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 manoeuvre 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 manoeuvre 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 manoeuvre, 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 manoeuvre, 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 manoeuvrability, likewise the terrain below the plane. Misjudgements can arise from a lack of situational awareness or from an error in reading instruments.
The reason for starting the Split S manoeuvre from the inverted position is that people tolerate accelaration ("gee force") applied from head to feet several times better than the reverse direction, as much as 9g versus 3g. A much tighter manouvre is therefore possible with the half roll.
The Split S without a beginning half-roll was a standard manouvre 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.
- 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.