Which term is described by the black markings on the ball instrument indicating its neutral position?

• A. Calibrated
• B. Cage
• C. Cabin altitude
• D. Bernoulli's Principle

Answer: (B)

The ball-in-cage slip/skid indicator shows coordination through a ball that sits inside a curved tube (the cage). The black markings on the ball line up with the center of the cage to indicate the neutral or zero-slip position. When the aircraft is not skidding or slipping, the ball is centered, which means coordinated flight. If the ball moves to one side, the aircraft is slipping or skidding and rudder input is needed to bring it back to center.

Other terms don’t describe this mechanism: calibrated relates to instrument accuracy, cabin altitude is about cabin pressure, and Bernoulli’s Principle explains lift, not the slip/skid indicator’s neutral position.