Behind the speeds stands CS-23 itself, whose performance sections require the manufacturer to demonstrate the stall speeds (a single-engine aeroplane cannot be certified with a VS0 above 61 kt), the take-off and landing distances to and from 50 ft, and the climb performance, all of which become the flight-manual data the operational rules then factor.
The light twin's failed engine
For a multi-engine propeller aeroplane the critical engine is the one whose failure hurts most, and propellers decide which that is. With both propellers turning clockwise seen from behind, the descending blade, which bites hardest, is on the right-hand side of each disc, so each engine's thrust line sits slightly right of its nacelle. The right engine's thrust line is therefore the further from the centreline: losing the left engine leaves that long-armed right engine pushing, the biggest yawing moment, and makes the left engine the critical one. Fail either engine and the story of module 3 replays harder: the power available halves while the power required rises (windmilling drag, and the rudder and bank needed to fly straight), so the climb collapses by far more than half.
The failure also has a control speed, VMC, the minimum at which the rudder can hold the live engine's yaw, and the syllabus asks how conditions move it. Thrust is the villain, so anything that weakens the live engine weakens the yaw: at higher pressure altitude or temperature the VMC decreases. That sounds like good news until you notice what does not move: the stall IAS. Hot and high, the two speeds close on each other, and the stall can arrive before the control problem does, which is its own kind of trap.
The distances defined
Take-off distance — from brake release to the point 50 ft above the surface; the ground roll is the part on the runway.
Landing distance — from 50 ft above the threshold to a full stop, its own airborne segment plus ground roll.
Maximum allowed take-off and landing masses — the lowest mass any applicable limit produces: field length, climb, obstacles or structure. The most restrictive limit is the one that counts.
What stretches a distance
Flap setting — take-off flap shortens the ground roll and take-off distance up to an optimum setting, at the price of climb; landing flap shortens the landing distance. More flap, shorter ground distances, poorer climb.
Runway slope — uphill lengthens the take-off and shortens the stop; downhill mirrors it.
Surface and elevation — wet, contaminated or grass surfaces cut acceleration and braking; higher elevation means thinner air and longer everything.
Pressure altitude and temperature — hot and high lengthen the take-off distance and landing distance and flatten the take-off and approach climbs.
The propeller — a fixed-pitch (cruise) propeller is inefficient at low speed and its thrust falls away along the run; a constant-speed propeller governs to fine pitch, holds thrust through the roll, and shortens it.
Brake technique — the certified distances assume take-off power set against the brakes before release; rolling the power on after brake release lengthens both the take-off distance and the accelerate-stop distance.
Wind: components, limits and accountability
Wind rarely blows down the centreline, so it is resolved into a component along the runway (head or tail) and one across it. The arithmetic is a right triangle: with θ the angle between wind and runway, the along-runway component is W·cos θ and the crosswind component W·sin θ. The clock code approximates sin θ when no graph is to hand: 30° off gives half the wind as crosswind, 45° gives 0.7, 60° gives 0.9, and at 90° it is all crosswind. Aeroplanes carry maximum crosswind limits because both the aerodynamic controls (rudder and aileron authority at low speed) and the undercarriage (tyre and gear side-loads) have finite strength.