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 and can also be written with a hyphen: Split-S. In basic terms, the Immelman and Split S 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 reasons for starting the Split S manoeuvre from the inverted position include the fact that people tolerate acceleration ("g-force") applied from head to feet several times better than the reverse direction, as much as 9g versus 3g. Most combat aircraft frames are also designed to pull more Gs in their positive aspects (upward to the aircraft), rather than negative (downward to the aircraft). A much tighter manoeuvre is therefore possible with the half roll.
The Split S without a beginning half-roll was a standard manoeuvre 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.