We used to fly these routinely down in the crummiest weather just hoping the needle wasn't going to be 40° off, because it would do that on its own, or that someone on the ground wasn't doing something with a power plant and would send the airplane into a mountain. (It would do that too, and did to a crew I knew.)
If you have to fly an NDB approach these days it probably means there isn't anything else left. In the United States you are now allowed to use GPS, provided the NDB is operational and monitored. (See New Overlay Procedures, below.) Overseas, many countries will allow you to use GPS instead. If the approach is valid under WGS-84 and the practice is allowed by the State pages, that's what I would do.
Flying an NDB without a computer's help is a lost art. Well, not exactly lost. Read on . . .
What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.
Instrument Flying Handbook, page 7-3] The nondirectional beacon (NDB) is a ground-based radio transmitter that transmits radio energy in all directions. The ADF, when used with an NDB, determines the bearing from the aircraft to the transmitting station. The indicator may be mounted in a separate instrument in the aircraft panel. The ADF needle points to the NDB ground station to determine the relative bearing (RB) to the transmitting station. Magnetic heading (MH) plus RB equals the magnetic bearing (MB) to the station.
An NDB approach is based off a non-directional beacon which is subject to environmental interference which often makes it unreliable. It is being phased out of use in much of the world and should always be your last choice in approach selection.
Instrument Flying Handbook, page 7-3] The airborne equipment includes two antennas, a receiver, and the indicator instrument. The “sense” antenna (nondirectional) receives signals with nearly equal efficiency from all directions. The “loop” antenna receives signals better from two directions (bidirectional). When the loop and sense antenna inputs are processed together in the ADF radio, the result is the ability to receive a radio signal well in all directions but one, thus resolving all directional ambiguity.
An instrument rating is needed, but the FAA is rapidly loosing interest in the entire concept of the NDB approach.
[FAA-H-8261-1, pg 5-57] "The long-term plan includes the gradual phase out of NDB facilities, and at some point in time, the NDB approach will become nonexistent."
Figure: Brunei NDB Approach, from Eddie's notes.
The key to understanding the NDB approach is knowing that needle does one thing and one thing only: point to the beacon. You need to keep in your head the correct bearing to the station, the wind direction, and a suitable drift correction which you need to correct as the approach progresses. The following example is to Bandar Seri Begawan, Brunei. As an Air Force pilot we didn't have NDB receivers installed in the T-37, T-38, or KC-135 and I never saw an NDB approach until I was in the Boeing 707. This was my first:
Figure: Brunei NDB Needle, from Eddie's notes.
You will find these approaches at a lot of airports around the world where there is nothing else available.
[Aeronautical Information Manual ¶1-1-1 d.] 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 Automatic Direction Finder (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.
[AIM 26 May 2016, Explanation of Changes.] a. 1−2−3. Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes
This change allows for the use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure (IAP) based on a VOR, TACAN, or NDB signal. The underlying NAVAID must be operational and monitored for the final segment course alignment.
[AIM 26 May 2016, ¶1-2-3.c.] Uses of Suitable RNAV Systems. Subject to the operating requirements, operators may use a suitable RNAV system in the following ways.
FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.
FAA-H-8261-1, Instrument Procedures Handbook, U.S. Department of Transportation, Flight Standards Branch, 2004