Automatic Direction Finder (ADF) — Page 99, Lesson 82

Let’s pick this up with tracking outbound from an NDB. I want you to picture the aircraft flying away from the beacon, and we’re going to use the relative bearing indicator, the RBI, to hold our track. First, the zero-wind case. Figure 7.14 shows the aircraft maintaining the required track outbound from the NDB with zero drift. The aircraft is heading 260°, and the relative bearing from the NDB is 180°. Remember, relative bearing is the angle measured clockwise from the aircraft’s nose to the beacon. So with the beacon dead astern, the RBI reads 180°. That’s your reference: outbound, zero drift, 180° relative. Now add crosswind. Figure 7.15 shows an aircraft maintaining a track of 100° in crosswind conditions where the drift is known. Here 23° of starboard drift is anticipated. Because the wind is pushing the aircraft to starboard, we subtract that drift from the track. So track 100° minus 23° gives a heading of 077°. And the relative bearing from the NDB becomes 203°. Notice the relationship: when you subtract drift from track to get heading, the relative bearing shifts accordingly. Now the opposite case, Figure 7.16. Here 20° of port drift is anticipated. Port drift is added, plus, to the track. So track 090° plus 20° gives an aircraft heading of 110°, with a relative bearing of 160° from the NDB. So the rule is simple: starboard drift is subtracted from track, port drift is added to track. That gives you the heading to fly, and the RBI shows the corresponding relative bearing. Now let’s move to drift assessment and regaining inbound track, Figure 7.17. This is the procedure when you’re flying toward the beacon. Initially, fly the aircraft on the required track with the beacon dead ahead, that’s 000° relative. Maintain the aircraft heading and watch the relative bearing indicator. If the relative bearing increases, the aircraft is experiencing port drift. So a rising RBI reading tells you the wind is pushing you to port. To regain track, alter heading, say 30° starboard. When you do that, the relative bearing will become 330° when track is regained. So you’ve turned into the wind to come back onto the line. Now assume a likely drift, say 10° port, and calculate a new heading to maintain track. When this heading has been taken up, the relative bearing will become 350°. If the drift has been correctly assessed, this relative bearing of 350° will be maintained until you’re overhead the NDB. If the relative bearing changes, however, further heading alterations and a new assessment of drift will be necessary. So the key check is: a steady relative bearing means your drift assessment is correct; a changing relative bearing means you need to reassess. Now let’s look at drift assessment outbound, Figure 7.18. This is the mirror image. With zero drift, the RBI indicates 180° relative. With 10° starboard drift, the relative bearing increases to 190°. With 10° port drift, the relative bearing decreases to 170°. So outbound, starboard drift increases the relative bearing, port drift decreases it. To assess drift by this means, the aircraft must maintain a steady heading from directly overhead the beacon. That’s the critical condition — you have to be flying a constant heading, starting from directly overhead. When the drift has been assessed, alter heading port or starboard, by say 30°, to regain track, until the correct relative bearing of 210° or 150° is obtained. So if you had starboard drift, you’d turn to get 210°; if port drift, you’d turn to get 150°. The aircraft is now back on track. The heading must now be altered to take into account the original assessment of drift. So you first turn to regain the track, then you reset your heading to the drift-corrected value to maintain it. Now let’s move to holding. When density of traffic or bad weather delay an aircraft’s landing at an airport, the air traffic controller directs it to a Holding Area. The area, also known as a ‘stack’, is organized over a ‘radio’ beacon. Each waiting aircraft flies a special circuit, and aircraft are separated vertically from each other by a minimum of 1000 ft. An aircraft drops to the next level as soon as it is free of other traffic, until it finally flies from the stack and comes in to land. So the stack is a vertical column of aircraft, each holding over the beacon, separated by at least 1000 feet, and they descend one level at a time as traffic clears.