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Like many of you, I bought my first airplane a long time ago, and the panel looked like something out of a South American locomotive. There was a very tired, crystal-controlled Narco VHT3 navcom that worked on alternate Thursdays when the moon was full. There was also an equally weary but still functional Bendix T12D ADF that looked as if it shouldn’t work but did.
At the time, I regarded the Bendix as a wonder box. It was the simplest possible form of radio navigation. The needle pointed at the station. End of directions.
Simplicity isn’t necessarily all it’s cracked up to be. ADF offers no distance or altitude information and no internal heading info, though following the needle could provide a bearing to the station. If you could receive two strong signals with the proper geometry, you could switch back and forth and sometimes triangulate a rough position. Very rough.
ADF was simple and primitive, but it worked…most of the time.
Anyone who’s read Rod Machado, William Kershner or any of a dozen other aviation textbook authors can probably recite chapter and verse as to why ADF has been all but abandoned as a primary nav aid for VFR and IFR flight.
The system dates back to the 1920s, and no, this isn’t a historical dissertation on the superiority of ADF over today’s satellite-based, supremely accurate navigation technology.
ADF operates in the LF band that ranges from 200 to 415 kHz and the AM, commercial broadcast band that plays between 550 and 1600 kHz. Unlike some clear channel, AM broadcast stations that boast power up to 50,000 watts, dedicated aviation LF stations generally make do with 200 watts or less.
Of course, as everyone learned in flight school, the LF signal is notoriously unreliable in convective weather. Any electrical discharge, typically a thunderstorm, can drive the needle crazy, most often pointing at the nearest lightning flash. Terrain errors, common in mountainous areas, can induce erratic readings. Ionosphere error during sunrise and sunset also can cause fluctuations in the readout. Bank error can compromise the DF function when the aircraft is in a turn.
The former owner of my 1946 Swift had a directory of commercial AM broadcast stations that he carried with him everywhere he flew, and he bequeathed that book to me when I bought his airplane. The info in the directory and my Bendix ADF helped me travel to pretty much anyplace I wanted to go.
Blessed with undeserved courage and limited aviation brain cells, I flew the Swift from Southern California to practically everywhere in North America except Mexico. Though my directory provided the lat/long of the transmission source, it didn’t relate to the airports’ location. I could usually determine that info from Acc-U-Quik or by consulting the chart.
I was reminded of all this a few weeks back when Dan Neil, auto reviewer for the Wall Street Journal, wrote a story in his Rumble Seat column about driving coast-to-coast across the lower U.S.A. in his minivan.
Neil waxed poetic about the joy of driving at night when some AM broadcast stations could reach out and touch him from 2,000 miles away. Obviously, Neil had no DF capability in his van, but that barely mattered since he was forced to follow the interstate highway system anyway.
When I flew at night over Southern California, I was a little amazed to sometimes tune WABC in New York (770 kHz) and have the stations come booming into my headset as if they were on the phone.
Moot point. The Swift had a range of about 350 nm, so those far-flung stations were never my destinations. At least, the ADF could point out the proper direction to the station, sometimes even in daylight.
When I was learning to fly in Alaska, navigation was mostly by pilotage, dead reckoning or ADF. Few trainers had VOR receivers. Mountains are practically everywhere in the 49th state, so even when VORs finally became available, their application was limited.
When ADF worked, it was the closest thing we had to long-range nav. Fortunately, most general aviation aircraft came equipped with ADF in that era, and it seemed there were NDBs everywhere.
When I began delivering airplanes overseas in 1980, ADF became even more valuable. LF beacons are relatively inexpensive to buy and maintain, and they’re still used extensively all over the world, especially in Africa and on many remote Pacific islands.
The advent of GPS has eclipsed much of the NDB’s application in the U.S., and the FAA is slowly decommissioning more and more of those low-frequency stations.
LF beacons and four-course ranges were standard in many places until VHF/VORs began to supplant them in the early 1960s. The VOR network was good for short distances over land but not so good over the ocean.
On the standard, 1,800-nm leg from Gander, Newfoundland, to Shannon, Ireland, we used to tune a commercial broadcast station (Radio 2 – 566 kHz, I think) that happened to have its transmitter located directly east of Shannon.
If atmospheric conditions were good, we could track that signal all the way across the Atlantic. Sometimes, when conditions were optimum, we could pick up Radio 2 while sitting on the ramp at Gander.
If we homed on the station with the needle, we’d sometimes prescribe a slight arc across the ocean because of frequent, northwesterly crosswinds. Most of the time, the added distance was less than 5 percent of our total. If we “tracked” to the station, using a wind correction angle, we could often reduce or eliminate the error.
More importantly for pilots who were smarter than to overfly oceans, ADF allowed tuning LF frequencies at airports across the U.S. These stations weren’t always very strong, but some offered NDB approaches that allowed IFR procedures when weather was marginal.
Coincidentally, a non-directional beacon (CPM–378 kHz) was mounted on the side of my hangar in Compton, California until a few years ago. One joke around the airport during instrument weather was that if I could call ahead and have someone open my hangar door, I could fly right into my hangar. Not!
ADF was usually a good friend. I used it religiously, even after the VHF/VOR network was introduced. Every airplane I’ve owned has been equipped with ADF, and the system has been a valuable backup more times than I can count.
Once, back in the ‘80s, well before the introduction of GPS, I was flying a 36 Bonanza outbound from Honolulu, final destination—Perth, Australia. I was halfway out on the second, 2,000-nm Pacific leg toward Majuro in the Marshall Islands, still 1,100 miles distant.
The Majuro NDB was strong and pointing straight ahead when the needle lost lock and wandered slowly around to the 90-degree-right, park position. I tried to check the identifier, but there was nothing on the frequency. Majuro’s beacon had gone off the air.
No cause for panic. For what it was worth, I still had my dead reckoning flight plan, but that was based on best guess winds aloft in Honolulu, now far behind me. My chances of finding tiny Majuro, a half-moon coral atoll about three miles across counting the lagoon in the center and still 1,000 nm away, were marginal, at best.
I called up San Francisco long range on HF and asked if they could check with Majuro on the problem with the NDB.
San Francisco called back a few minutes later and said they were still trying to contact Majuro. Apparently, there’d been a major power outage, and even San Francisco couldn’t raise anyone at Majuro.
Meanwhile, with no better plan in place, I kept doing what I’d been doing, hoping that someone could get the NDB back on the air. Power failures are common on mid-Pacific atolls, and they usually don’t last long, I kept telling myself.
Sure enough, a half-hour later, the needle came back to life and rotated back to top center. At the same time, San Francisco called and said someone at Majuro had forgotten to refill the generator’s emergency fuel tank. When the power failed, the voltage drop was supposed to kick the generator back on and keep the NDB on the air. No gas, no signal.
Perhaps sadly, the advent of GPS has eclipsed ADF in both OEM and the aftermarket. Most aircraft manufacturers still offer ADF as an option, but many buyers don’t bother to check that box, because GPS is regarded as invincible.
Don’t believe it. I was flying a Cessna Grand Caravan from Guam to Seoul, Korea, on the last leg of a ferry delivery from Santa Barbara a decade after the Majuro incident and was treated to a GPS “blink,” a brief signal loss on both panel-mount GPSs and my two portables.
I was flying in strong headwinds toward Hiroshima, Japan, and the airplane was 1,700 pounds over gross with ferry fuel. Plus, with a cargo pod hanging down, the Caravan was about as aerodynamic as a stagecoach, so I was only grounding (watering?) 120 knots. Plus, Dumbo, the dancing bear in the left seat, hadn’t checked the ADF before departing from California. It turned out the ADF was stone cold dead.
Japan and Korea are hard targets to miss if you can merely read a compass, but I was still grateful the GPS signal came back online in about 20 minutes.
Before you sign on to your MacBook or Dell to ask, yes, I’m a big fan of GPS. I have two of them on my current Mooney’s panel, plus a semi-permanent Garmin backup on top of the dash, and they’re my primary nav sources. I also have a small Terra ADF tucked into the lower right corner of the panel, and it’s on most of the time during every flight, just in case GPS decides to blink at me again.
Check out more Cross-Country Log flying stories from ferry pilot and Senior Editor Bill Cox.