For decades, the non-directional beacon (NDB) approach flown with an automatic direction finder (ADF) was a staple of instrument flying. Today it is nearly extinct. Most pilots training for an instrument rating will never fly one in an actual airplane. The FAA even managed to finally remove questions about NDBs and ADFs from the knowledge tests a few years ago.
But some of us remember. I admit I am old enough to remember doing these in my instrument rating and CFI-I training, and not just a few of them in actual IFR to get home to an airport.
Particularly, the home airport I learned to fly at had an NDB approach that used an ADF. Specifically, it was the NDB runway 29 at Oconto, WI (KOCQ) on a frequency of 388. That is still burned into my mind.
Eventually, the plate for it got updated to the one I found an old copy of here, the NDB (wait for it…) or GPS Rwy 29. It was in a period of time when new navigation systems were taking over and kicking out, even on the same chart. This plate has long disappeared, replaced with a modern RNAV GPS RWY 29 that is much easier to fly. But I still think I could fly the old NDB that my first instructor drilled into me if it was ever there again. Probably by memory if I had to.
This particular historical approach plate captures a classic late-era example of an NDB approach that utilized an ADF.
The OCQ NDB was on 388 kHz and sat on the field and served as both the initial approach fix and the missed-approach holding fix. Final approach course was 301°, there was no depicted final approach fix, and the MDA was 1,240 feet (636 feet height above touchdown). The missed approach required a climbing left turn to 2,300 feet via a 180° heading, then a left turn direct to the NDB. Notes called for the Green Bay altimeter setting and pilot-controlled lighting. Straight-in and circling minima were published, with Category D not authorized. The chart title itself—“NDB or GPS”—marked it as a product of the transitional years when GPS was being grafted onto the existing NDB infrastructure.
On an approach such as this one, pilots were lucky. The NDB was on the field and you could more easily “home” to the location. You would fly to the station, get station passage, then fly the outbound leg, conduct a procedure turn, then fly back inbound. If you passed the station again and hadn’t broken out, it was time to go missed. In cases where the NDB was based off the field, the pilot might cross a station and then proceed inbound, following a bearing, and using timing to identify a missed approach point. Those were less fun, and less accurate. I don’t miss them.
NDB/ADF procedures were a genuine improvement over the four-course “A-N” radio ranges that dominated the 1930s and 1940s. Those earlier ranges forced pilots to fly by ear, listening for the steady on-course tone created by overlapping Morse A and N signals while managing quadrant ambiguity. The NDB used a simple single-antenna transmitter and an airborne ADF whose needle pointed continuously toward the station. Pilots could track to or from the beacon, fly procedure turns or holding patterns, and conduct an approach with far less aural workload. For many years this combination delivered practical instrument access to smaller airports that lacked VOR or localizer service.
The limitations remained significant. Low- and medium-frequency signals suffered from thunderstorm static, night effect, coastal refraction, and terrain reflection. The ADF needle could swing or reverse. Without collocated DME there was no direct distance information. Continuous wind correction and relative-bearing arithmetic were required, and minima were typically higher than those later available with GPS. Despite these shortcomings, NDB approaches remained operationally useful well into the GPS era because the beacons were inexpensive to install and maintain, especially at general-aviation fields, and because they offered a simple backup.
A key transitional phase occurred in the 1990s and early 2000s. As IFR-certified GPS receivers proliferated, the FAA launched the GPS Overlay Program. Existing NDB (and VOR) approaches were amended by adding “or GPS” to the title. This allowed pilots to fly the same tracks, altitudes, and procedural elements using GPS as the primary navigation source, without needing a working ADF or even monitoring the NDB signal in many cases. The Oconto plate is a textbook illustration of that era. These overlays accelerated the acceptance of satellite navigation, reduced dependence on aging ground equipment, and gave operators a graceful path away from analog ADFs that were already disappearing from panel space. Over time most of the overlays themselves were replaced by standalone RNAV (GPS) procedures that no longer referenced the NDB at all.
The Oconto NDB or GPS Rwy 29 followed that path. Today the airport is served only by RNAV (GPS) approaches to Runways 11 and 29. Across the National Airspace System not many pure NDB or NDB/DME approach charts remain, and they are going away quickly. The NDB itself is no longer required for the instrument rating practical test, and the majority of training aircraft no longer carry functional ADFs.
The ADF approach was never elegant, but for a long stretch of aviation history it was essential. The “NDB or GPS” plates of the early 2000s represent the brief middle chapter in which the old technology was allowed to share the stage with the new. That chapter has closed. New instrument pilots will encounter the concept in textbooks and simulators; very few will ever chase a real needle toward a real NDB on a published procedure.
It has been a while since I have had to use an actual NDB approach. In fact, the most common use I had for ADF devices, before the advent of bluetooth headsets and the ability to play music to my own headphones from my phone, was tuning into 720 AM and listening to Cubs games. I know lots of fellow pilots that most frequently in the last days of ADFs used them to listen to various AM radio stations in their area of operations.
Never flown an NDB approach? Or even seen an ADF in an aircraft (that works)? Check out the simulator below here to play with how it might look as you move the aircraft was moved around near it and your compass heading was changed.
Click here to show this NDB/ADF simulation in its own page as a standalone.
