Class A · the take-off speed chain
Connect the speed constraints to the decisions and physical events they protect.
Why can you not choose V1 independently?
During ground acceleration, net accelerating force is thrust minus aerodynamic drag, rolling resistance and the along-runway component of weight. Lift progressively unloads the wheels; mass both increases inertia and raises required lift-off speed. Flap can shorten the ground phase while adding airborne drag. The certification demonstration uses specified crew technique, engine-failure timing and aircraft configuration.
Follow the events in order
During ground acceleration, net accelerating force is thrust minus aerodynamic drag, rolling resistance and the along-runway component of weight. Lift progressively unloads the wheels; mass both increases inertia and raises required lift-off speed. Flap can shorten the ground phase while adding airborne drag. The certification demonstration uses specified crew technique, engine-failure timing and aircraft configuration.
VEF is the assumed critical-engine failure speed. V1 includes the recognised failure/reaction interval and defines the latest speed for the first stopping action in the demonstrated rejected take-off; it also supports continuation after the assumed failure. It is not a speed at which the crew begins an unbounded discussion. VR begins rotation; VLOF is actual lift-off; V2 is the take-off safety speed to be achieved by the specified point. VMU is the minimum unstick speed established by demonstration.
Understand the floors and ceilings
VSR is the reference stall speed; VSR1 refers to the specified configuration; VS1g describes the 1-g stall concept. VMC is minimum control speed with the critical engine inoperative; VMCG is the ground-control case and VMCA the airborne case. VMBE limits brake-energy capability, while maximum tyre speed limits wheel ground speed. These limits can bind before field length.
VR must not be below V1 and has additional CS-25 VMC/VMU and manoeuvre margins. V2MIN includes both stall and control constraints; common twin-jet CS-25 cases use at least 1.13 VSR and 1.10 VMC, with different categories using their applicable stall factor. Select the approved schedule rather than enforcing an incomplete universal ratio. High density altitude or tailwind can make a tyre-speed limit critical even at a legal indicated VLOF.
Check the pavement and start point
Alignment consumes runway before the performance reference start. Use the declared intersection and the applicable alignment deduction. ACN compares aircraft pavement loading with PCN strength under compatible pavement/subgrade assumptions; ACN above PCN needs the aerodrome’s overload policy/authorisation and can risk pavement damage. The newer ICAO ACR/PCR system uses a different technical method and its numbers must not be mixed with ACN/PCN. EASA’s ECQB 2026 update explicitly distinguishes the EU/exam pavement baseline from ICAO’s newer system.
Can you make the performance decision?
Four original questions. Work from the stated conditions and review the reasoning.