What additional requirement exists for aircraft flying under IFR besides VFR and night equipment?

The requirement for at least a slip-skid indicator is grounded in the need for pilots operating under Instrument Flight Rules (IFR) to maintain precise control over their aircraft, especially in conditions where visibility may be severely limited. A slip-skid indicator provides crucial information about the aircraft's lateral balance and helps prevent dangerous situations such as uncoordinated flight or a stall.

While flying under IFR, pilots are often managing complex flight conditions and must be aware of their aircraft's performance. The slip-skid indicator aids in making necessary adjustments to ensure that the aircraft is not skidding or slipping during turns, which is vital for maintaining safe flight operations in low visibility situations. Enhanced situational awareness promotes better control over the aircraft, which is of utmost importance under IFR conditions.

The other options—distance measuring equipment, electronic flight instrument systems, and emergency locator transmitters—are important for various operational aspects and safety protocols, but they do not specifically address the requirement concerning the essential flight indicators needed for maintaining control during IFR operations.