Double-Checking the Vector

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A special supplement to J. Ross Russo's article "Vectors to Final."

Near the end of a long trip, actual IMC and perhaps flown with no autopilot, being vectored to final can be a great relief. But there's a downside, too. A pilot anxious to get established on the approach and on the ground might be lulled into allowing position awareness to lapse at a time when he or she can least afford it. It's all too easy to simply tune the localizer and wait for the needle to center.

What the pilot should be doing, of course, is verifying that the controller's vectors will result in an intercept far enough outside the final approach fix (or approach gate) to allow for a stabilized approach. As you fly toward the inbound, on your final vector, the angle that you'll intercept the localizer is equal to the difference between your vector groundtrack (not the heading) and the localizer course. Ideally, this should be between 20 and 30 degrees. Of course, you probably won't know what the wind is doing so at best, you'll have only a vague idea of the groundtrack.

On an ILS equipped with a LOM, the best way to monitor the intercept angle is to tune the LOM and track bearings on the ADF. To achieve a 20 to 30 degree intercept (and one that'll intersect somewhere outside the marker) the ADF needle should be about 20 or 30 degrees off the nose and on the opposite side as the localizer. In other words, if the localizer says fly left, the ADF needle should be off the right side of the nose, and vice versa.

Since the relative bearing equals the intercept angle at the moment of interception, the ADF will give some idea of how soon the intercept will occur. In the example shown here, the relative bearing is 20 degrees, the intercept angle about 30 degrees. Ten degrees difference; a ways to go yet. At five degrees difference, watch for the localizer needle to come alive.

Ask for another vector if the ADF needle is on the same side as the localizer needle (interception inside the marker) or right off the nose, signaling intercept at the marker. An HSI simplifies the intercept somewhat. The vector heading should result in the lubber line being more or less off the tip of the CDI. This will assure a reasonable intercept angle. But monitoring the ADF is still the only way to tell where the vector will intercept the final course.

A sure source of confusion is to get a vector opposite the direction you expect. This sometimes happens when you've been vectored overtop the airport outbound but aren't sure which side of the localizer you're on. A quick method of reorienting yourself is to imagine the words Black-On-White written across the VOR face, with black on the left, white on the right. If the CDI needle is on the black side, you're on the shaded side of the ILS feather.