Navigate in Flight Using an Automatic Direction Finder
An Automatic Direction Finder (ADF) is used for exactly what it says: to find the direction of not only a radio source (i.e: a radio tower), but also to use the bearings (locations) of two radio sources and the headings (directions of travel) of their signals in order to pinpoint the location of the aircraft receiving them.
An example of one type of ADF indicator is the RMI (Radio Magnetic Indicator) pictured in Figure 1, which is the version discussed in this article. An RMI consists of a magnetic compass backing with two indication needles: one, generally double-bared or thicker, and another thinner needle, both of which are tuned via the ADF to home into the signals being transmitted from two separate radio stations as the aircraft approaches them from the air. The small orange symbol at the center along with the orange arrow aligned with it and set into the compass degrees indicate the current orientation of the aircraft, in relation to the signals being received.
By FAA definition, it is: “Electronic navigation equipment that operates in the low- and medium-frequency bands. Used in conjunction with the ground-based non-directional beacon (NDB), the instrument displays the number of degrees clockwise from the nose of the aircraft to the station being received.”
For the beginning pilot, particularly those flying in aircraft without more advanced navigation systems, an ADF system represents a component for in-flight navigation over long distances. For an AME (Aircraft Maintenance Engineer), particularly one specializing in an aircraft's electrical systems, it is a system that leaves no room for missteps in its upkeep and repair, where even the smallest of mistakes on their part could result in a margin of error literally miles wide. However, in both cases, before either can progress to using it for themselves or to learning to maintain it, the essential first step is to understand the fundamentals of how the system functions and how it is designed to be operated.
The following are steps in the basic operation of an RMI-ADF system in a typical aircraft, as they would begin once lift off has been achieved, and the aircraft is on a steady, level flight path.
Steps
- Begin with locating a single radio source by looking first to the ADF control panel (Figure 2) to select the proper setting; push the button labeled 'ADF'.
- Next, select the radio station you wish to locate based on those selected for your flight path in your previously determined flight plan, and utilize the tuning knob (seen on panel in Figure 2) in order to tune into that station's frequency.
- At this point, in aircraft where it is automated (as it is in the majority of newer models), the aircraft's loop antenna (Figures 3 & 4), contained in a casing often found on the underside of the aircraft, will receive the signal being projected from the station it is tuned into and produce an audio output for the you or your navigator to hear, in the form of Morse code.
- The designation for the station being received comes through as a series of short and long beeps, representing the dots and dashes involved in Morse code, which you or your navigator must interpret with the purpose of confirming the station.
- Once this is confirmed, the one interpreting the signal must look to the RMI, which will at this point indicate on the compass the bearing of the station in degrees with regards to the aircraft's relative position (Figure 5).
- Once the bearing is read, record it on a navigation chart for further reference.
- In order to continue on to find the aircraft's position, you, the pilot, or your navigator must then select the frequency of a second station, using the same method as with the first. (The setting needed to read both frequencies at the same time varies based on the system installed in a given cockpit.)
- Determine the bearing of the second station; the thinnest needle will remain tuned into the first frequency, and the second and thickest needle on the RMI will indicate the new bearing, also in degrees.
- Read the bearing, then be plot it onto the navigation chart with the first.
- With the bearings of both stations determined, you must now draw straight lines from both locations to indicate the headings of the radio signals. The point in which they intersect will be the location of the aircraft in flight.
- In order to track the positioning of the aircraft throughout flight, repeat the above steps in the same manner, accounting for the change in frequency with each new radio station.
Tips
- The layout and certain operating features of the ADF system can vary depending on the aircraft, so if you're going to be navigating a plane for the first time, using this system, do your homework, and know the system's layout beforehand.
Things You'll Need
- (Installed in cockpit) RMI + ADF Control Panel
- Navigation Chart (i.e: map) + Pen/Pencil
Related Articles
References
- Maury Markowitz, Wikipedia Article, Radio Direction Finder; October 19th, 2010; https://en.wikipedia.org/wiki/Radio_direction_finder
- Andrew Herd, Flight Simulation Tutorial, Understanding Approach Plates; October 20th, 2010
- Thomas K. Eismin, Aviation Technology Series, Aircraft Electricity & Electronics – Fifth Edition; Glenco/McGraw-Hill Inc.; Columbus, Ohio, 1995; Chapter 15, Pages 303 – 306.
