Learning to Fly the NDB

Before you go off on the “new and better technology” available to instrument pilots, no one is disputing that. But the fact is that NDB approaches, while increasingly rare, are still out there. And there are still airplanes equipped with ADFs that enable the pilot to fly these NDB approaches—if only someone taught them how. 

NDBs are still mentioned as nonprecision approaches in FAA documentation and are still covered in the FAA’s Instrument Flying Handbook, therefore they are fair game during check rides for instrument candidates. If you have an aircraft capable of flying an NDB approach, it behooves you to learn how to do it, just in case the other tech fails or is failed by a DPE on a check ride. 

Recently I had the opportunity to teach this skill to a CFI-I in training. She has a friend seeking an instrument rating who owns a Cessna 150 equipped with a working ADF. Since the CFI earned her instrument rating after 2015, after many of the NDB approaches were phased out in favor of satellite-based WAAS, she’d never flown a NDB approach but was eager to add this arrow to her quiver.

We used a Redbird FMX for the lesson, because the nearest NDBs are 64 and 90 nm away at Washington state’s Arlington Municipal Airport (KAWO) and Friday Harbor Airport (KFHR). Neither one of us had the time to make that long of a flight in a Cessna 150, so we went into the box and through the magic of computers—learning took place. 

Let’s start with the basic understanding of the components of the NDB/ADF approach. The NDB transmits in the frequency range of 190 to 535 kilohertz (kHz).

The NDB is a ground-based aid to navigation that operates in the same range but also extends to 1,750 kHz, modulation, which puts it into the AM radio frequency band. You can listen to commercial AM radio stations as you fly, and sometimes when you are tuned in to the NDB frequency, there may be some bleed over from commercial stations. 

On the sectional, the NDB looks like a magenta puffball. It has a five-letter name like GRAYE, or there is a three-letter Morse code identifier that appears on the sectional. 

The ADF is the receiver in the aircraft. Unlike a VOR, there is no “off” flag with the ADF receiver, so when you use the NDB for navigation, you have to continually listen to the identifier to ensure the station is still transmitting.

Many NDBs are located near former military bases as they used to be state-of-the-art navigation when IFR flight was in its infancy decades ago.

The entire system consists of the transmitting ground station (from the air it often looks like a wooden shed set out in the middle of a field), an ADF receiver, and an antenna that includes a loop antenna to determine the aircraft’s magnetic bearing from the station, and the sense antenna that provides direction information and the bearing indicator.

The ADF pointer is a needle. Some ADFs have a compass rose fixed on the face of the instrument. This is known as a “fixed card.” Later models had a feature that allowed you to reposition the card with the twist of a knob like you do with the heading indicator. This is known as a “movable card.”

When the NDB is transmitting and the ADF is receiving, the head of the needle points to the station. This can help with situational awareness.

With a fixed card, the pilot needs to determine the station’s relative bearing by noting the angle between the nose of the airplane to the station—where the head of the needle is pointing. If the arrow is pointing 30 degrees to the right of the nose, that is the relative bearing.

To determine the magnetic bearing to the station, the pilot takes the magnetic heading and adds it to the relative bearing, resulting in the magnetic bearing to the station.

For example, let’s say you are flying on a magnetic heading (MH) of 020, and the relative bearing (RB) to the station is 040. 

If we add those together, we get 060, so if we turn the aircraft to 060, we should be heading directly for the station. MH 020 + RB 040 = MB 060

If the value is greater than 360, subtract 360 from the resultant to get the magnetic bearing to the station.

Remember the math using the mnemonic device: Mary Had Roast Beef. Mary Barfed (MH + RB = MB)

The easier way to determine your MB is to make a peace sign with your hand and line your fingers up so your index finger is on the head of the needle and middle finger on the tail. Now without changing your hand posture, move your hand to the heading indicator. Your index finger will be on the magnetic bearing to the station.

There are some limitations when using an NDB. It is not limited to line of sight, which permits reception at low altitude over long distances because of ground waves.

It has greater range at night because radio waves can be reflected back by the ionosphere. However, this can cause fluctuations 30 to 60 nm from the transmitter, especially just before sunrise and just after sunset.

The radio waves follow the curvature of the Earth, and terrain such as hills and mountains can reflect radio waves, giving erroneous readings, especially if they contain magnetic deposits.

Precipitation static (like during a rainstorm) can cause the course deviation needle to twitch. It’s also a low budget lightning detector. During an electrical storm, the needle will momentarily deflect toward the strike then return to normal.

The low-frequency radio waves will refract or bend near a shoreline, especially if they are close to parallel to the shore, so don’t make any course corrections until you are well past it.

For an NDB approach, the NDB is often directly on the airport (especially if it was used by the military during World War II) or located a few miles away, often off the extended centerline of a runway.

If the NDB is located a few miles from the airport, the pilot flies over the NDB maintaining a specific ground speed and timing the approach. Station passage is observed when the head of the needle falls so it is pointing straight down, indicating the aircraft has flown over the ground station sending the signal.

If the pilot maintains a published ground speed, for a published amount of time, and was at the published altitude if the conditions were right, they would see the airport in time to allow them to make a normal landing.

Course corrections for wind are fairly straight forward—determine which way you are getting blown and correct into the wind. Start with a 10-degree correction and allow at least a minute—and I mean timed by a clock to evaluate if you need more of a correction. When the amount of needle deflection equals the amount of input correct, you are on course. 

Tracking away and tracking to an NDB use the same principle. Blown off course? Correct into the wind, then take out the intercept angle when you’re on the desired bearing. Experiment to get a wind correction angle to stay on course. 

When I fly NDB approaches, I still hear my WWII-trained CFI chanting, “Put the needle on the nose and dance! Dance!”—meaning do what you need to do to keep the needle pointing to the station. If you can determine the crosswind components— for example, 5 degrees—double that for the correction to avoid simply paralleling the course.

Is it old technology? Absolutely. Are they fun to fly? Yes! And it gives you a skill set that not many people have anymore, like the ability to drive a stick shift. It will set you apart from other pilots and might even come in handy someday when you ferry a vintage machine across the country.

This column first appeared in the October Issue 951 of the FLYING print edition.