When must an air carrier airplane be equipped with DME or a suitable RNAV system?

The requirement for an air carrier airplane to be equipped with DME (Distance Measuring Equipment) or a suitable RNAV (Area Navigation) system is primarily tied to the operational need for navigation accuracy and reliability. In this context, the correct answer indicates that DME or a suitable RNAV system is necessary whenever VOR (VHF Omnidirectional Range) navigation equipment is mandated.

When VOR navigation is required, it's essential to possess the capability to measure the distance to the VOR station accurately. DME provides this functionality by enabling pilots to determine their precise distance from the VOR, thereby enhancing navigation accuracy and situational awareness, especially in complex airspace or during precision approaches. Similarly, a suitable RNAV system offers the necessary navigational precision which aligns with the requirements set forth by aviation regulations.

This requirement is particularly vital for air carrier operations given that safety and operational efficiency are paramount. Ensuring that the aircraft is equipped with DME or an RNAV system enhances the overall navigation capabilities, thereby contributing to successful flight operations.

Options that suggest other scenarios—such as operations in Class E airspace or specific altitudes—do not adhere to the broader regulatory framework which mandates DME or RNAV capabilities when VOR navigation systems are needed,