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Spoon Feeding: Behind the Scenes On Vectored Approaches

Getting vectored onto an IAP close to the final approach fix definitely saves time, and it makes efficient use of busy terminal airspace -- but now you’re actually working under another set of rules that most of us don’t know too much about.Getting vectored onto an IAP close to the final approach fix definitely saves time, and it makes efficient use of busy terminal airspace -- but now you’re actually working under another set of rules that most of us don’t know too much about. When you get right down to it, most instrument approaches are helped along with radar vectors from ATC. (This is especially true in the northeast corridor where I fly.) There are some “behind the scenes” considerations that are worth looking at...

Procedure turns are charted for most approaches (because the possibility of radar failure always exists), but are more often not needed due to radar vectors. You should still always be ready to fly the full procedure on a moment’s notice, however. Let’s start out with some precision approach anatomy.

  • Instrument pilots already know about the localizer, which is 3º to 6º wide, with 5º being average;
  • the glide slope;
  • the outer marker (which is from four to seven miles from the runway and is usually about where you normally intercept the glide slope); and
  • the middle marker (about a half-mile from the runway and normally about where you’ll reach Category I decision height).
But there is also something called an approach gate along every final approach course, before the final approach segment, whenever radar vectoring is in progress. It’s one nautical mile outside the final approach fix (FAF) or five nautical miles from the runway -- whichever is greater. (This isn’t the same thing as a TRACON’s arrival gate, which is another procedural term altogether.) Then at least two nautical miles outside of that, there’s something called an intercept point.

Inside Information: When you get vectored onto an approach, they’re actually shortening the intermediate segment of that approach for you, and that’s what they’re directing you towards. Normally, the intermediate segment is at least five miles long, but it can be cut down to three, and if the ceiling isn’t lower than 500 feet above the minimum vectoring altitude and the visibility at the airport is at least three miles, it can be even less. If you ask, they can vector you right to the FAF. If it’s not VMC though, you’ll get sent to that intercept point.

The controller consideration for vectored approaches is that they must ensure your interception of the approach course at a shallow enough angle to allow for a smooth transition. The controller’s handbook says that if they vector you to a point more than two nautical miles from the approach gate, that intercept angle can be no greater than 30º. And if it’s less than two miles outside the gate (and the above weather minima must exist for this), that intercept angle maximum becomes 20º. (This intercept can be no closer than the approach gate, unless the pilot requests it, in which case it still can’t be inside the FAF.) The “normal” limit in most situations is 30º.

A guy could get spoiled! (In fact, collectively, we probably are; vectors save time, but the price is usually a lower level of positional awareness.) But don’t get too cozy. If you do the math you’ll see that you could still blow right through that localizer if your scan is rusty.

Example: On a “short” approach with four miles to the outer marker and a 30º intercept at two miles from the gate, a five degree localizer is about six tenths of a mile wide, and at 30º, you’d cross it at exactly half of your ground speed. If your “FAF to MAP” is 120 knots, you have 18 seconds between your needle’s first stirring until dead center. With a 20º intercept two miles closer in, that “blow through” time winds up being about the same.

Translation: Be more than 10 seconds behind the airplane, and you have to bust standard rate to catch up. Though this beats a 45º intercept, it’s still no time to slack off.

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