What does the DMEH special function switch do?

The DMEH special function switch is responsible for holding the current DME NAVAID frequency. This function is crucial for pilots, as it allows them to maintain a stable navigational reference while in flight. By locking in the frequency, pilots can ensure that they are accurately receiving distance information from the DME facility without inadvertently switching to a different frequency, which could lead to navigation errors.

Maintaining the proper DME frequency is particularly important in busy airspace or during complex flight operations, where relying on precise distance measurements can significantly impact flight safety. The DME system provides essential information for determining the aircraft's distance from a DME station, aiding in navigation and approach procedures.

While the other functions mentioned play important roles in aircraft operations, they do not specifically relate to the capabilities of the DMEH special function switch. For instance, switching VHF communication modes, adjusting audio control, and controlling transponder visibility are handled through different systems or controls within the aircraft.