The NDB is used in modern aviation mostly for instrument approaches.
In precision approaches, there will often be an NDB co-located with
the Outer Marker (OM). In this case the marker is then referred
to as a LOM (Locator Outer Marker). In non-precision approaches,
the NDB may be one of the fixes used in the approach. A third
use of NDBs is as a navaid forming one or more fixes for an intersection.
Last, there are non-precision NDB approaches.
NDBs are more difficult to track for most pilots since they do
not directly indicate the course to fly to reach the NDB. In a no
wind situation, this does not present any major problem. However,
with crosswinds, or worse, variable direction winds, it takes more
mental gymnastics to accurately track an NDB.
From AIM 1-1-2. NONDIRECTIONAL RADIO BEACON (NDB)
- A low or medium frequency radio beacon transmits nondirectional
signals whereby the pilot of an aircraft properly equipped can
determine bearings and "home" on the station. These facilities
normally operate in the frequency band of 190 to 535 kHz and transmit
a continuous carrier with either 400 or 1020 Hz modulation. All
radio beacons except the compass locators transmit a continuous
three letter identification in code except during voice transmissions.
- When a radio beacon is used in conjunction with the Instrument
Landing System markers, it is called a Compass Locator.
- Voice transmissions are made on radio beacons unless the letter
"W" (without voice) is included in the class designator (HW).
- Radio beacons are subject to disturbances that may result in
erroneous bearing information. Such disturbances result from such
factors as lightning, precipitation static, etc. At night radio
beacons are vulnerable to interference from distant stations.
Nearly all disturbances which affect the ADF bearing also affect
the facility's identification. Noisy identification usually occurs
when the ADF needle is erratic. Voice, music or erroneous identification
may be heard when a steady false bearing is being displayed. Since
ADF receivers do not have a "FLAG" to warn the pilot when erroneous
bearing information is being displayed, the pilot should continuously
monitor the NDB's identification.