What additional equipment is required for aircraft operating under IFR?

The requirement for a slip-skid indicator in aircraft operating under IFR (Instrument Flight Rules) relates to the need for pilots to maintain control of the aircraft during flight, particularly in situations where flight conditions could lead to a yawing motion. This additional equipment helps the pilot understand the aircraft's rate of turn and balance, providing critical information for maintaining proper flight attitudes. In IFR conditions, where visibility may be compromised, such instruments are essential for ensuring safe operations, especially in turns, to prevent stalling and to maintain the correct flight path.

While other options, such as dual VOR receivers, GPS systems, and weather radar, may enhance navigational capabilities or safety, they are not strictly required for IFR operations as a slip-skid indicator is. Regulations focus on ensuring that pilots have the tools necessary to manage the aircraft effectively under challenging conditions, with the slip-skid indicator playing a vital role in maintaining flight control and achieving safe operation during turns.