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The Two Fundamental Concepts that Greatly Simplify NDB Approaches

By Doug Daniel

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Published: 28Jan2009
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It was an instrument lesson and an actual approach. We were flying a Piper Arrow into the Monterey Peninsula Airport just north of the famous Pebble Beach Country Club. The localizer was out of service. The only available approach was the NDB (non-directional beacon) approach using the outer compass locater nestled up against the outer marker on the breakwater where the peninsula met the Pacific. Flying the Monterey Bay coast is one of the most beautiful sights in aviation. Above the cloud layer the sky is dazzling. Green and brown hills push up through the seemingly perpetual low stratus more than a thousand feet above the cloud tops. On that day, the tops were less than 1,000 MSL.

One fundamental principle of NDB approaches is to fly the course that takes you from the radio beacon to the missed approach point. The other fundamental principle is that the accuracy is limited by the accuracy of your gyrocompass. Any drift or uncompensated deviation degrades your ability to fly an accurate course.

The NDB approach requires that you use your gyrocompass for heading reference and your radio compass (automatic direction finder, a.k.a. ADF) for orientation. The fatal mistake is to simply follow the needle. The seductive part of an NDB approach is that half of the time, following the needle works just fine. That half is until you pass over the beacon. Once your needle points back at 180°, all the radio compass tells you is that your tail is pointing at the radio beacon. You could be flying in any direction.

I decided to let Ralph try the approach and told Approach Control that we would execute a missed approach at or before the missed approach point (MAP). Ralph and I had talked about NDB approaches. I was not sure he understood them. Here was a place where a mistake could be fatal. So during our procedure turn outbound and still above the clouds, I told him to be prepared for me to take over and execute the missed approach.

We descended into the clouds. Shortly after passing the NDB, we were 30° degrees off course and headed into high terrain. I asked Ralph what heading would get us to the airport. He could not tell me and said we were doing just fine. He started to argue with me about flying a missed approach and who should be flying. I applied full power, retracted flaps and gear, and told ATC that we were missed. Ralph decided I was serious and capitulated. Once we were on top Ralph stared at the mountains and turned pale.

Assume the course from the NDB to the missed approach point is 100°. Make it easy by assuming that the inbound course to the NDB is also 100°. Fly a 30° intercept to the inbound course. This would be either 70° or 130° depending on the approach procedure. If it is 130°, your heading is 30° to the right of the inbound course. Your ADF would read 30° to the left of straight ahead or 330° when you intercept the course that you want to follow. If you are two miles or more from the NDB, start the turn inbound just a few degrees early so you can roll to wings level just as the ADF centers. Initially fly the magnetic heading shown on the approach procedure. In a few moments the ADF will start to move to either the right or left. It always does.

To interrupt the narrative, how do you know when you are on the path that takes you over the NDB to the airport? The answer is when the ADF needle is off to one side and the gyrocompass if off by an equal angle to the other side. Earlier your gyrocompass was off by 30° to the right and your ADF was off by 30° to the left so you were on the course but not on the proper heading. We'll try to nail this down next.

Continuing to track inbound to the NDB, fly trial headings and watch the ADF. Your first try is the heading on the chart. If the wind blows you to one side, turn back far enough that the ADF is pointing to the other side. Suppose the wind blows you to the left until the ADF points to 005°, turn right until it points to -005° which is really 355°. This would require a 10° turn changing your heading from 100° to 110°. Hold that course until the ADF moves left to -010° or 350°. If the ADF moves right, then you did not turn far enough, but let's say you did. Once again the radio compass is off an equal but opposite angle from the gyrocompass. You are on course but not on the proper heading. You know that the correction angle is between zero and ten degrees because zero was taking you to the left of the inbound course and ten would have taken you to the right of it if you had continued. If the wind compensating angle is between zero and ten degrees, then the heading to fly is between 100° and 110°.You may as well try half way between. That would be 105°. It won't work either exactly, so keep refining your heading.

This is the procedure that you follow all the way to the missed approach point. Once past the NDB your corrections are left or right of straight back rather than straight ahead.

To put it in a nutshell, for an NDB approach or any other kind of approach, fly headings, don't chase needles.

Doug Daniel is a long time pilot, flight instructor, software engineering manager and author. His department developed the software for the out-the-window-displays for the space shuttle, F-117, RS-71 and numerous other exotic airplanes. 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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