On Autopilot

Fighting wrong-way needles

Everybody hates a back course (BC) instrument approach because it can be such a confusing, demanding experience, and because many instrument landing system (ILS)-equipped airports don't use back course approaches. So, needles that give wrong-way steering information—and a lack of practice—combine to make back course approaches avoided. However, if you have a horizontal situation indicator (HSI) and know how your autopilot handles a back course approach, you can make more precise, safer approaches. Trouble is, most autopilot manuals and supplements to pilot operating handbooks do a poor job of explaining how to program for a back course approach.

BC basics in brief

Think of the signals that make up an ILS as having two parts: the front course and the back course. The front course provides very precise lateral and vertical guidance to published decision heights—using localizer (for lateral) and glideslope (for vertical) needle steering indications. It's the very precise localizer (about four times more sensitive than a VOR) and glideslope guidance that let you fly precision (i.e., having vertical guidance) approaches on an ILS front course.

The localizer component of the ILS uses VHF signals, modulating them at 150 Hz for one side of the inbound final approach course and 90 Hz for the other. (Older texts often call the 150-Hz half of the localizer beam the blue sector. The 90-Hz half is the yellow sector. Some older VOR navigation heads depict these two colors on the instrument face. Why? We'll see shortly.)

Glideslope information is broadcast over a UHF frequency. These VHF and UHF frequencies are paired and transmitted from two different antenna sites. The localizer antenna is situated some 1,000 feet or so from the departure end of the runway served by the ILS front course; glideslope antennas are close to the approach end of an ILS runway.

When flying an ILS front course, lateral course deviations from the localizer show up as standard left-needle, right-needle deflections. To stay on course, you'd fly toward the deflected needle: needle left, fly left; needle right, fly right. The glideslope needle gives fly-up (needle rising from the central "bull's eye") or fly-down (needle descending) commands. Keep both the localizer and glideslope needles centered, and you'll descend at a constant rate toward the decision height (DH). This point typically puts you at a point in space 200 feet agl and one-half mile from the approach end of the runway.

But for every localizer front course there's a mirror-image back course. When flying inbound to a runway served by a back course approach, you'll see reverse sensing of the localizer needle. Here, a left-needle deflection is a command to turn to the right, and vice versa. That's the psychologically confusing aspect of flying inbound on a back course, or outbound on a front course (as you might do as part of a full ILS approach procedure).

For those of you with the old blue-yellow markings on your nav heads, just remember that the localizer needle always points to the sector you're flying in. The colors were added to improve situational awareness on front course and back course final approach courses.

To sum up, when flying inbound on the front course or outbound on the back course you fly toward the needle to intercept the localizer. When flying outbound on the front course or inbound on the back course you fly away from the needle to intercept the localizer.

Real sensitive

Needle sensitivity on the ILS front course is acute. On the back course it's extreme. That's because on a back course approach the localizer antenna site is in front of the runway threshold. As you fly toward the runway, the back course signal width becomes narrower and narrower, and progressively more difficult to track if hand-flying.

By comparison, the front course signal width is wider—the antenna's off the end of the runway, remember? All installations differ somewhat, but at the outer marker (or final approach fix) each dot's worth of localizer deflection equals 300 feet; at the middle marker, it's 100 feet.

On the back course, these values can be cut by as much as half. This means smaller—and more frequent—corrections when tracking the back course inbound.

Bare-bones BC

Most back course approaches don't provide glideslope information, making them nonprecision localizer approaches. Even so, you may see the glideslope needle come alive and seem to provide vertical guidance cues during a back course approach. These indications should be ignored—unless the approach is specifically identified as a back course ILS. That would mean that a second, separate glideslope antenna has been set up for use on the back course.

Usually, however, you'll see the warning "disregard glideslope indications" printed in bold letters on a localizer back course's approach plate.

Glideslope signals are typically designed for use on the ILS front course only. They are meant to guide you down a defined glide path to the runway served by an ILS—and no other runway. Any glideslope needle activity you might see on a back course approach merely means that your glideslope receiver has picked up some unusable lobes of that signal. The indications may seem to make sense, and even provide you with some trackable vertical cues—but if you follow them they could put you on a dangerously low approach path.

HSI to the rescue

If your airplane has an HSI then you're in luck. The HSI provides a way to get rid of wrong-way needle deflections on back course approaches, and it's easy to do. Simply rotate the course arrow's pointer so that it's set to the front course's inbound heading. So if you're shooting a back course approach to Runway 9, set the course arrow's head at 270 degrees. This reverses the HSI's needle sensitivity and gives you normal steering commands: needle left, steer left; needle right, steer right. Now you can intercept and track a back course localizer the same way you'd track a front course localizer.

This won't work with a plain-Jane VHF navigation display head. You can spin the OBS knob all you want, but wrong-way steering commands will still come up when you fly a back course procedure.

Let George do it

To return to the HSI scenario, a typical back course approach sequence of events might go as follows (please consult the flight manual supplement or operating handbook for the specifics pertaining to the autopilot you use—the information provided here is general in nature and must not be used for operational guidance):

• Prior to intercept. Tune in the localizer frequency. Use the heading bug to set up your intercept. Let the autopilot fly, using the heading (HDG) function. Hold the proper altitude, using Altitude Hold (ALT) if available and desired. Turn the HSI's course arrow to the front course. The final approach course will now appear under the "tail" of the course arrow, and you'll have proper needle sensing. Select Navigation (NAV) or Approach (APCH) in your autopilot mode selector (many autopilots automatically switch to APCH from NAV when a localizer frequency is selected). You should see "Armed" appear in the autopilot annunciator panel. Push the BC or REV (reverse sensing) buttons so that the autopilot roll servos will reverse the localizer signal sectors (the needles may indicate properly, but the blue/yellow sectors are still flip-flopped, and that's what the autopilot will try to follow unless you hit the BC or REV button). Slow the airplane and configure it for a nonprecision approach descent profile.
• Intercept. By rotating the course arrow 180 degrees from the back course, the tail of the course arrow becomes the new course pointer. When the airplane closes on the final approach course, the "Captured" annunciation should illuminate (CAPT in some installations). The other element of approach guidance—the left-right needle—is taken care of by the BC/REV signal processing. With retractable-gear airplanes, lower the landing gear at the final approach fix and make sure all landing checklist items are complete.
• On final approach course. If it's been selected, turn off altitude hold. The autopilot will track the back course inbound. Any glideslope signals are suppressed—this suppression being another aspect of the BC/REV function. Use power and pitch to control airspeed and descent rate.
• Missed approach. Set the heading bug for the initial missed approach heading, and set your second nav head for any close-in initial missed approach course guidance. Apply climb power and climb to the published missed approach altitude. Without an HSI, autopilots rely more heavily on the heading bug for final approach course information:
• Prior to intercept. Tune the localizer frequency. Select NAV or APCH. Use altitude hold if available and desired. In some autopilot installations it's recommended that you fly an intercept heading using the heading bug in HDG mode. While flying the intercept heading, set the nav head's compass card to the inbound course (for reference purposes—remember, the nav head itself isn't sensitive to course settings), and get ready to set the heading indicator's heading bug to the back course inbound. Select BC or REV on the mode selector. Slow the airplane and configure it for a nonprecision approach descent profile.
• Intercept. Fly the airplane—in HDG or NAV mode—on a heading that will result in a 30- to 45-degree intercept angle with the final approach course. Course deviation indications will be wrong-way, but because you've selected BC or REV, the airplane's roll servos will turn the airplane so as to properly track the back course. Complete the prelanding checklist.
• On final approach. Disengage altitude hold. Left-right corrections are made based on the amount of needle deflection from the course centerline. Use power and pitch to control airspeed and descent rate. In retractables, lower the landing gear at the final approach fix (FAF) inbound.
• Missed approach. Select the HDG or NAV mode, as appropriate, to follow the published missed approach procedure. Apply climb power and climb to the initial published missed approach altitude.

Autopilots can make back course approaches a breeze, but practice is still necessary for maximum proficiency. So is a good familiarity with the autopilot you'll be flying. Study the back course functions in the autopilot operating manual, seek out an instructor who has familiarity with your autopilot, and if you're rusty, schedule some time for quality instruction in autopilot approach functions. Soon you'll be using your autopilot for flying inbound and outbound on both front and back courses—with the course deviation needle glued to dead-center.

Links to additional information on using the autopilot can be found on AOPA Online ( www.aopa.org/pilot/links/links0010.shtml). E-mail the author at [email protected].