Which factor increases V1 during takeoff?

During takeoff, V1 is the critical speed at which a pilot must make the decision to continue takeoff or abort in case of an emergency. A longer runway increases V1 because it provides additional distance for the aircraft to reach this critical speed safely, allowing for more takeoff options and adjustments to weight and balance. With a longer runway, there is also a greater margin for error, especially in situations where the aircraft may need more time or distance to reach the necessary speed for a safe takeoff.

The other factors mentioned influence V1 differently. Increased temperature typically reduces air density, which can lead to a higher takeoff distance and a potentially lower V1. A contaminated runway can negatively impact aircraft performance, often requiring longer distances to reach takeoff speeds, thereby generally decreasing V1. Increased headwind, while beneficial for lifting the aircraft off the ground sooner by adding extra lift at lower speeds, is more about shortening the required takeoff distance rather than increasing V1 itself. A longer runway clearly provides the best support for increasing the V1 speed, enabling safer transition through the critical takeoff phase.