What indicates station passage at an NDB?

When navigating with a Non-Directional Beacon (NDB), the indication of station passage is typically identified by the first swing of the aircraft's heading indicator through 90 degrees. This occurs as the aircraft passes almost directly over the NDB. The behavior of the aircraft's automatic direction finder (ADF) needle reflects changes in the bearing to the station as the aircraft approaches and subsequently moves away from it.

As the aircraft enters the area of the NDB's signal, the ADF will show a specific bearing that represents the direction of the beacon relative to the aircraft's position. When the aircraft is at the closest point of approach—essentially passing directly over the NDB—the ADF needle will dramatically change direction, resulting in a swing of approximately 90 degrees. This shift is a reliable cue to the pilot that they have successfully passed the NDB.

Understanding this concept is crucial because it helps pilots determine their exact position relative to navigational aids in the environment, facilitating better navigation and situational awareness. While the other potential indications are relevant in flight operations, such as signal strength or range finding, they do not directly correspond to the moment of station passage as clearly as the 90-degree swing does.