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NDB Approaches Made Easy

By Doug Daniel

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Published: 06Mar2009
Word count: 986
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The NDB approach is the oldest instrument approach and the most difficult to fly. For me, it is the approach of last resort. A non-directional beacon is a very simple AM, low power radio transmitter located near an airport. It sends out a Morse code signal that you can listen to for identification. That's all; no azimuth or range information.

The instrument that tells you what direction you are headed is the gyrocompass after it has be aligned with the magnetic compass. The instrument that tells you where the NDB is relative to your airplane is the automatic direction finder (ADF). The ADF looks like the gyrocompass but the compass rose has a different meaning. 360° means straight ahead of the airplane, 90° means to the right, and so on around. You have to use both of the compasses to fly a successful NDB approach.

Let me set up a hypothetical and straightforward NDB approach. Assume that the NDB is three nautical miles from the missed approach point (MAP); the wind is from the northwest at 14 knots; the approach speed is 100 knots; the minimum descent altitude (MDA) is 350' MSL; and the final approach course is 360° magnetic. The approach procedure tells you to pass over the NDB at 950' MSL. The NDB also serves as the final approach fix (FAF).

Knowing all this, you do some mental arithmetic. The 14 knot wind has a 10 knot crosswind component and a 10 knot headwind component. We can see that the ground speed on final approach will be 90 knots. Fortunately 90 knots is 1.5 miles per minute. So if you must go 3 miles from FAF to MAP, the approach will take 2 minutes. The FAF altitude is 600 feet above the MAP altitude, so the final approach vertical speed should be 300 feet per minute (fpm).

Let's start after you have descended to 950'MSL and have turned inbound to intercept the final approach course. You fly a heading of 30° until the ADF reads 330°. The airplane is now directly south of the NDB. A turn to 360° puts the plane on a bearing directly to the NDB and both the gyrocompass and ADF read 360°. As the airplane approaches the NDB the crosswind component blows you off course to the right.

The ADF tells you that the plane is off course by pointing to the left of straight ahead by a few degrees. You cannot turn the airplane by those few degrees and head directly toward the NDB again and hope to intercept the inbound course because the wind will just blow you off course again. You know that you need to turn into the wind to some degree if you are to find the crab angle that will keep you on course to the NDB. If the ADF told you that the NDB is 5° to the left, you need to turn to a course that is more than 5° to the left of the NDB. Read this next paragraph very carefully. It is tricky.

You know that you must turn into the wind to compensate for its drift. The ADF told you that you have moved to the right relative to the NDB. Therefore you must turn to the left. If you turned 5°, you would be pointing directly toward the NDB. You also know that you must be heading to the left of the NDB to compensate for the crosswind. If you did not move any closer to the NDB during your turn, you might turn 5° to the left of the NDB and be on a ground track that would take you directly over the NDB. But you have moved closer to the NDB and you want to intercept the inbound course before you get to the NDB so you have more time to fine-tune your inbound heading.

You turn the airplane 15° to the left. Both the ADF and the gyrocompass respond. The ADF changes from 355° to 010°. The gyrocompass changes from 360° to 345°. You want to stay on that heading until intercepting the inbound course. You would like to see the ADF change from 010° to 015°. You monitor the ADF to see if it starts to swing back to the right. If it doesn't, you need to increase the correction angle. If the needle does swing to the right, you will be on course when the ADF points the same number of degrees but in an opposite direction from the apparent error on the gyrocompass. In this case, the ADF would read 015° and the gyrocompass would read 345°. Once you are on course, turn right to a heading of 355°. That is based on the belief that a 5° correction to the left is the proper drift correction angle. You continue to fine-tune your approach heading all the way to MAP at MDA.

Here is a mental trick that works for me that you might try: When looking at the ADF, I think of 345° as -15° or as 15° to the left of straight ahead. I visualize it as one big mark and one small mark to the left of straight up.

As you approach the NDB, you stabilize the airspeed at 100 knots indicated airspeed (KIAS). When the ADF starts to be extremely sensitive, you know that you are very close to the transmitter and simply fly what you believe to be the best heading. When the ADF suddenly reverses itself, you note the time or push a timer button. You reduce power to a setting that should yield a 300 fpm descent. You continue to track toward the airport, maintaining a very constant airspeed. You adjust the power as needed to establish your 300fpm or slightly greater descent.

Ideally you should be at MDA before you reach the MAP, otherwise you may never descend to MDA. You are very careful to never descent below MDA.

When the timer indicates that the plane has flown for two minutes past the FAF, and you do not have the airport environment in sight, you must execute the missed approach procedure.

Doug Daniel is a long time pilot, flight instructor, sailor and author. His passion is sharing his insights with all who love to fly. His writing focuses on flying techniques designed to make flying easier and safer. If this was interesting, visit his website at http://www.FlyingSecretsRevealed.com/flying_questions/

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