What is ADF?
ADF, an abbreviation for Automated Direction Finder, is one of the oldest, simplest, most valuable and most reliable navigation instruments we could have. The system components necessary for a functional ADF system are:
How ADF Works
I can't imagine an effective navigation system that is less complicated than the ADF: As we discussed in earlier material, the NDB (Non-Directional-Beacon) station emits radio signals in every direction at the same time, much as a light bulb emits light-beams in every direction at the same time. When the ADF radio is tuned to an NDB station that is within range of the aircraft, the ADF needle locks onto the strongest signal from that station and holds it. In an ADF gauge, straight-up with the needle is equal to straight-ahead with the aircraft...so we simply turn the aircraft until the needle is pointing straight-up, and follow it to the airport! Think of the NDB station as a magnet, if you like, and think of the ADF gauge as a compass...In fact, the ADF system was once (and still is) called precisely that; a radio compass. As with any navigation radio system, it's probably worth mentioning that NDB stations are not always positioned dead-center of the airport, and in fact rarely are, because the tower-height presents a risk to air traffic. Plus, being line-of-sight communications, added height means added range for reception, so don't be shocked to discover an NDB station is on a hill several miles from the airport.
Below are some examples of what you should expect to see under similar flight situations.
Using ADF
Well, let's see; we already know that using the ADF amounts to this:
- Tune in an NDB station that's within range of the aircraft (generally 40-60 miles).
- Keep your aircraft turned so the ADF needle is pointing straight-up, and follow it to the airport.
So...rather than spew more gibberish you don't really need, why don't you just try a few practice flights using the ADF, and see how simple it really is? Download a Practice Flight to your PC, unzip it, click on the little airplane icon, and away you go!
A word about Practice Flights:
Some practice flights begin in the air, so be ready with the throttles; flights start with the radios turned off; remember to Power them up. Flight instructions are in the Kneeboard Briefing...PLEASE READ THE BRIEFING CAREFULLY!
Download ADF Practice Flight-1
Download ADF Practice Flight-2
Download ADF Practice Flight-3
Once again, it's important to understand that ADF is non-directional; it doesn't care where I am or runway headings...it simply tells me where the NDB station is with respect to the current heading of the aircraft.
Pros & Cons of ADF
We've discussed the advantages of ADF: Simple to learn and to use, and often considered a last-ditch gauge when all else fails. Now let's talk about the downsides associated with ADF, and I can only think of a few. One, ADF operates in the LF frequency range (550-1700 Kilohertz, also the AM radio band) and thus is subject to noise and static from electrical storms, atmospheric interference, and interference from the zillions of AM radio stations that exist around the planet. Two, the LF band is more subject to blockage from physical obstructions (mountains, etc) than the VORs and other radio systems that operate in the VHF (Very High Frequency) region. ADF doesn't care about altitude, of course, so you may very well be 8,000 feet above the actual tower, and ADF doesn't care...it brought you home; it's your responsibility as a pilot to land the plane.