What type of indicator is the "Ball" in IFR required equipment?

The "Ball" in IFR (Instrument Flight Rules) refers to the slip-skid indicator, which is an important tool for pilots operating in instrument flight conditions. This indicator helps pilots maintain proper coordination and control of the aircraft during flight. Specifically, the slip-skid indicator shows whether the aircraft is correctly coordinated in turns.

When a pilot makes a turn, the aircraft tends to naturally want to either slip or skid, depending on the balance of forces acting on it. The ball within the indicator moves to one side if the aircraft is slipping and to the other side if it is skidding. A centered ball indicates that the aircraft is in a coordinated turn, which is essential for safe operations in IFR conditions, where visual references outside the aircraft might be limited.

Having a functional slip-skid indicator is critical for maintaining optimal performance and control, especially in conditions where visual cues are not available. Thus, this type of indicator is deemed required equipment for IFR operations.