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Paul Bertorelli |
This originally appeared in IFR MAGAZINE and is reproduced here by permission of Belvoir Publications.
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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.
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