Automatic Direction Finder (ADF) — Page 99, Lesson 84

I want to walk you through how we actually use the NDB and the ADF in the real world, and then the things that can degrade its accuracy. We're starting with runway instrument approach procedures. Most aerodromes have NDB runway instrument approach procedures. That means the pilot flies the published procedure — the charted track, altitudes, and timing — in order to position the aircraft in poor weather conditions for a visual landing. The NDB may also be used in conjunction with other runway approach aids for the same purpose. So the NDB isn't always the sole aid; it can back up an ILS or a VOR approach. Now, the critical part: factors affecting ADF accuracy. The first one is the Designated Operational Coverage, or DOC. The DOC of NDBs is based upon a daytime protection ratio — a signal/noise ratio of 3:1 — between wanted and unwanted signals that permits the required level of bearing accuracy. Let me unpack that. The protection ratio is the margin between the strength of the signal you want and the noise or interference you don't want. A 3:1 ratio means the wanted signal is three times stronger than the unwanted one. That ratio is what allows the bearing to be accurate. At ranges greater than those promulgated — that is, beyond the published DOC range — bearing errors will increase. And adverse propagation conditions, particularly at night, will also increase bearing errors. Next, static interference. There are two types that can affect ADF performance. The first is precipitation static. It's generated by the collision of water droplets and ice crystals with the aircraft. It causes a reduction in the signal/noise ratio, which affects the accuracy of the bearings and can, in extreme circumstances, completely mask the incoming signal. The indications on the RMI or RBI — that's the Radio Magnetic Indicator or Radio Bearing Indicator — will be a wandering needle, and the audio will have a background hiss, which is also likely to be present on VHF frequencies. So you'd hear that hiss on your VHF comms too. The second type is thunderstorms. They have very powerful discharges of static electricity across the electromagnetic spectrum, including LF and MF — low frequency and medium frequency, which is exactly where the NDB band lives. These discharges cause bearing errors in the ADF. A static discharge in a cumulonimbus cloud — a Cb — will be heard as a loud crackle on the audio, and the needle will move rapidly to point to the Cb. When there are several active cells close together, it's possible for the needle to point to them for prolonged periods. Care must be taken in the use of ADF when Cb activity is forecast. It's been said that during Cb activity the only sensible use of the ADF is to indicate where the active cells are. So the needle becomes a storm locator rather than a navigation aid. Finally, the night effect. By day, the D-region of the ionosphere absorbs signals in the LF and MF bands. At night, the D-region disappears, allowing sky wave contamination of the surface wave. That's the mechanism behind the increased night errors I mentioned earlier — the sky wave bounces off the ionosphere and interferes with the direct surface wave, corrupting the bearing. Let me show you what these procedures and drift assessments look like on the charts. So to tie it together: the DOC gives you the reliable envelope, static interference and night effect are the enemies that push you outside that envelope. Know your DOC, watch for Cb activity, and remember that at night your bearings are more suspect.