Say Again? #58: ATC 207 — The ILS

Want an easy and relatively safe way to get into an airport that's IMC? Let your friendly air traffic controller give you vectors to the localizer and take that ILS right down to your port. But could there be some subtle tricks waiting to catch you when talking to ATC? This wouldn't be a column by AVweb's Don Brown if there weren't.


I‘m always fascinated by how little I know about the things I use every day. The ILS is one of those things. The ILS is the “bread and butter” of the instrument approaches. If there’s a choice, most pilots and most controllers will go with the ILS. (Actually, most would prefer a visual approach, but that’s a whole ‘nother column.)Let’s take a look at what the AIM says about an ILS:

1-1-9. Instrument Landing System (ILS)a. General1. The ILS is designed to provide an approach path for exact alignment and descent of an aircraft on final approach to a runway. 2. The ground equipment consists of two highly directional transmitting systems and, along the approach, three (or fewer) marker beacons. The directional transmitters are known as the localizer and glide slope transmitters. 3. The system may be divided functionally into three parts:

(a) Guidance information: localizer, glide slope; (b) Range information: marker beacon, DME; and (c) Visual information: approach lights, touchdown and centerline lights, runway lights.

Guidance, range and visual. Once again I’ll remind you I’m a Center controller. I use the guidance and range part of an ILS but I never get involved with the visual (or lighting) part of an ILS. To be honest, I don’t have much involvement with the range part, either. Let me explain.

Remote Radar

Unlike our Approach Control counterparts, the radar sites Center controllers use aren’t necessarily located in places conducive to vectoring for the ILS. As a matter of fact, the radars themselves aren’t primarily designed for this type of work. There are two classes of radars used in ATC.

ASR — AIRPORT SURVEILLANCE RADAR — Approach control radar used to detect and display an aircraft’s position in the terminal area. ASR provides range and azimuth information but does not provide elevation data. Coverage of the ASR can extend up to 60 miles.ARSR — AIR ROUTE SURVEILLANCE RADAR — Air route traffic control center (ARTCC) radar used primarily to detect and display an aircraft’s position while en route between terminal areas. The ARSR enables controllers to provide radar air traffic control service when aircraft are within the ARSR coverage. In some instances, ARSR may enable an ARTCC to provide terminal radar services similar to but usually more limited than those provided by a radar approach control.

See? It says so right in the book: “ARSR may enable an ARTCC to provide terminal radar services similar to but usually more limited than those provided by a radar approach control.” We’ll get into all the details in a second, but for now, Center controllers think mostly about the glideslope and the localizer when they think about an ILS approach.

The Incredible, Flexible Vector

The most obvious reason controllers and pilots like the ILS is the vectors to the final approach course. We all tend to view vectors to the localizer as common as clouds and the section in the ATC “bible” (FAA 7110.65) explaining how it is done is guaranteed to make your eyes glaze over:

Section 9. Radar Arrivals5-9-1. VECTORS TO FINAL APPROACH COURSEExcept as provided in para 7-4-2, Vectors for Visual Approach, vector arriving aircraft to intercept the final approach course: a. At least 2 miles outside the approach gate unless one of the following exists:

1. When the reported ceiling is at least 500 feet above the MVA/MIA and the visibility is at least 3 miles (report may be a PIREP if no weather is reported for the airport), aircraft may be vectored to intercept the final approach course closer than 2 miles outside the approach gate but no closer than the approach gate.2. If specifically requested by the pilot, aircraft may be vectored to intercept the final approach course inside the approach gate but no closer than the final approach fix.

EXCEPTION. Conditions 1 and 2 above do not apply to RNAV aircraft being vectored for a GPS or RNAV approach.

As you’ll notice, this section deals with more that just vectors to the ILS. Don’t get too excited about that “vectored for a GPS or RNAV approach” part either. I know Approach Controls vector for those but I don’t know of any place in Atlanta Center where we do. Your milage may vary in your neck of the woods.One of the critical differences between Center and Approach Control starts to become apparent in the next section of the 7110.65.

5-9-1. VECTORS TO FINAL APPROACH COURSEExcept as provided in para 7-4-2, Vectors for Visual Approach, vector arriving aircraft to intercept the final approach course:b. For a precision approach, at an altitude not above the glideslope/glidepath or below the minimum glideslope intercept altitude specified on the approach procedure chart.

For this discussion, the critical element is “not above the glideslope …” Don’t vector an aircraft to join the localizer at a point that will be above the glideslope. The MIAs (minimum IFR altitudes) are the lowest useable altitudes in the Centers. Just so I don’t have to go on another tangent and explain the differences between MIAs, MVAs, and MEAs, I’ll make a blanket statement that Center controllers have to keep airplanes shooting an approach up higher than Approach Control controllers. Because they’re up higher and because you can’t join the localizer above the glideslope, you wind up being vectored out further away from the airport.Whew! I told you it would make your eyes glaze over. There is one more section I have to mention about Centers.


5-9-1. VECTORS TO FINAL APPROACH COURSEd. EN ROUTE. The following provisions are required before an aircraft may be vectored to the final approach course:

1. The approach gate and a line (solid or broken), depicting the final approach course starting at or passing through the approach gate and extending away from the airport, be displayed on the radar scope; for a precision approach, the line length shall extend at least the maximum range of the localizer; for a non-precision approach, the line length shall extend at least 10 nm outside the approach gate; and2. The maximum range selected on the radar display is 150 nm; or3. An adjacent radar display is set at 125 nm or less, configured for the approach in use, and is utilized for the vector to the final approach course.

The “EN ROUTE” means it only applies to Centers. If you remembered the blurb about the ASRs from above (coverage of the ASR can extend up to 60 miles) you probably said, “Big duh,” when you read, “The maximum range selected on the radar display is 150 nm.” If you ever get to visit a Center be sure to get a controller to show you what a 60-mile range looks like compared to a 150-mile range on the radar scope.There’s one more piece of the puzzle I have to mention. You can look at the chart if you’d like but to keep it simple, the approach course interception angle isn’t supposed to be greater than 30 degrees. Any of you who have been vectored for the ILS when the wind was a little squirrelly will probably get a chuckle out of that one.After all that, when it’s all said and done, the visual is so simple I can draw it without a graphics program:


X” is the airport, the dashed line is the Final Approach Course and the “arrowhead” is the closest point that we can vector an aircraft onto the localizer and still be below the glideslope at the minimum IFR altitude. To be honest, I don’t know if the “arrowhead” is a local adaptation for Atlanta Center or not. The segment on either side is drawn at 30 degrees to the Final Approach Course so we’ll know what 30 degrees looks like. We know we aren’t supposed to join any closer than that point so we always aim for a point just outside it. It really is simple. Approach Controllers reading this are probably thinking all this sounds a lot like “Vectors to the Final Approach Course for Dummies.”


Where were we? Ah yes, as I hope all that showed you, Center guys don’t deal much with the “Range” aspect of an ILS, either. By the time you get to outer marker, middle marker, compass locater, NDB, whatever … we’ve terminated your radar service and shipped you over to the Tower or common traffic advisory frequency. I know that sounds a little casual, so don’t fall asleep on me. There is something really important that should have jumped out at you right there.Did you catch it? It’s really hard for us to issue a low altitude alert (LAA) when you’re not on the frequency. Furthermore, we don’t tend to watch airplanes after they are established on the final approach course. Remember? We’re the guys working on the big-range settings. When you’re approaching TAWBA intersection 500 feet low at Hickory, N.C., I’m looking at the guy I’m vectoring onto the localizer at West Jefferson. I could be working on things as far away as Knoxville, Tenn., or Roanoke, Va. The chances that I’ll see that you are too low are slim; and no, the computers at the Center won’t issue me an LAA in this situation, either.

Hickory, Dickory, Dock

(122 Kb)
Hickory, N.C. (KHKY) ILS Rwy 24
(Click for larger version)

Let’s run through an example and see how it all works. I’ll use (what else?) the ILS RWY 24 at Hickory, N.C.We’ll say you’re coming in from CRW (Charleston, W.V.) That is to the north for the geographically challenged. And let’s say the pilot has filed “direct HKY” like most folks do these days. You might have read my columns on the subject (and perhaps follow my advice), but most pilots don’t.From CRW to HKY is bearing 177 degrees. That would be an eastbound altitude so this pilot (again, like most) picks 7,000 as a cruising altitude. That will work until the mountains and then the controller will climb you to 8,000 to clear the 7,500 foot MIA east of HMV. Right around the Virginia-North Carolina line, he’ll switch you to the Wilkes sector on 125.15.

“Atlanta Center, Cessna one two three four five with you [cringe] at eight thousand.”

“Skyhawk one two three four five, Atlanta Center, Hickory altimeter three zero, zero, zero.”

“Three triple zero [ouch] three four five.”

“Skyhawk one two three four five, expect radar vectors ILS runway two four Hickory.”

“Cessna three four five roger.”

It will take a long time to reach HKY from the aircraft’s current position so the controller can put him on a vector for about 50 miles (after the aircraft crosses the sector border) or he can wait. Me personally? I’ve already figured out what kind of pilot I’m working and I’ll just wait. I won’t get into all the mental processes I use but let’s just say I’d rather he be responsible for his navigation than putting him on a vector and making it mine.About 30 miles from HKY I’ll take another look at the strips to check for other inbounds and start making my final plan. A jet could depart from CLT or some place close and still beat this Skyhawk to HKY but it’s time to act.Again, the bearing to the airport is 177 degrees so I’ll take a guess that his heading is around 185 or so. The thing about vectoring for an ILS is that you get plenty of opportunities to adjust the heading so I won’t worry about being precise at this point.

“Skyhawk one two three four five, fly heading one four five radar vectors ILS runway two four Hickory, descend and maintain four thousand three hundred.”

“Uhhh, OK, one forty five and forty three hundred.”

“Verify that was Skyhawk one two three four five reading back the clearance.”

“Uhhh three four five,yeah that was us, sorry”

Vector Angle

I’ll still got a lot of time to adjust the heading — at least several minutes. At this time, I’m actually aiming for a point about five miles northeast of the “arrowhead.” If you’ll think about the mechanics of it — the angles — from a 145 heading to a 241 degree final approach course is a big turn. It’s a turn that we don’t normally give at one time. We usually divide it up into sections. In addition, we have some altitude to lose.The MIA in this area is 4,300 MSL. That won’t drop any lower until about five miles from the localizer at the current angle of his heading. So I’m giving myself a little extra room to work on the altitude.When he’s about five miles northeast of the “arrowhead” — the turn-on point — he’ll be crossing into the area where the MIA is 3,400 MSL.

“Skyhawk one two three four five, turn right heading one eight zero, descend and maintain three thousand four hundred.”

“… eighty and thirty four hundred three four five.”

Things start speeding up from this point, for the controller and the pilot. The localizer will start to come “alive” for the pilot. The controller has committed himself to the timing. He can’t let anything else distract him because the window of time to make a good turn onto the localizer is narrow. Now is not the time you want to hear, “Atlanta Center, this is Cherokee two three four five six, we’re on the ramp down here at Statesville and it looks like we’re going to have to get a clearance off the ground. We want to go over to Asheville — that’s Aye Vee El — and we’re going to need about a 10-minute void time.”

“Cessna one two three four five, four miles from MIRTY, turn right heading two two zero, maintain three thousand four hundred until MIRTY, cleared ILS runway two four approach Hickory.”

“Heading two twenty to join, thirty four hundred until established, cleared for the approach, three four five.

Final Exam

Oh well, close enough for government work. “Until established” is habitual. Pilots hear it so often that they always seem to read it back no matter what you say. It’s like “fly runway heading” on a departure. It’s just a habit. Pilots hear it so often they’ll insert it into their readback even if you don’t say it. The “three thousand four hundred until MIRTY” has been a source of debate in my Area for 20 years. As you might imagine, I have my reasons for issuing it. If you’d like to debate it, you’ll have to meet me after class because we’re almost out of time. [Click the “Post your thoughts” button at the end of the column.]I’ll keep glancing back at the target to make sure he gets established but from this point on, my job is basically done. After I see a few updates on the screen that show the airplane is established on the localizer, I’ll switch him to the Tower.

“Skyhawk one two three four five, radar service terminated over MIRTY, contact Hickory Tower one two eight point one five.”

“… ty eight fifteen, so long.” [sigh]

Extra Credit

Here is an assignment for extra credit. Instead of filing direct to the airport, let’s say you filed CRW..HVQ.V133.BZM..HKY. When you checked on frequency 125.15, level at 8,000, you were told to expect the full ILS RWY 24 HKY. As you are approaching the MULBE intersection (46 miles north of BZM), you are given the following clearance:

“Skyhawk one two three four five, five miles from MULBE, maintain eight thousand until MULBE, cleared ILS runway two four approach Hickory.”

See how that works for you. It works for me. Think of all the transmissions saved. Think about how it eliminates the time-critical nature of a vector to the Final Approach Course for the controller. It can be a real time-saver for the controller working five frequencies on the midnight shift. And when you’re past BZM on a 219 degree bearing to TAWBA, see if you think you need to make a procedure turn at TAWBA. Some folks think you don’t. I think you do.Have a safe flight.

Don Brown
Facility Safety Representative
National Air Traffic Controllers Association
Atlanta ARTCC

Want to read more from air traffic controller Don Brown? Check out the rest of his columns.