What does the term 'cage' refer to on the ball instrument in flight instruments?

• A. The spherical housing of the instrument.
• B. The black markings on the ball instrument indicating its neutral position.
• C. The pointer inside the instrument.
• D. The scale on the instrument glass.

Answer: (B)

In the ball-type inclinometer used for slip-skid indication, your reading comes from the ball’s position inside a curved, fixed reference. The “cage” refers to those fixed black markings that surround or accompany the ball and define the neutral position. When the airplane is in coordinated flight, the ball sits in the center of that cage. If the ball slides to one side, you have a slip or skid and you need to apply rudder or adjust inputs to bring the ball back to center. The cage isn’t the spherical housing, the pointer, or the glass scale—the cage is the set of fixed markings that show the neutral/center position.