Aviation Theory

INTRODUCTION

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)

1. 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.
    
2. When a radio beacon is used in conjunction with the Instrument Landing System markers, it is called a Compass Locator.
    
3. Voice transmissions are made on radio beacons unless the letter "W" (without voice) is included in the class designator (HW).
    
4. 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.