Airframe structures

This course covers the Airframe rating: the structure, its repair, and the systems that keep the aircraft flying, stopping and comfortable. The General course comes first; this lesson recaps the words the rest uses.

Five stresses

• Tension: pulling apart (a control cable).
• Compression: squeezing together (a landing gear strut on touchdown).
• Torsion: twisting (a wing in a roll, a shaft).
• Shear: layers sliding past each other (a rivet holding two sheets).
• Bending: a mix of tension on one side and compression on the other (a wing spar in flight).

How fuselages are built

Older and light aircraft often use a truss: welded steel tubes (4130) forming triangles, covered with fabric. In a monocoque ("single shell") the skin carries all the load, like an egg; it's strong but any dent weakens it. Most aircraft are semimonocoque: the skin is backed by formers and bulkheads (rings that give the shape) and stringers and longerons (lengthwise members), so the load is shared and the structure can take damage.

A wing's main members are the spars, which run from root to tip and carry the bending load. Ribs run front to back and give the airfoil its shape, and stringers stiffen the skin between them. Primary structure carries flight loads and its failure could cause loss of the aircraft; secondary structure doesn't.

Designs plan for failure in different ways. Safe-life parts are retired after a set number of hours or cycles. Fail-safe structure has more than one load path, so if one part cracks the others carry the load until an inspection finds it. Damage tolerance designs combine this with inspections timed to find cracks before they grow too far.