Flying a localizer approach is pretty straight forward, but flying a back course approach is a whole different animal...

What Is A "Back Course" Approach?

When you fly a localizer back course approach (LOC BC), you're navigating to the runway using horizontal guidance off of a localizer system, but in the opposite direction that you'd normally use the localizer. Navigation is very similar to a localizer-only approach, but with a few key differences.

Two signals are transmitted laterally from a localizer (LOC), one that's 90 Hz and one that's 150 Hz. The LOC receiver interprets the overlap of the two frequencies to determine which side of the localizer course the airplane is flying on, or if it's flying perfectly down the middle of the course.

The localizer transmits a "front course" and a "back course" from the antenna system. The "front course" is the LOC navigation used to fly a standard ILS or LOC approach. When flying standard approaches, the localizer is situated at the departure end of the runway you're landing on.

When you're using LOC BC approach, your receiver references signals emitting from opposite side of the localizer antenna.

Just like a standard LOC approach, the localizer back course signal is only guaranteed to be accurate up to 10 degrees on either side of the runway to 18NM. At an angle of 35 degrees on either side of runway centerline, the useful volume is limited to 10NM. You might receive localizer signals outside of the service volume, but you can't rely on them for navigation.

Where You'll Find "Back Course" Approaches

You'll often find LOC BC approaches on runways with an ILS installed, but pointing in opposite direction. In Grand Forks, ND (KGFK), winds are predominantly out of the northwest. The ILS for KGFK's north-facing Runway 35L is also used to create a LOC BC approach to Runway 17R. Winds out of the south aren't common in Grand Forks, so it saves money to install just one ILS system.

Speaking of money, that's another reason you'll find LOC BC approaches. Even at airports with winds that frequently change direction, there may be just one ILS approach installed.

Take Kona, Hawaii for example. There's an ILS approach to Runway 17, and a LOC BC to Runway 35. It's not uncommon when flying into Kona for wind changes to shift traffic flow a few times during a matter of hours.

Another unusual example of LOC BC is the missed approach procedure for Aspen, Colorado (KASE). The localizer approach into Aspen requires pilots to chop and drop nearly 1,900 feet in just 3.1 NM. The approach angle is 6.59 degrees. It terminates with a missed approach that turns you onto a remotely located mountaintop localizer for a back-course departure, through a valley, to the waypoint LINDZ.

Keep in mind, just because a back course exists, doesn't mean you should use it. As stated in the FAA's AIM, "do not use back course signals for approach unless a back course approach procedure is published for that particular runway and the approach is authorized by ATC." If an approach isn't published, you're putting yourself at risk for losing safe separation with terrain and obstacles.

How To Set Up A "LOC BC" (Without HSI)

For pilots flying a LOC BC approach using a course deviation indicator (CDI) with an omni bearing selector (OBS), you must pay close attention to "reverse sensing" associated with the back course. According to the AIM, "when flying inbound on the back course it is necessary to steer the aircraft in the direction opposite the needle deflection when making corrections from off-course to on-course. This 'flying away from the needle' is also required when flying outbound on the front course of the localizer."

It doesn't matter whether you select the front course or the back course with the OBS knob (in the image below, 174 degrees or 354 degrees, or any OBS setting for that matter), the needle will work the same. However, it's easier for many pilots to dial the final approach course with the OBS to remember which general direction they're flying the approach.

When you're established on the LOC BC approach, you will always "reverse sense" using a CDI with OBS. If you're in this situation, try saying out loud "fly away from the needle."

In the image below, and CDI is set up for the Localizer Back Course approach to runway 17R in KGFK (174 degree final approach course). The CDI needle is deflected to the left. You need to turn right (fly away from the needle) to get back on course for this localizer back course approach.

How To Set Up A "LOC BC" (With HSI)

If you're lucky enough to fly with a horizontal situation indicator (HSI), flying LOC BC approaches gets significantly easier. The HSI combines a heading indicator with CDI needles. As long as you tune the front course for the localizer, you won't get reverse sensing. When configured properly, you can fly the LOC BC approach with normal sensing, and fly "to the needle" like you do in all other navigation.

For example, if you were flying the LOC BC to runway 17R in Grand Forks, instead of dialing the back course of 174 degrees, you'd dial the front course of 354 degrees, and fly to the needle to navigate the approach.

Flying The LOC BC Approach

Once you're established on a LOC BC approach, flying is as simple as any non-precision approach. You'll reference step down fixes and DME before leveling off at your minimum descent altitude (MDA).

Unlike most localizer approaches, expect a missed approach point on a LOC BC approach to be located more than half a mile before the runway. The reason? It would be impossible to fly all the way to the runway threshold at MDA on a LOC BC approach. As you get closer and closer to the localizer antenna, it becomes increasingly difficult to stay on course due to increasing signal sensitivity. Since the localizer system you're using for the back course is located near the runway threshold, as opposed to the departure end, the missed approach point is farther away from the runway.

The FAA offers a warning about flying back course approaches: "False glide slope signals may exist in the area of the localizer back course approach which can cause the glide slope flag alarm to disappear and present unreliable glide slope information. Disregard all glide slope signal indications when making a localizer back course approach unless a glide slope is specified on the approach and landing chart."

Using An Autopilot? PUSH THE "BACK COURSE BUTTON"

When you fly an autopilot-coupled LOC BC approach, ALWAYS push the "back course button" if there's one installed for your autopilot system. By pushing this button, you're telling the autopilot to turn inbound on the back course instead of outbound. Here's the scenario...

If you're flying with an HSI, you should have tuned the front course for the localizer to avoid reverse sensing. Your CDI needle will thus point away from the runway. If ATC clears you to track inbound on the localizer and you click "NAV" mode on the autopilot, the autopilot will turn the wrong way, outbound!

The autopilot doesn't know that you're flying a back course approach. If you've engaged NAV mode, it will intercept and fly the course you've set. By pushing the "BC" button, you're telling the autopilot to track inbound on the back course, which will avoid you making an inadvertent and potentially dangerous outbound turn.

Think you're ready to fly the approach? Take this quiz: Could You Fly This Localizer Back Course Approach?

How many times have you had to fly a LOC BC approach? Tell us in the comments below.