When must the Terrain Awareness alerting be inhibited?

The correct answer is that terrain awareness alerting must be inhibited when the Flight Management System (FMS) is operating in Database Reference (DR) mode. This is crucial because, in DR mode, the FMS is unable to access terrain data that is essential for the proper functioning of the terrain awareness system. The system relies on accurate terrain databases to provide timely warnings of potential ground proximity hazards. When the FMS is in DR mode, it does not have the necessary terrain information, which could result in inaccurate or misleading alerting.

Inhibiting the terrain alerting in this case helps to prevent unnecessary and potentially confusing alerts that could distract the crew and lead to poor decision-making during critical phases of flight. Understanding the operational modes of the FMS and their implications for systems like terrain awareness is key for safe aircraft operation.