NDB (ADF) Approaches

NDB (ADF) Approaches

NOTE: The assumption is made that the reader has an understandingof the material covered in the previous tutorials, UnderstandingIFR, what it is, how to do it, VORApproaches, and VOR/DMEApproaches.

This can be the least exact of instrument approaches. For thatreason, usually the minimums for descent altitude and visibilityare higher than for other types.

Here is an NDB approach plate forthe old Denver, Stapleton International Airport

For this approach, we will join the outbound leg on the "airportside" 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'sinstruments look at each point on the approach.

Also, note that in the case the NDB is the LOM: Locator, OuterMarker.

Your position relative to an NDB is known as a "bearing",not a "radial". If you are due South of an NDB, youare on the 180 degree bearing from the NDB, you are not on the180 degree radial.

As an aside, your chances of making this full of an NDB approachat a busy field like Denver, are as likely as... well, they'renot very likely.

For this exercise, we will use a "non-slaved" ADFdisplay. A "non-slaved" ADF display is an ADF instrument/cardthat does not show the same magnetic heading as the HSI. As theaircraft turns to various headings, the ADF display remains fixedand does not rotate with the change in the aircraft's heading.In order to change the displayed heading, you must reach downand rotate the card with the knob that is on the lower left ofthe display.

Tomake this tutorial less confusing, each time the aircraft's headingchanges, we will "reach down" and change the ADF displayedheading to match the aircraft's heading.

OK, now that the preliminary stuffis out of the way, let's shoot this approach into the old DenverStapleton 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", levelat 9 thousand feet, heading 030 degrees.

"Red Baron 123, Denver Approach. Turn left heading020,intercept the 258 degree bearing of the Altur Outer Marker andtrack it inbound to the Marker. Descend to and maintain 8,000."Roger, Red Baron 123. 020 on the heading, join the bearing andtrack it outbound. We're out of ten for eight, Red Baron 123."

At "1" your HSI and ADFwill 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 sothat the reading agrees with the HSI. Leveling at 8,000 feet Approachcalls. "Red Baron 123, I show you just coming up on the Altur258 bearing. Turn right heading 080. You're cleared the NDB 26Left approach into Denver Stapleton Airport. Report ProcedureTurn Inbound." "Cleared the approach, we'll call theturn inbound, Red Baron 123." You turn right to a headingof 080, and rotate the ADF card so that it agrees with the HSI.

You are at position "2".

At "2" your HSI and ADFwill look like this

Level at 8,000 feet you hear the sound of the Middle Marker"dah, dah, dah" and see the blue "OM" lightflashing. You are crossing over Altur, and remember that the NDBis located at the Outer Marker. In fact, Altur is classified asan LOM, Locator Outer Marker. At this time the ADF needle willswing around 180 degrees, and point behind you at the recedingNDB. Note the time. Just like when flying a VOR approach withoutDME information, this will be a timed approach. Let's say thatyour trusty Red Baron uses an approach speed of 120 knots, andthat you are at that speed as you cross Altur. The plate showsthat the Procedure Turn must be completed within 10 nautical milesof 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 ADFwill look like this

There is a strong Southerly flow of air aloft, and you seethat 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 beon the 078 bearing. You must get back on course. For starters,"double the error", in other words you are 10 degreesoff course, turn right 20 degrees to a new heading of 100. Youwatch as the ADF needle comes back showing that you are once againon the 078 degree bearing. Now, you know that a heading of 080will allow you to drift off of course to the left. A heading of100 will bring you back to course. You cannot maintain a headingof 100 as that would put you right of course. So, halve the 20degree correction that you used earlier. Turn back left to a headingof 090 degrees. You note, with satisfaction, that this does thetrick and that you are now once again tracking on the 078 bearing.You make a mental note that after the Procedure Turn, when youare inbound, that you will probably have to steer to the left(South) to compensate for drift. At this point, your HSI and ADFwill look like this:

Notethat although the ADF needle is not straight up and down, it doesshow that you are on the 078 bearing from the NDB. You are oncourse, thanks to your correction for the wind drift. You arelevel at 8,000 feet and on course. Two and one half minutes havenow passed since you crossed over Altur. Time to start your ProcedureTurn. Turn left to a new heading of 038 degrees. Also, rememberto turn the ADF card to the same new heading. Why not as the chartshows, 033 degrees? Sure, it's that Southerly flow of air. Youknow that it will drift you to the North, might as well throwin a little Kentucky Windage to compensate. Note the time. Youwill need to make your 180 degree turn in one minute, or less.

You are at position "4".

At "4" your HSI and ADFwill look like this

Youknow that you are being pushed North by the wind. No sense incompounding the error by flying this heading for a full minute.Start your course reversal in, say, 50 seconds. After 50 secondson this heading, start your course reversal. Turn 170 degreesto a new heading of 208 degrees. (5 degrees left to compensatefor that old devil wind.) Turn the ADF card to the same heading.

You are now at position "5".

At "5" your HSI and ADFwill look like this

Youare now tracking back towards the final approach bearing. In abouta minute you can expect the ADF needle to show the intercept ofthe 078 degree bearing from the Altur NDB. The ADF needle slowlytracks clockwise as you close on intercept. As the ADF needlecrosses through the 080 degree bearing start your turn right toestablish yourself on the inbound bearing of 078 degrees. Keepingin mind the left to right wind aloft, and the fact that a 5 degreecompensation worked for you outbound, make your heading 253 degrees.Reset the ADF card. You are now Procedure Turn Inbound. Time tocall Approach. "Denver Approach, Red Baron is the turn inbound."(Approach uses the Procedure Turn Inbound as a metering pointfor traffic.) "Roger Red Baron 123, contact the Tower atAltur." "Tower at Altur, 123." According to theplate, you may now descend to 7,000 feet.

You are now at position "6".

At "6" your HSI and ADFwill look like this

Youare on the 078 degree bearing, on course. Level at 7,000. Your5 degree heading to the left of course is doing the job compensatingfor the left to right drift from the Southerly winds aloft. TheMarker Beacon receiver starts to come alive again. You are nearingAltur once more. Approach power is set. Drop your first notchof flaps. If you have your power set correctly, this drag shouldtake 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, 5,880feet, 547 feet above the Stapleton runway. "Approach, RedBaron is Altur inbound." "Roger Red Baron 123, contactStapleton 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 toland runway 26 Left. And say, did you pick up any ice on the approach?""Cleared to land, 26 Left, and no, negative icing on theapproach." "OK, thank you 123, and you're cleared toland."

You are now at position "7".

At "7" your HSI and ADFwill look like this

Youpass beneath the cloud bases going through 6,000 feet and breakout into visibility greater than 10 miles. The runway is rightin front of you. A perfect ADF approach! Good for you. Full landingflaps, adjust the power as necessary, cross the threshold of 26Left and set your trusty bird down. You and your partner havedone a good job. Taxi into the Combs FBO and have that cup ofcoffee you promised yourself when you were at the Procedure TurnOutbound.

This ends the NDB/ADF tutorial.

Corrected, 1/8/2001: ADF bearing was incorrect at position#5. Thank you to J. Paul Espy for pointing this out.

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