A class A take-off is scheduled around a failure that almost never happens: CS-25 assumes the critical engine quits at the worst moment of every departure, and the whole apparatus of this lesson, fifteen named speeds and three defined distances, exists so that the aeroplane has a safe answer whichever side of the decision speed the failure falls. Learn the chain once, properly, and the next four lessons are applications.
The physics of the run
During the ground run the accelerating force is the thrust minus two bills: the aerodynamic drag, growing with speed, and the wheels' rolling friction, which acts on the weight not yet carried by lift and therefore fades as speed builds. A higher thrust-to-weight ratio accelerates the mass harder and shortens the ground roll directly; take-off flap shortens it by a different route, lowering the lift-off speed the run must reach. CS-25's General and Take-off performance sections turn this physics into obligations: the manufacturer must establish and publish the take-off speeds, distances and climb data for the whole envelope of mass, altitude, temperature, wind and runway conditions, always with the engine-failure case accounted.
One ground-side pairing rides with this LO set:
Aircraft classification number (ACN) — the load the aeroplane imposes on a pavement.
Pavement classification number (PCN) — the load the pavement can bear.
Operating where the ACN exceeds the PCN is an overload: it risks rutting or breaking the pavement and stressing the landing gear, and it is permitted only within the narrow occasional-use concessions the aerodrome authority publishes.
The speed chain
The certified speeds arrive as a family, and their definitions are pure LO material:
VSR / VSR1 — the reference stall speed, based on the 1-g stall speed VS1g (the speed at which lift can just equal weight at 1 g); VSR1 is the same reference in a specific configuration.
VMC, VMCG, VMCA — the minimum control speeds with the critical engine inoperative: generally, on or near the ground, and airborne in the take-off climb.
VEF — the speed at which the engine failure is assumed to occur for scheduling; it may not be below VMCG.
V1 — the decision speed: the failure at VEF is recognised by V1, the last speed at which the stop may be begun and the first at which the take-off can be continued.
VR — the rotation speed, at least 1.05 VMCA, chosen so V2 is reached by 35 ft.
VMU and VLOF — the minimum unstick speed the aeroplane can physically leave the ground at, and the scheduled lift-off speed kept a margin above it.
V2 and V2MIN — the take-off safety speed flown from 35 ft, no less than V2MIN, itself at least 1.13 VSR (1.08 VSR for four-engine turboprops) and 1.10 VMCA.
VMBE and V max tyre — the brake-energy ceiling that V1 may not exceed, and the tyre-speed ceiling that VLOF may not exceed.