Landing Distance Estimate

Landing distance: d ≈ V_TD²/(2·a)

Landing Distance Required (LDR) adds the air distance from threshold to touchdown to the ground roll, with a flare margin folded in. A rough estimate runs on d = V_TD²/(2·a_dec), where V_TD is touchdown speed and a_dec is the average deceleration you get from reverse thrust, ground spoilers and wheel brakes working together. RBAC 121 caps LDR at 60% of the available runway for jets and 70% for turboprops. To anchor the numbers: a Cessna 172 needs roughly 180 m on a dry runway, while a Boeing 737-800 wants about 1500 m at a typical landing weight. A wet runway pushes LDR up by 15–30%, and a contaminated or icy one by half or more. EASA OPS handles landing performance along the same lines. Plug in V_TD = 55 m/s and a = 3 m/s² and you get d = 55²/(2·3) ≈ 504 m.

Applications

ANAC landing planning, calling a go-around when the runway condition turns bad, reading a NOTAM about a shortened or wet runway, dispatch under RBAC 121/135, bush operations into short fields, hot-and-high airfields where the braking margin thins out, and training simulators.

FAQ

Why does a wet runway need more distance? Less friction between tyre and pavement means weaker braking deceleration. Above a critical speed the wheels can hydroplane, which kills braking almost entirely until they regain contact.

Do reverse thrust and spoilers replace brakes? No, they work alongside the brakes. Spoilers dump lift onto the wheels so they grip harder, reverse thrust adds aerodynamic drag, and below roughly 80 kt the brakes still do most of the slowing.

What if LDR exceeds the available runway? The crew has to divert, hold until conditions improve, or bring the landing weight down by burning fuel or jettisoning it, so that LDR fits back inside the regulatory margin.