How can you determine initial station passage when navigating via an NDB?

Determining initial station passage when navigating via a Non-Directional Beacon (NDB) is critical for accurate navigation and course management. The correct answer focuses on the relationship between the bearing pointer and the inbound course.

When the bearing pointer passes 90 degrees to the inbound course, it indicates that the aircraft is effectively crossing the radial of the NDB. At this point, the aircraft has passed over the station, leading to initial station passage. This is a vital moment in navigation, as it indicates that the aircraft is on the other side of the NDB, allowing the pilot to adjust their navigation plans accordingly.

Understanding this concept helps pilots recognize their position relative to the NDB and make informed decisions about any necessary course corrections or navigational tasks they need to perform as they continue their flight. The other choices provide scenarios that are not aligned with the moment of initial station passage. For instance, the "To-From" indicator change signifies a crossing of the course but doesn't specifically indicate the position relative to the NDB, making it less precise for determining initial station passage. Similarly, a range indicator reaching zero or DME indicating a steady number pertains to distance but not to the angular position crucial for confirming station passage.