not well suited to a short-haul role, largely because:

• Its structure is given a fatigue life based on its intended usage pattern, and short-haul flying accumulates pressurization and landing cycles much faster relative to flight hours
• Checklists would take too long to complete on a short flight
• Its fuel tanks would remain empty for most of a short flight, straining the structure
• It would simply use too much fuel on short sectors, with no structural implication

Explanation: Airframe fatigue life is calculated against an assumed usage pattern - a long-haul design's fatigue life is typically expressed in flight hours, based on relatively few but long flights. Flown short-haul instead, the aircraft accumulates the cyclic loading of pressurization and landing gear cycles far more often per flight hour than intended, which can use up its structural fatigue life much faster than its hours alone would suggest.

Airframe & Structures - Structural Fatigue Life