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

Let’s start with the big picture. The Automatic Direction Finder, or ADF, is the airborne equipment that points at a ground transmitter called a Non-directional Beacon, the NDB. The NDB itself is just a ground radio station that sends out a signal equally in all directions — that’s why it’s called non-directional. The ADF on your aircraft receives that signal and tells you the direction to the beacon. So when we talk about ADF work, we’re really talking about using NDBs for navigation. First, let’s cover the uses of the NDB, because that tells you why we bother with this system at all. There are four main jobs. One: en route navigational bearings — that means using the NDB to get a bearing to or from the beacon while you’re cruising between places. Two: homing to, or flying from, the NDB when you’re maintaining airway centre lines — so when you need to stay exactly on the centre line of an airway, the NDB gives you the reference. Three: holding overhead at an assigned level in a race-track pattern — that’s the standard holding pattern you fly over a fix, and the NDB can be that fix. And four: runway instrument approach procedures — NDBs are used to guide you down to a runway when you can’t see it, in poor weather. Now, before we get into the flying techniques, I want to touch on plotting ADF bearings. The full plotting is covered in depth in the Navigation General syllabus, so here we just need the reminder. The key point is this: the bearing you get from the ADF is measured at the aircraft. So when you want to convert that to a true bearing — a bearing relative to true north — you must apply variation at the aircraft. Variation is the difference between true north and magnetic north, and because the bearing is measured at your position, that’s where the correction goes. You also have to account for convergency between the aircraft meridian and the beacon meridian — that’s the way the meridians, the lines of longitude, come together as you move toward the poles. On a chart, that convergence affects how the bearing line is drawn. So: variation applied at the aircraft, and convergency between the two meridians taken into account. Now let’s move to track maintenance using the Relative Bearing Indicator, the RBI. The RBI is the instrument that shows you the relative bearing — the angle between your aircraft’s heading and the direction to the beacon. You need to maintain track — that is, hold a constant ground path — in several situations. When flying the airway centre line between NDBs. When holding over an NDB or a Locator — a Locator is a low-power NDB used near 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 a specific track or another aircraft’s path. So now the heart of it: homing. Homing is the simplest technique — you just keep the relative bearing at 360°, which means the beacon is dead ahead, and you fly straight at it. Let me walk you through Figure 7.10. Here the aircraft is maintaining a relative bearing of 360° in zero wind, so zero drift. The aircraft is heading 077°, and because there’s no wind pushing it sideways, it will track inbound on 077° — the heading equals the track. That’s the clean case. But now look at Figure 7.11. Same idea — the aircraft maintains a relative bearing of 360° — but now there’s a crosswind from the left. The wind pushes the aircraft sideways, so even though you keep the beacon dead ahead, you don’t fly a straight line. Instead, a curved track is followed. You end up flying a curve toward the beacon, not a straight line. That’s the weakness of plain homing: it gets you to the beacon, but not on a constant track. So to do it properly, we use tracking inbound. Tracking means you hold a constant ground track, not just a constant relative bearing. To achieve a required track inbound to an NDB with a crosswind, the correct method is to allow for the anticipated drift, and that keeps you on a constant track. Let me show you the two examples. In Figure 7.12, 20° of starboard drift is anticipated. Starboard means to the right. So because the wind is pushing you to the right, you subtract 20 from the track. The aircraft is heading 060° with a relative bearing of 020°. Let me make sure you see the relationship. The relative bearing is 020°, which means the beacon is 20° to the right of your nose. Your heading is 060°. So the bearing to the beacon is 060 plus 020, which is 080° — that’s your track. And you’ve subtracted the 20° of drift from the track to get your heading. So the heading is the track minus the drift. Now Figure 7.13 is the mirror image. Here 28° of port drift is anticipated — port means to the left. Because the wind pushes you left, you add the drift to the track — the text says it’s added, Plus, to the track value. The aircraft is heading 108° with a relative bearing of 332°. Let’s check that. A relative bearing of 332° means the beacon is 28° to the left of your nose — because 360 minus 332 is 28. Your heading is 108°. So the bearing to the beacon is 108 minus 28, which is 080° again — same track. And here the heading is the track plus the drift. So the rule you want to remember: when drift is to starboard, subtract it from the track to get your heading. When drift is to port, add it to the track. In both cases you end up tracking the same inbound bearing, 080°, but your heading is adjusted to cancel the wind. That’s the difference between homing — where you just chase the needle — and tracking — where you hold a constant ground path by allowing for drift.