The Split S is an air combat maneuver mostly used to disengage from combat. To execute a Split S, the pilot half-rolls his aircraft inverted and executes a descending half-loop, resulting in level flight in the exact opposite direction at a lower altitude.
The Split S is taught to be used in dogfighting when the pilot has the opportunity to withdraw from battle. Contrary to popular belief, this maneuver is almost never used to evade target-locked air-to-air missiles. However, it can be an effective tactic to prevent an enemy behind (between eight o'clock and four o'clock positions) from gaining a missile lock-on while one is disengaging from a fight.
The Split S maneuver is contrasted with the Immelmann turn, which is an ascending half-loop that finishes with a half-roll out, resulting in level flight in the exact opposite direction at a higher altitude. The Split S is also called a reversed Immelmann turn, or can be listed with a hyphen Split-S. In basic terms, they are very similar maneuvers, both accomplishing the same goal, but the Split S exchanges altitude to gain speed, while the Immelmann turn exchanges speed to gain altitude.
The Split S, being a descending maneuver, means that the pilot must always ensure that he/she is starting high enough to complete the half-loop; the exact altitude needed depends on factors like the aircraft's speed, weight and manoeuverability, likewise the terrain below the plane. Misjudgments can arise from a lack of situational awareness or from an error in reading instruments.
The reason for starting the Split S maneuver from the inverted position is "If you are flying straight and level and push the nose of the plane down, you will experience your weight lessening. The harder you push the nose down, the more "weightless" you will feel; you are experiencing negative Gs. These negative Gs result in blood rushing up into the head, which is the opposite of positive Gs where blood rushes to the lower extremities. However, while the body can stand up to 9 positive Gs without severe consequences, blood vessels in the eyes would start to rupture when already applying 2 up to 3 negative Gs. This is the redout effect. A pilot who pushes too many negative Gs will be seeing the world through bloodshot eyes."
However, the Split S without a beginning half-roll was frequently used in early WWII by German pilots seeking to evade British fighters. The Merlin engine used in British fighters was carburetted, and the float valves would malfunction under negative g-force leading to reduced power or a stalled engine (The German fighters were not subject to this problem since they used fuel injection). This could be prevented by quarter-rolling the aircraft before starting the dive, but doing so took up enough time to give the German pilots an excellent chance of escaping. The beginnings of a solution was provided by "Miss Shilling's orifice", a fuel-flow restriction device, and was finally solved by changing from the original S.U. carburetters to Bendix-Stromberg pressure carburettors, and later to S.U. injection carburettors.
See also 
Pop culture 
- In Top Gun, "Charlie" makes reference to footage of an aircraft performing a Split S.
- Numerous video games involving air combat typically provide tutorials on ACMs, including the Split S.