032 Performance topic guide
V1 and the Balanced Field
V1 is the decision speed for a multi-engine take-off: the last speed at which a critical failure still leaves the crew able to stop within the accelerate-stop distance available, and at the same time the earliest speed from which a rejected take-off is no longer the guaranteed safe choice. It always sits inside a bounded range rather than being picked freely: V1 can never be lower than VMCG, the minimum control speed on the ground, and it can never be higher than VR, the rotation speed.
Balanced field length is the specific case where the choice of V1 makes accelerate-stop distance and accelerate-go distance equal, so one runway length limits both possible outcomes of a failure by the same amount. Change the mass, the runway surface, or the clearway and stopway on offer, and the balanced V1 moves to keep that equality, which is why the same aeroplane can be dispatched with a different V1 from the same runway on different days.
Why V1 is bounded, not chosen freely
The lower bound exists because directional control on the ground, with an engine failed and asymmetric thrust from the live engine, depends on rudder authority that only builds with speed. Below VMCG a crew cannot reliably keep the aeroplane on the runway centreline if an engine fails, so continuing is not an option regardless of how much runway remains, and V1 can never be set below it.
The upper bound exists because VR is where rotation begins. A decision speed set above rotation would ask the crew to decide whether to reject after they have already started to fly, which defeats the purpose of having a decision speed at all. Every V1 quoted in a question should be checked against this band before anything else is calculated.
The accelerate-stop and accelerate-go trade
Choosing V1 anywhere inside that band is a trade between two distances. Accelerate-stop distance required is the distance to reach V1, recognise the failure, and brake to a stop; it grows the later V1 is set, because the aeroplane carries more kinetic energy when the reject begins. Accelerate-go distance required is the distance to reach V1, continue on the remaining engine, and climb to the screen height; it shrinks the later V1 is set, because less further acceleration on one engine is needed afterwards.
Balanced field length is the point where these two opposing trends are made equal by the choice of V1, so a single published runway length limits both the reject case and the continue case by exactly the same margin.
What moves V1 on the day
Nothing about V1 is fixed for the aeroplane type; it is recalculated for every dispatch from the conditions on the day, and two of the changes below move V1 by extending a declared distance rather than by changing the aeroplane's own performance.
- Heavier mass: both distances grow, but VR and V2 climb too, since they are built from stall speed, and the balanced V1 usually rises with them.
- Wet or contaminated runway: braking is less effective, so the accelerate-stop credit shrinks and the balanced V1 tends to fall.
- Clearway added: extends the accelerate-go side, so a higher V1 can still leave enough distance to reach the screen height.
- Stopway added: extends the accelerate-stop side, so a higher V1 can still stop within the runway plus stopway.
Worked example
Worked example: adding a stopway
A twin-engined aeroplane is dispatched from a runway to which a stopway has been added beyond the existing runway end, with the physical runway length, aircraft mass, and all other conditions unchanged. Compared with a dispatch calculation using the runway length alone, what happens to the balanced take-off V1?
- AV1 decreases, because the stopway shortens the distance available for acceleration
- BV1 increases, because the extra distance now available for stopping lets the balanced point move to a higher speed
- CV1 stays the same, because a stopway only changes the landing case, not the take-off case
- DV1 decreases, because a higher V1 would need the stopway to be usable for acceleration
Show the answer and walkthrough
Correct answer: B
- A. This treats the stopway as if it reduces a distance. A stopway is added distance, for stopping only, and it never subtracts from anything.
- B. Correct. Stopway extends ASDA, the accelerate-stop distance available, so the accelerate-stop distance required can grow before it exceeds what is available, and that requirement grows with V1.
- C. Stopway is a take-off concept: it extends ASDA for a rejected take-off. It has no role in landing distance at all.
- D. It is correct that stopway is never used for acceleration, but that is precisely why it only ever expands the stopping side of the calculation, and an expanded stopping side allows V1 to rise, not fall.
Step by step
- Recall what each declared distance feeds: clearway extends TODA, the accelerate-go side, and stopway extends ASDA, the accelerate-stop side only.
- With more ASDA available, the accelerate-stop distance required can grow before it would exceed what is available, and that requirement grows the later V1 is set.
- The accelerate-go side is unaffected by the stopway, so the balancing point, where the two required distances are made equal by the choice of V1, moves to a higher V1.
- That higher V1 is still bounded above by VR, so the increase can never push V1 past rotation speed.
Common mistakes
Treating V1 as a fixed number for the aeroplane type
V1 is recalculated for every dispatch from mass, runway, wind, slope, and surface condition, so carrying over a remembered number from a different question is a reliable way to lose the mark.
Assuming V1 must equal VR or must equal VMCG
V1 sits inside the band, at whichever point balances the two distances or satisfies a stated limiting case. A stem that quotes both VMCG and VR is testing whether you can place V1 between them, not at either end.
Attributing the wrong effect to clearway or to stopway
Clearway raises the achievable V1 by extending the continue side, and stopway raises it by extending the stop side. Swapping which extension does which is a very common one-mark loss.
Related topic guides
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Last reviewed July 2026