Non-Directional Radio Beacon

• According to International Civil Aviation Organization (ICAO) Annex 10 the frequency range for NDBs is between 190 and 1750 kHz, and transmit a continuous carrier with either 400 or 1020 hertz (Hz) modulation
• The NDB frequency can sometimes bleed over to the AM Radio frequency band and likewise radios can bleed over onto the NDB frequency

Twilight Error (Night Effect):

• Radio waves can be reflected back by the ionosphere and can cause fluctuations 30 to 60 NM (approx. 54 to 108 KM) from the transmitter, especially just before sunrise and just after sunset

Terrain Error:

• High terrain like hills and mountains can reflect radio waves, giving erroneous readings especially if they contain magnetic deposits

Electrical Error:

• Electrical storms, and sometimes also electrical interference can cause the ADF needle to deflect toward the electrical source

Shoreline Error:

• Low-frequency radio waves will refract or bend near a shoreline, especially if they are close to parallel to the shore

Bank Error:

• When the aircraft is banked, the needle reading will be offset

• By tuning to low frequency (LF) radio stations such as NDBs, or to commercial broadcast (AM) stations, a pilot may use ADF for navigation in cross-country flying
• Some major commercial broadcast station locations and frequencies are shown on sectional aeronautical charts

• Primarily for air navigation, the LF/MF stations are FAA and privately operated non-directional radio beacons
• Some broadcast stations operate only during daylight hours, and many of the low powered stations transmit on identical frequencies and may cause erratic ADF indications

• That is, when the bearing pointer is on the nose position, the station is directly ahead of the airplane; when the pointer is on the tail position, the station is directly behind the airplane; and when the pointer is 90° to either side (wingtip position), the station is directly off the respective wingtip

• In this type, the bearing pointer shows only the station's relative bearing, i.e., the angle from the nose of the airplane to the station [Figure 3]
• A more sophisticated instrument called a Radio Magnetic Indicator (RMI) uses a 360° azimuth dial which, being slaved to a gyro compass, rotates as the airplane turns and continually shows the Magnetic Heading at the top of the instrument
• Thus, with this rotating azimuth referenced to a magnetic direction, the bearing pointer superimposed on the azimuth indicates the Magnetic Bearing to the station

• Since the ADF pointer always points to the station, the pilot can simply head the airplane so that the pointer is on the 0° or nose position. The station, then will be directly ahead of the airplane. With a crosswind, however, the airplane would continually drift to the side and, unless a corrective change in heading is made, would no longer be flying straight to the station. This would be indicated by the pointer moving to the upwind side of the nose position on the dial. By periodically turning the airplane into the wind (toward the head of the pointer) so as to continually return the pointer to the 0° position, the airplane can be flown to the station, although in a curving flightpath as a result of wind drift. The lighter the crosswind and the shorter the distance from the station, the less the flightpath curves. Upon arrival at and passing the station, the pointer will swing 180° from a nose position to a tail position

• The number to which the bearing indicator points on the fixed azimuth dial has no directional meaning to the pilot until it is related to the airplane's heading. To apply this relationship, the magnetic heading must be observed carefully when reading the Relative Bearing to the station. Any time the airplane's heading is changed, the Relative Bearing will be changed an equal number of degrees

• When the pointer is on the nose position, the airplane is heading straight to the station and the Magnetic Bearing can be read directly from the magnetic compass. If the pointer is left or right of the nose, the pilot should note the direction and number of degrees of turn that would (if the airplane were to be headed to that station) move the pointer to the nose position, and mentally apply this to the airplane's heading. For example, in Fig. 12-11, when the airplane is headed 090°, the pointer is 60° to the left of the nose position. A turn 60° to the left would place the pointer on the nose position. If the airplane were to be turned 60° to the left, the heading would be 030°. The bearing from the station is the reciprocal - or 210°

1. After the course has been intercepted, maintain the heading that corresponds to the Course To or Bearing From the station
2. If a 10° course devision is indicated (off the nose of tail relative to the needle) then re-intercept by beginning with a change toward the "head" of the needle that is 20°
3. Maintain the intercept heading until the angle of deflection from the nose or tail is 20° and then turn to a new course heading by taking out half of the intercept angle
   • This new heading is the new relative bearing
4. If the aDF needle deflects toward the nose or away from the tail, re-intercept by beginning with a 10° change in heading (intercept heading) toward the needle deflection
5. Maintain the intercept heading until the deflection angle equals the intercept angle (deflection = correction), and then turn back to a new course heading by taking out half of the heading change
6. Note that larger correction angles can be used if the wind requires

• Pilots should disregard any navigation indication, regardless of its apparent validity, if the particular transmitter was identified by NOTAM or otherwise as unusable or inoperative