An automatic direction finder is a basic navigation aid common in light aircraft but one that students may not learn to use. The ADF is a radio navigation system that shows the direction to a nondirectional radio beacon (NDB) transmitter. ADFs and NDBs are sometimes confused because they are part of the same system. The important thing to remember is that the ADF is in the airplane and the NDB is on the ground.
ADF displays typically consist of a compass card that may or may not be moveable and an indicator needle. The head of the needle always points to the NDB. To get to the transmitter, simply fly in the direction in which the needle is pointing. When the needle is pointing straight ahead, you are headed for the station, presuming a no-wind situation.
As with a VOR, it's important to select a course heading that's working and stick with it because as you get close to the station the needle will begin to move erratically, eventually dropping approximately 90 degrees to one side, indicating station passage.
Of course, ADFs are good for more than homing to a station. They can also be used to fly a specific bearing, as part of a cross-check to identify your position, and as guidance for nonprecision instrument approaches. While ADFs are slowly being replaced by more modern systems, it's a good idea for any pilot to learn how to use them.
Elizabeth A Tennyson
Senior Director of Communications
AOPA Senior Director of Communications Elizabeth Tennyson is an instrument-rated private pilot who first joined AOPA in 1998.
