NDB (ADF) Approaches
© Hal Stoen, September 9, 2000
NOTE: The assumption is made that the reader has an understanding of the material covered in the previous tutorials, Understanding IFR, what it is, how to do it, VOR Approaches, and VOR/DME Approaches.
This can be the least exact of instrument approaches. For that reason, usually the minimums for descent altitude and visibility are higher than for other types. Installing a NDB (Non-Directional Beacon) is the least expensive way for an airport to get themselves an instrument approach- except for using existing GPS. For that reason most ADF approaches are at small airports. The ADF is also the oldest of the various approach types and requires no ground calibration. In addition, the NDB is a wonderful orientation devise as you shoot other types of approaches. If you can fly a full NDB approach: approach the NDB, fly over it and intercept the outbound bearing, make a procedure turn, fly back to the NDB on the approach bearing, cross over the NDB again and fly the outbound bearing to the published minimums you can fly any approach with confidence.
Here is an NDB approach plate for the old Denver, Stapleton International Airport
For this approach, we will join the outbound leg on the "airport side" of the Beacon, fly outbound, make a procedure turn, join the final approach course and fly it inbound to a landing. Because the presentation of the ADF display can be confusing, we will annotate the approach plate and examine how the aircraft's instruments look at each point on the approach.
Also, note that in the case the NDB is the LOM: Locator, Outer Marker.
Your position relative to an NDB is known as a "bearing", not a "radial". If you are due South of an NDB, you are on the 180 degree bearing from the NDB, you are not on the 180 degree radial.
As an aside, your chances of making this full of an NDB approach at a busy field like Denver are as likely as... well, they're not very likely. However, there are many small airports that only have an ADF approach into them.
For this exercise, we will use a "non-slaved" ADF display. A "non-slaved" ADF display is an ADF instrument/card that does not show the same magnetic heading as the HSI. As the aircraft turns to various headings, the ADF display remains fixed and does not rotate with the change in the aircraft's heading. In order to change the displayed heading, you must reach down and rotate the card with the knob that is on the lower left of the display. (I'm doing it this way only to make this more of a mental exercise in order to better understand how the ADF display works. If you wish to use a "slaved" ADF display simplly ignore the remarks about setting the ADF card- the displays on this tutorial always show the HSI display and the ADF card on the same heading.)
To make this tutorial less confusing, each time the aircraft's heading changes, we will "reach down" and change the ADF displayed heading to match the aircraft's heading.
OK, now that the preliminary stuff is out of the way, let's shoot this approach into the old Denver Stapleton Airport
The ADF tuner head is set to the frequency of the NDB "Altur", 362 KC:
You are at the position shown with the red "1", level at 9 thousand feet, heading 030 degrees.
"Red Baron 123, Denver Approach. Turn left heading020, intercept the 258 degree bearing of the Altur Outer Marker and track it inbound to the Marker. Descend to and maintain 8,000." Roger, Red Baron 123. 020 on the heading, join the bearing and track it inbound. We're out of ten for eight, Red Baron 123."
At "1" your HSI and ADF will look like this
(NOTE: For clarity purposes, the CDI will not be shown in the HSI illustrations.)
You start your descent to 8,000 feet. Turn the ADF card so that the reading agrees with the HSI. Leveling at 8,000 feet Approach calls. "Red Baron 123, I show you just coming up on the Altur 258 bearing. Turn right heading 080. You're cleared the NDB 26 Left approach into Denver Stapleton Airport. Report Procedure Turn Inbound." "Cleared the approach, we'll call the turn inbound, Red Baron 123." You turn right to a heading of 080, and rotate the ADF card so that it agrees with the HSI.
You are at position "2".
At "2" your HSI and ADF will look like this
Level at 8,000 feet you hear the sound of the Middle Marker "dah, dah, dah" and see the blue "OM" light flashing. You are crossing over Altur, and remember that the NDB is located at the Outer Marker. In fact, Altur is classified as an LOM, Locator Outer Marker. At this time the ADF needle will swing around 180 degrees, and point behind you at the receding NDB. Note the time. Just like when flying a VOR approach without DME information, this will be a timed approach. Let's say that your trusty Red Baron uses an approach speed of 120 knots, and that you are at that speed as you cross Altur. The plate shows that the Procedure Turn must be completed within 10 nautical miles of Altur. At 120 knots you will cover 10 nautical miles in 5 minutes. Plan on starting your Procedure Turn in two and one half minutes.
You are at position "3".
At "3" your HSI and ADF will look like this
There is a strong Southerly flow of air aloft, and you see that you are drifting to the left of course. Before you correct, your HSI and ADF will look like this:
You are on the 068 bearing, whereas you are supposed to be on the 078 bearing. You must get back on course. For starters, "double the error", in other words you are 10 degrees off course, turn right 20 degrees to a new heading of 100. You watch as the ADF needle comes back showing that you are once again on the 078 degree bearing. Now, you know that a heading of 080 will allow you to drift off of course to the left. A heading of 100 will bring you back to course. You cannot maintain a heading of 100 as that would put you right of course. So, halve the 20 degree correction that you used earlier. Turn back left to a heading of 090 degrees. You note, with satisfaction, that this does the trick and that you are now once again tracking on the 078 bearing. You make a mental note that after the Procedure Turn, when you are inbound, that you will probably have to steer to the left (South) to compensate for drift. At this point, your HSI and ADF will look like this:
Note that although the ADF needle is not straight up and down, it does show that you are on the 078 bearing from the NDB. You are on course, thanks to your correction for the wind drift. You are level at 8,000 feet and on course. Two and one half minutes have now passed since you crossed over Altur. Time to start your Procedure Turn. Turn left to a new heading of 038 degrees. Also, remember to turn the ADF card to the same new heading. Why not as the chart shows, 033 degrees? Sure, it's that Southerly flow of air. You know that it will drift you to the North, might as well throw in a little Kentucky Windage to compensate. Note the time. You will need to make your 180 degree turn in one minute, or less.
You are at position "4".
At "4" your HSI and ADF will look like this
You know that you are being pushed North by the wind. No sense in compounding the error by flying this heading for a full minute. Start your course reversal in, say, 50 seconds. After 50 seconds on this heading, start your course reversal. Turn 170 degrees to a new heading of 208 degrees. (5 degrees left to compensate for that old devil wind.) Turn the ADF card to the same heading.
You are now at position "5".
At "5" your HSI and ADF will look like this
You are now tracking back towards the final approach bearing. You are on (about) the 065 bearing from the NDB. In about a minute you can expect the ADF needle to show the intercept of the 078 degree bearing from the Altur NDB. (You are north of your desired bearing from the NDB, the 078 bearing. That means that you are on a bearing from the NDB that is "less" than 078- in this case 065.) The ADF needle slowly tracks clockwise, pointing toward the NDB, as you close on intercept. (On your heading of 208 degrees you are flying towards bearings from the NDB of "increasing degrees." In other words, right now a heading of 245 degrees- no wind- would take you to the NDB. In a minute or so a heading of 258- the 078 bearing from the NDB- will take you to the NDB. If you cross through that bearing, the bearings from the NDB will become greater and greater.) As the ADF needle crosses through the 080 degree bearing start your right turn to establish yourself on 078 degree bearing. Keeping in mind the left to right wind aloft, and the fact that a 5 degree compensation worked for you outbound, make your heading 253 degrees. Reset the ADF card. You are now Procedure Turn Inbound. Time to call Approach. "Denver Approach, Red Baron is the turn inbound." (Approach uses the Procedure Turn Inbound as a metering point for traffic.) "Roger Red Baron 123, contact the Tower at Altur." "Tower at Altur, 123." According to the plate, you may now descend to 7,000 feet.
You are now at position "6".
At "6" your HSI and ADF will look like this
You are on the 078 degree bearing, on course. Level at 7,000. Your 5 degree heading to the left of course is doing the job compensating for the left to right drift from the Southerly winds aloft. The Marker Beacon receiver starts to come alive again. You are nearing Altur once more. Approach power is set. Drop your first notch of flaps. If you have your power set correctly, this drag should take you into your gear extension speed on the airspeed indicator. "Dah, dah, dah". The ADF needles swings around 180 degrees. You are at Altur. Gear down. Start your descent to your MDA, 5,880 feet, 547 feet above the Stapleton runway. "Approach, Red Baron is Altur inbound." "Roger Red Baron 123, contact Stapleton Tower at this time. Good day!" "Good day Sir." "Good afternoon Tower, Red Baron 123 is with you, Altur inbound." "Red Baron 123, Stapleton Tower, not in sight, cleared to land runway 26 Left. And say, did you pick up any ice on the approach?" "Cleared to land, 26 Left, and no, negative icing on the approach." "OK, thank you 123, and you're cleared to land." No one knows better about the conditions "out there" than you do, that's why the Tower asked you about the icing conditions- they'll be able to pass that information on to the other crews that will follow you.
You are now at position "7".
At "7" your HSI and ADF will look like this
You pass beneath the cloud bases going through 6,000 feet and break out into visibility greater than 10 miles. The runway is right in front of you. A perfect ADF approach! Good for you. Full landing flaps, adjust the power as necessary, cross the threshold of 26 Left and set your trusty bird down. You and your partner have done a good job. Taxi into the Combs FBO and have that cup of coffee you promised yourself when you were at the Procedure Turn Outbound.
This ends the NDB/ADF tutorial.
© Hal Stoen, September 9, 2000
Corrected, 1/8/2001: ADF bearing was incorrect at position #5. Thank you to J. Paul Espy for pointing this out.
Corrected, 12/31/06: Added additional remarks about the ADF approach. Wording change to "inbound" on initial approach clearance. NDB bearings at positions 4 and 5 corrected. Wording added at position 5 to help clarify the aircrafts position relative to the NDB. Thank you to Scott Lea for pointing this out.
Corrected 9/3/2007. Wrong ADF image at position #6, Thank you to Steven M. Smith for calling this to my attention.
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