Everything the last two lessons built ends at a few hand-widths of rubber. Tyres carry the whole aircraft across every load the structures module computed, and they absorb the heat the brakes cannot keep. This short closing lesson covers how a tyre is built, the fuse hidden in the wheel, why tyre speed is limited, and what the walkaround must find before the runway does.
How a tyre is built
Three construction terms sort the types:
Tubeless — no inner tube: an airtight liner bonded inside the casing holds the pressure, with fewer parts to fail and less heat generated. The standard for modern aircraft tyres.
Diagonal (cross-ply) — the casing's fabric plies run diagonally, alternate layers crossing each other: a robust, stiff-walled construction, long the aviation standard.
Radial (circumferential bias) — the plies run across the tyre from bead to bead, with a stiff belt running around the circumference under the tread: lighter, cooler-running and longer-wearing, at a higher price.
And four vocabulary terms are defined by the syllabus:
Ply rating — an index of the tyre's strength, expressed as the number of cotton plies that would give the same strength: a 24-ply-rating tyre does not contain 24 plies, it carries the load 24 plies once did.
Tyre tread — the grooved wearing surface that meets the runway, its circumferential grooves cut to drain water from the contact patch and delay aquaplaning.
Tyre creep — the slow rotation of the tyre around its own rim, driven by the torques of braking and wheel spin-up. Painted alignment marks across tyre and rim make it visible: marks that have walked apart mean the tyre is creeping, and a tyre that creeps far enough can tear out its own inflation valve (the reason tubed tyres feared it most).
Retread (cover) — a worn tread replaced on a sound, inspected carcass. Aircraft tyres are built for this: a casing may safely be retreaded several times, and retreads are standard airline practice, not an economy compromise.