5.ndb-and-adf. NDB and ADF
The Non-Directional Beacon (NDB) is a low or medium frequency ground-based radio transmitter that broadcasts a continuous omnidirectional carrier signal in the 190–535 kHz band. Because the signal is non-directional, the airborne Automatic Direction Finder (ADF) receiver does the work of determining bearing to the station. NDBs are identified by a one-, two-, or three-letter Morse code identifier transmitted on the assigned frequency. NDBs are classified by power output: Compass Locators (less than 25 W, used with ILS outer/middle markers, ~15 NM range), MH (less than 50 W, ~25 NM), H (50–1999 W, ~50 NM), and HH (2000 W or more, ~75 NM).
The ADF receiver consists of a loop antenna, a sense antenna, a receiver unit, and a bearing indicator. By electronically combining the figure-eight pattern of the loop antenna with the omnidirectional pattern of the sense antenna, the receiver resolves a cardioid pattern that produces a single null pointing toward the station. The bearing pointer (needle) on the indicator points directly to the NDB.
Three types of indicators are common:
- Fixed-card ADF — The card is fixed with 0° (north) always at the top. The needle indicates Relative Bearing (RB), the angle from the nose of the aircraft, measured clockwise to the station.
- Rotatable-card (Heading-card) ADF — The pilot manually rotates the card so the present Magnetic Heading (MH) is under the index. The needle then indicates Magnetic Bearing (MB) TO the station directly.
- Radio Magnetic Indicator (RMI) — The compass card slaves automatically to the heading system. The ADF needle continuously displays MB TO the station; the tail displays MB FROM.
The fundamental ADF formula is:
MH + RB = MB to the station
If the result exceeds 360°, subtract 360°. To find the bearing FROM the station, add or subtract 180°. Example: MH 040°, RB 280°. MB = 040 + 280 = 320° to the station; bearing from = 140°.
Orientation and Homing. Homing means turning to keep the ADF needle on the nose (RB 0°) and following it to the station. In a crosswind, homing produces a curved track and a longer flight path. Tracking corrects for wind by establishing a heading that holds a desired bearing. Procedure to track inbound:
- Turn to a heading equal to the desired MB.
- Observe needle drift; if the needle drifts right, the wind is from the right—turn right (toward the needle) by an estimated wind correction angle (WCA).
- The needle should then point left of the nose by an amount equal to the WCA, indicating you are on the desired bearing with drift compensation. Use the rule: double the drift, then halve it to bracket the wind.
Intercepting a bearing. To intercept a desired inbound MB:
- Determine MB to the station (MH + RB).
- Determine the difference between present MB and the desired MB.
- Turn to an intercept heading, typically twice the difference (not exceeding 90°), on the side that will close on the desired bearing.
- As the needle approaches the intercept value, turn to the inbound heading and apply WCA.
Station passage is indicated by the first positive, complete reversal of the needle from the nose to the tail, not by needle oscillation alone. Over the cone of confusion the needle may swing erratically; the time of passage is when the needle settles on or near the tail.
Limitations and errors. Because NDB signals are in the LF/MF band, ADF reception is subject to several errors the instrument pilot must recognize:
- Thunderstorm effect — Lightning produces strong LF energy; the needle deflects toward thunderstorms. Useful for avoidance, unreliable for navigation in convective weather.
- Night effect — After sunset, sky waves reflected from the ionosphere interfere with ground waves, causing the needle to fade, swing, or oscillate, especially within 30–60 minutes of sunrise/sunset.
- Terrain/Mountain effect — Reflection and refraction over mountainous terrain induce bearing errors.
- Shoreline effect — LF waves crossing a shoreline at angles less than 30° refract along the shore, biasing the indicated bearing.
- Bank error — A banked attitude tilts the loop antenna, introducing temporary needle error; level the wings to read accurate bearing.
- Electrical interference — Precipitation static, alternators, and onboard electronics can cause needle disturbance.
Preflight and inflight checks. Tune the published frequency, identify the station by Morse code (do not rely on a missing or unmodulated signal), set ADF function (not ANT or TEST), and verify needle response. The Morse identifier is the only positive confirmation of station identity—if the ident is absent or the station is broadcasting a continuous tone for maintenance, the navigation information is not reliable. Many older NDB approaches have been decommissioned, but ADF remains a required piece of equipment on certain IFR routes and approaches outside the contiguous U.S., and it is an excellent backup to GPS where installed.