Automatic Direction Finder (ADF) — Page 90, Lesson 80

I want to walk you through the Automatic Direction Finder, or ADF, and specifically how we use the Non-Directional Beacon, the NDB, for navigation. Let's start with the big picture: what is an NDB actually used for in professional flying? There are four primary uses. First, it gives you en route navigational bearings — that's your basic position-fixing and track-keeping between beacons. Second, you use it for homing to, or flying from, the NDB when you're maintaining airway centre lines. Third, it's used for holding overhead at an assigned level in a race-track pattern — that's the standard oval holding pattern you fly over a beacon. And fourth, it's used in runway instrument approach procedures — the NDB can be the primary navaid guiding you down to the runway. Now, before we get into the flying techniques, I need to remind you about plotting ADF bearings. The full plotting is covered in depth in the Navigation General syllabus, but at this stage there's one critical point: the bearing is measured at the aircraft. That means when you convert it to a true bearing, you must apply variation at the aircraft — not at the beacon. And you also have to account for the convergency between the aircraft's meridian and the beacon's meridian. Convergency is the angle at which the meridians converge toward the poles, and it matters because your bearing line and the beacon's reference line aren't parallel over long distances. Now let's talk about track maintenance using the Relative Bearing Indicator, the RBI. The RBI shows you the relative bearing — the angle from your aircraft's heading to the beacon. You're required to maintain tracks in several specific situations: when flying the airway centre line between NDBs; when holding over an NDB or a Locator — a Locator being a low-power NDB used at airfields; when carrying out a let-down procedure at an airfield based solely on NDBs or Locators, or NDBs and Locators combined with other navaids; when ATC requests you to intercept and maintain a track or airway centre line; and when carrying out interceptions — that's when you're being vectored to intercept another aircraft's track. Let's now look at homing. Figure 7.10 shows an aircraft maintaining a 360° relative bearing in zero wind, meaning zero drift. The aircraft is heading 077°, and because there's no wind, it will track inbound on 077° — heading and track are identical. That's the simplest case: point the nose at the beacon and you fly straight to it. But now look at Figure 7.11. Here we have the same 360° relative bearing, but with a crosswind from the left. The aircraft keeps pointing at the beacon, but the wind pushes it sideways. The result is a curved track — you're not flying a straight line to the beacon; you're flying a curve because you're constantly correcting heading to keep the needle on 360°. That's the classic mistake of homing without allowing for drift. So how do we do it properly? That's tracking inbound. To achieve a required track inbound to an NDB with a crosswind, the correct method is to allow for the anticipated drift, thereby maintaining a constant track. Let me walk you through the two examples. In Figure 7.12, 20° of starboard drift is anticipated. Starboard means the wind is pushing you to the right. So you subtract 20 from the track value. The aircraft is heading 060° with a relative bearing of 020°. Let's check that: track is 080°, you subtract the 20° starboard drift to get a heading of 060°. The relative bearing of 020° means the beacon is 20° to the right of your nose — and that's exactly the drift angle, so the needle is offset by the drift, and you hold that constant offset to maintain the straight track. In Figure 7.13, we have the opposite: 28° of port drift is anticipated — wind pushing left. So you add, or plus, the drift to the track value. The aircraft is heading 108° with a relative bearing of 332°. Track here is 080°, you add the 28° port drift to get 108° heading. The relative bearing of 332° means the beacon is 28° to the left of your nose — again, the needle offset equals the drift angle, and you hold that constant. The key principle to remember: when drift is starboard, you subtract it from track to get heading; when drift is port, you add it. And the relative bearing you hold is exactly equal to the drift angle — on the side the wind is pushing you. That's how you maintain a constant track inbound to the NDB rather than flying that curved homing path.