Definition
Uncoordinated flight conditions in a turn, indicated by the inclinometer ball being off-center. A slip occurs when the aircraft's bank angle is too steep for the rate of turn, causing the ball to fall to the inside (low) side of the turn. A skid occurs when the rate of turn is too fast for the bank angle, causing the ball to slide to the outside (high) side of the turn. Both result from a mismatch between bank, rudder input, and turn rate, and both indicate that the aircraft is not flying coordinated.
Plain English
When you turn an airplane, the nose, wings, and rudder all need to work together. If they don't, the aircraft slides sideways through the air instead of carving a clean turn. A slip is sliding toward the inside of the turn; a skid is sliding toward the outside. The little ball on the instrument panel shows you which one is happening — if it's off-center, you're not flying a clean turn.
Context Anchor
Seen in instrument flying when discussing coordinated turns, the turn coordinator, and the body’s false sensations during uncoordinated flight.
Derivation
Slip comes from the Old English slipan, meaning to slide or glide smoothly. Skid comes from a Norse word meaning a piece of wood used to slide something along — later applied to anything sliding sideways without grip. Both words describe sideways motion that isn't supposed to be there, which fits exactly what the aircraft is doing.
Why Pilots Care
Following the false sensations of a skid or slip instead of the instruments can quickly produce spatial disorientation in IMC.
Grounding Statement
Picture a car taking a curve. If it leans into the turn too hard, it slides toward the inside — that's a slip. If it doesn't lean enough and the rear swings wide, it slides toward the outside — that's a skid. The aircraft does the same thing in the air, and the ball shows you which is happening.
Intuition Check
Do not think of skids and slips as runway tire problems here. In flight, they mean the airplane is moving sideways through the air because the turn is not coordinated.
Example Sentence 1
During the steep turn, the instructor pointed out that the ball was off-center and asked the student to identify whether the aircraft was in a skid or a slip.
Example Sentence 2
A slight slip kept the airplane aligned with the runway centerline in the crosswind.