Why is higher V1 preferred over lower V1 in takeoff performance calculations?

Question

I am using a takeoff performance application that seems to prioritize a higher V1 speed whenever there is extra runway available. It does this as long as the Acceleration-Stop Distance (ASD) remains within the runway limits. I understand that V1 is the critical speed below which a takeoff can be rejected and the aircraft can be safely brought to a stop within the remaining runway distance. However, I am curious to know why a higher V1 is generally considered better than a lower V1.

Intuitively, it seems like a lower V1 would allow for a shorter ASD, which would be advantageous in terms of runway usage. Is there a specific reason why higher V1 speeds are preferred? Are there any safety or performance benefits associated with higher V1 speeds?

Additionally, are there any drawbacks to using a higher V1 speed? Could it compromise safety or lead to any operational limitations?

I am trying to understand the rationale behind the computation of V1 in this context as well as the implications for takeoff safety and performance. Any insights or references to relevant guidelines/regulations would be greatly appreciated.

Answer 1

To amend John K:s answer, choice of V1 is basically matter of two things: do you want to minimize Accelerate-Stop distance or Continued Takeoff distance in case of engine failure. By choosing low V1, there will be more runway available in case of engine failure but as described by John, in case of an engine failure just above low V1 you still have to accelerate to Vr and then carry on towards V2 before runway end. Also if there are obstacles beyond end of the runway low V1 will put you closer to the obstacles as acceleration from V1 to V2 is lower with single engine.

High V1 increases the Accelerate-Stop distance, but shortens the takeoff distance in case of engine failure, leaving more runway to to rotate and leaves more room regarding the close-in-obstacles.

The only reason I would see to choose lower V1 is if runway is contaminated to the extent that you are doubtful if you would actually achieve reported braking action. Especially the very end of an icy runway tends to be more slippery than the average, reported runway condition. Then again, I wouldn't go to close to VMCG (minimum control speed on ground) either as it assumes dry and calm conditions. Should engine failure occur there, the controllability on ground might become an issue.

Answer 2

Generally you want the decision speed V₁, above which you are supposed to carry on and fly no matter what (with rare exceptions), as close as possible to rotation speed. That is, preserve the ability to remain on the ground, if something goes wrong, as late as possible without running off the end, to minimize the potential rolling acceleration phase that has to be done with a dead engine.

To visualize, take it to an extreme. V₁ is a function of accelerate/stop. Say that because the brakes on your plane were a very poor design so you needed a LOT of runway to stop on a rejected takeoff, so bad that V₁ had to be set at 100kts to be able to reject and stop in time on a given runway, with a rotation speed of 150kts.

An engine quits at 110kt, well past V₁ and because of that you have to keep going and take off because of your crappy brakes. But rotation is at 150, so you have to sit there accelerating to rotation speed for an eternity with one engine surging or vibrating or on fire. So V₁ will be set to the highest speed that allows a safe stop following a reject, to minimize the rolling acceleration phase to rotation, and you only reduce it because a short runway or degraded runway friction or some other parameter forces you to.

At whatever speed that is set for V₁, a number of allowances or fudge factors are built in, such as an assumed delay on getting on the brakes, to ensure the calculated stop distance can be achieved. A high energy reject like that is considered high risk, but still less than the risks built into a single engine takeoff, so if you can stay on the ground and stop, you stay on the ground and stop.

On the flip side of that, once past V₁, stopping on the runway is no longer a consideration, and you can actually take your time rotating. Inexperienced pilots can think they need to get in the air as fast as possible at Vᵣ with the end of the runway looming, and can tend to over-rotate by rushing the rotation.