Selecting a zero V/S while in vertical speed mode engages ____ mode.

Selecting a zero vertical speed while in vertical speed mode engages altitude hold mode. This action indicates to the aircraft's autopilot system that the current altitude should be maintained. Essentially, when the vertical speed is set to zero, the autopilot interprets this as a command to stop any climbing or descending, thereby providing a stable altitude hold.

In this scenario, the aircraft maintains its current altitude without any changes, which is crucial for operations requiring precise altitude management, such as during long flights, approaches to landing, or when needing to maintain a specific flight level in busy airspace. Understanding the functionality of vertical speed and its relation to altitude hold is essential for effective aircraft operation.