turn to the north), they will be traveling toward each other along the same turn circle. This type of engagement is known as "one-circle flow". If the aircraft turn in opposite directions (e.g.: one turns north but the other turns south), they will move away from each other, flying around to engage each other on separate turn circles. This is called "two-circle flow". One-circle flow will result in another merge, unless an angular advantage can be obtained. During one-circle flow, the fighter with the smaller turn radius will have the advantage. Pilots will often pitch up out of plane while increasing thrust, to help minimize turn radius. Because it does not really matter where the two fighters meet in the circle, turn rate is of little importance during one circle flow. Therefore, it is often called a radius fight. An out-of-plane maneuver, such as a displacement roll, is a viable option for reducing turn radius. Two-circle flow will also result in another merge. In two-circle flow, turn radius is of little importance, because what matters is which fighter can get back to the merging place first. Two-circle flow is a turn rate fight, and the angular advantage usually goes to the aircraft with the higher turn rate at its corner speed. Pilots will often slice turn in order to maximize their turn rate. A third option is called vertical flow, in which one or both fighters turn toward the vertical plane. If both fighters go up or down, the fight becomes one-circle flow. If one fighter goes up or down, while the other turns horizontally, it is really a modified version of one-circle flow. However, if one fighter goes up while the other goes down, it becomes two-circle flow. In both types of flow, the closest possible merge is desirable to keep the enemy at an angular disadvantage. Although circle flow is often described using neutral merges, the concept applies anytime two aircraft maneuver in relation to each other and the horizon. For instance, the "flat scissors" is an example of one-circle flow, while the "rolling scissors" is an example of two-circle flow. Maneuvers Combat spread The combat spread is the most basic of maneuvers used prior to engagement. A pair of attacking aircraft will separate, often by a distance of one mile horizontal by 1500 feet vertical. The fighter with the lower altitude becomes the defender, while the wingman