Definition
A bearing line from a non-directional beacon (NDB) that intersects an aircraft's course or flight path, used to identify a specific point along that course when the primary fix-identifying equipment is inoperative or unavailable. The pilot determines position by noting when the relative bearing to the NDB indicates the aircraft is crossing the published bearing line.
Plain English
An imaginary line running outward from an NDB ground station. When the aircraft flies across that line, the ADF needle shows it, and the pilot knows they are at a known point along their route.
Context Anchor
Seen in instrument procedures and equipment-substitution discussions, especially when a pilot is using an NDB bearing to identify a fix or point along an approach or route.
Derivation
Crossing' means passing through or over. 'NDB' is a non-directional beacon — a ground radio station that transmits in all directions. 'Bearing' comes from the old sense of 'carrying' or 'direction held,' and in navigation means a direction line from one point to another. Together: the direction line from the NDB that the aircraft passes through.
Why Pilots Care
It provides a reliable way to identify a fix or step-down point on an approach without relying on the primary navigation aid.
Analogy
It is like drawing one line on a map for your road and another line from a landmark. Where the two lines cross is the point you are trying to identify.
Intuition Check
Do not read “crossing” as meaning you fly over the NDB itself. Here, the NDB bearing line crosses another line, such as your course. Do not read “bearing” as your heading. A bearing is a direction line to or from the beacon, not necessarily the direction the airplane is pointed.
Example Sentence 1
With the DME inoperative, the pilot used a crossing NDB bearing to identify the final approach fix.
Example Sentence 2
ATC cleared the aircraft to begin the approach at the crossing NDB bearing of 270 degrees.