Do you want to understand the segments of an approach? It's simple -- just design a fictitious IAP from scratch and you'll never forget.
March 5, 2003
This article appeared in the March 2002 issue of IFR Refresher magazine, and is reprinted here by permission.
You probably haven't been called on lately to define the segments along an approach course, but you might remember a time when it would have been handy to know why the designer created such an obstacle course to a final approach fix. Or perhaps you have been part of a hangar-flying session that tried to pin down just when it is that a flight is expected to report "established." If you were to design an instrument approach to an airport by trial and error, you quickly would realize that what you are trying to design is a navigation course from the en route altitudes, from some MEA, down to an obstacle-free altitude where an airplane can slow down and get configured to follow navigation guidance to a runway. It would occur to you that it would be hard to design such a course the way it is flown and that it would be sensible to start at the airport and work backward, creating first the final approach course to the runway.
|Flying the System
An Obstacle-Free Surface
First you'll need to account for the obstacles along the final approach course and decide what kind of navigation guidance that you can afford. Standing midfield and looking up the final approach course, you will note that at some point you will have to establish a minimum altitude at which the final approach can begin and end, again taking obstacles into account and yet providing an inbound flight with a reasonable chance of descending to the runway surface somewhere near the beginning of the runway. As you sketch your design, you might label the beginning of this final segment the "final approach fix" and end the course at the runway or at some point and height above the ground where the flight could launch a missed approach if it wasn't in a position to land.
Getting To Final
It will occur to you that airplanes will need some kind of positive navigation guidance to get from the en route structure to your final approach course. Airplanes will need a segment to descend and get configured before flying down the final course. So you've got to go back to your sketch pad and draw in another leg or "intermediate segment" from the final approach fix that is long enough to give the crew time to slow down and get set up for the final. Five miles gives a Cessna 172 pilot about two minutes to slow down; a hot Bonanza pilot might appreciate a 10-mile intermediate segment. You'll need to figure out an altitude that will keep airplanes and obstacles apart, and you might choose 500 feet for this segment. Finally, you need to connect this intermediate segment to the en route structure, and you might do that by providing your sketch with a course of, say, 20 miles to allow aircraft a chance to descend from MEA. You might call this leg the "initial approach segment," and it will terminate at the intermediate segment. You might begin the initial segment at some navigational fix that can be positively identified in the airplane. You will create an obstacle clearance for this segment that is similar to the en route -- 1,000 feet -- unless you're in the mountains, where you'll need to establish 2,000-foot obstacle clearance. Good job. You have worked out an instrument approach that lets a pilot leave the en route structure MEA and fly to an initial approach fix, descend and fly to the intermediate segment, where he'll get slowed down and configured for an approach that will begin at the final approach fix. If you're still standing midfield, you might look upwind for a moment and sketch in a way for the airplane to fail the approach and climb to a safe altitude, again avoiding obstacles. And you'll need to create a navigation fix for the airplane that has missed the approach and will need a fix out there to hold.
Lots Of Geography
Looking at your sketch pad, you may have drawn a straight line from the en route structure to the initial approach fix, then to the intermediate fix and on for 10 more miles or so to the final approach fix, then down to the runway and beyond to some fix where an airplane can fly when it has missed your approach. You may have drawn a line that is, say, 50 miles long, which is fine for airplanes coming from the direction of your initial approach fix but a bit awkward for flights approaching from the other direction, considering that they will have to fly a 50-mile cross country before they can even reach your IAF, and then they've got to turn around and come back. It won't be a popular approach. And there may be mountains or tall towers along that straight-line course as well, which will affect your allowances for obstacle clearance and lengthen the segments even more. So you may want to redesign the approach in a way that tucks all of the segments into, say, a 10-mile area. There are a lot of options. If you set up the nav facility on the airport, for example, you could create an initial approach fix there, several thousand feet above the runway, and run the initial segment out, say, five or six miles, which would give most small airplanes a couple of minutes to descend. Now you'll have to figure out a way to turn airplanes around for the approach. You could turn them left or right, depending on obstacles, for a minute and then turn them again 180 degrees so that they would be in position to intercept your intermediate segment, get established, slow down, descend and drive on down to the runway along the final approach course.
Fly The Funnel
You can see that there are some pretty important decisions to be made before you publish your approach, so it might be worth your trouble to fly this funnel that you have created. The big end of the funnel may have several initial approach fixes to accommodate traffic that will arrive from various directions. The obstacle clearance out here is 1,000 feet -- or 2,000 if you've got mountains to worry about. You are going to use the initial approach segment to descend the airplane at 500 feet per nautical mile to the intermediate segment. There are three ways to get an airplane from the initial approach fix to the intermediate. The airplane can be radar vectored to the final approach course so that you arrive and get established on the intermediate course several miles from the final approach fix. Or the initial approach segment might be a procedure turn, a way to align the airplane with the intermediate course. And the third type of initial approach segment is one that doesn't require a procedure turn and where the initial approach is a part of the en route structure. Your 50-mile approach course would fall into this category of initial approach segment, and so would an arc, a kind of slick way to open an initial approach fix to traffic coming from many directions flying a specific track to an intermediate segment.
The intermediate segment of an approach exists simply to allow transition from en route airplane configuration to final approach configuration, and it has an optimum descent gradient of 150 feet per nautical mile to a maximum of 300 fpnm. There is 500 feet of obstacle clearance in the primary area of the intermediate segment -- as you slide into the throat of the funnel -- and the area that you have on either side of the centerline also gets smaller as you get closer to the final approach fix. As you notice, you can be a little sloppy out there in the initial segment, but you've got to sit up and fly right by the time that you're navigating the intermediate. The final approach segment starts at the final approach fix and ends at the runway or missed approach point, whichever comes last. Your navigation needs to be right on the money here, and the descent gradient and obstacle clearance criteria will depend on the nav guidance, as you can well imagine. The missed approach segment will begin at decision height on a precision approach and at some specified point on a non-precision procedure. The missed approach segment has got to be simple, with a specified altitude sufficient to hold or begin en route flight. Whenever it's possible, the missed approach course ought to be a continuation of the final approach course, and you'll need a clearance limit. Of course, there can't be any obstacles penetrating the missed approach segment, which begins at the missed approach point and ascends uniformly at the rate of one foot vertically for each 40 feet horizontally, or 40:1. When the 40:1 surface reaches 1,000 feet below the missed approach altitude, you can abandon the 40:1 rule.
So there you have the segments, backward and forward, and though you may decide to get out of the approach-design business after reading this, you'll never again forget the segments of an approach course.