Definition
A flight condition in a multiengine airplane with one engine inoperative in which the airplane is flown with a small bank toward the operating engine and a small amount of rudder, such that the relative wind strikes the fuselage straight on with no lateral airflow across it. This configuration produces the lowest total drag and the best single-engine climb performance, and is achieved by combining wings-level rudder input to control yaw with approximately 2 degrees of bank toward the live engine.
Plain English
When one engine quits on a twin, the airplane wants to swing and slip sideways through the air. Zero sideslip is the precise combination of rudder pressure and a slight bank toward the working engine that stops that sideways slipping, so the airplane is flying straight through the air with the least possible drag.
Context Anchor
Seen in multiengine training when learning how to control the airplane after one engine fails.
Derivation
Sideslip' describes air flowing across the side of the fuselage rather than straight along it. 'Zero sideslip' simply means that sideways airflow has been reduced to none. Naming it this way makes the goal obvious: eliminate the slip.
Why Pilots Care
Zero sideslip reduces drag, improves climb rate, and helps the airplane fly as efficiently as possible on the remaining engine.
Grounding Statement
With zero sideslip, the airplane is not being flown slightly sideways through the air.
Intuition Check
Zero sideslip does not always mean the cockpit ball is perfectly centered. In engine-out multiengine flight, the best zero-sideslip condition may show the ball slightly off center while the airplane flies cleanly through the air.
Example Sentence 1
After the right engine failed, the pilot established zero sideslip by applying left rudder and banking about 2 degrees into the left engine to maximize climb performance.
Example Sentence 2
The instructor checked that the inclinometer ball was centered to confirm the airplane was in zero sideslip.