LOC, LDA and Back-Course Approaches

Localizer, Back Course, and LDA approaches

At this point, we have learned about ILS, VOR, NDB and GPS approaches. In the sim world we commonly fly into the larger airports which almost always have an ILS. Since it provides the most accurate course guidance available, we quickly become accustomed to flying this type of procedure to the exclusion of all the others. I find it fun to fly other approach types from time to time. It adds a little variety and challenge. In real-life, there are many other types of instrument approach systems, most of which are rare. These are listed at the right.

Precision Approach Radar uses special radar equipment on the ground and is, well, a precision approach. The controller will give the pilot headings to fly, to direct them to, and keep their aircraft aligned with the extended runway centerline. For glidepath, they are given information for how far above or below the glidepath they are in terms of "slightly" or "well." You might hear "well above the glidepath, coming down rapidly." Range from touchdown is provided every mile. Pilots are expected to not readback these directions. Just listen and adjust. I have seen this simulated once as a VATSIM event. Special client software was provided allowing the controller to "see" the aircraft in a precise way on his radar scope. It's rare but pretty cool.

As a rule, approaches with localizer-type systems (ILS, LOC, LDA) have the lowest minima, so look for one of these when weather is bad at your destination airport. VOR procedures would be next, followed by NDB and then RNAV procedures.

Approaches You Won't See in MSFS

The Simplified Directional Facility is non-precision, and similar to a localizer. It is less accurate having a wider course beam than a localizer (which is 3-to-6 degrees in width).

Microwave Landing Systems were to be the replacement for ILS until GPS came along. While GPS has yet to achieve precision status, research and investments are focused in that area. The MLS is very similar to ILS, except it uses radio frequencies in the microwave spectrum which reduces susceptability to interference. TACAN is similar to ILS, but created for and used by the military who have special requirements (for example, runways that don't stay sitll as in aircraft carrier!).

Requiring no special onboard equipment are ground-radar approaches such as the Airport Surveillance Radar approach. ASR is non-precision and the controller will vector the pilot to the final approach course, then provide azimuth and distance information along the approach.

Localizer Approach -

ILS systems have four components: (1) The localizer transmitter, which provides precise azimuth information. This is similar to a VOR radial, but where VOR's will give you a course width of 20-24 degrees wide (each dot of deflection is 2-2.5 degrees) a localizer course width is very narrow, usually 5 degrees wide (full-left to full-right deflection). Much more precise. (2) A glideslope transmitter, (3) Inner, middle and outer marker beacons, and (4) Approach lighting systems. Should the ILS glideslope component be out of service, the approach reverts to a non-percision LOC approach with MDA altitudes.

In some cases, a localizer is installed with no glideslope. These are LOC approaches, and are flown just like a VOR approach, observing step-down altitude fixes and Minimum Descent Altitudes (MDA). Consider these to be a more precise version of a VOR approach. Another difference between a VOR signal and a localizer signal is that OBS setting has no impact on deviation indication. You will see the same amount of needle swing for a given course deviation regardless of the OBS setting. This is because unlike a VOR, a localizer does not transmit 360 radials. OBS has no meaning here. It is however useful to rotate the OBS to put the LOC inbound course under the course index for situational awareness.


Oxnard ILS 25. Note the maltese cross for the FAF and the step-down minimums. These are for use in the event the glideslope is out of service. Note also minimums listed for S-ILS (straight-in ILS), S-LOC (straight-in localizer) and CIRCLING.	KSAN LOC 27. Click to enlarge.

Back Course Localizer Approach -

An interesting variant of the LOC approach is the LOC-BC or "back course" approach. Localizer antennae are sited at the opposite end of the runway from the approach direction and are aligned with the runway centerline. Every localizer transmitter radiates a signal in two directions, one being the "front course" and the other is the "back course." For example, an ILS RWY 26 with a centerline heading of 260, the antenna is located at the RWY 8 end of the runway and radiates a front course in the direction of 080 for runway 26. The back course radiates in a direction of 260. If an IAP has been charted for this back course, then you would fly it just as you would a LOC approach, with one important exception: your Course Deviation Indicator (CDI) will "reverse sense." You must fly away from the needle in order to fly toward the courseline. This takes conscious effort as it is contrary to your natural instinct.

On IAP charts, the graphic representing a localizer will be shaded on the right side when a front course is used, and the left side when a back course is used, as shown here. Contrast this diagram to any ILS or front course localizer chart. If you are using a coupled auto-pilot for a back course approach, you must use the BC (sometimes labeled REV or BCRS) mode rather than the LOC or APP mode if you hope to track the back course.

Localizer-type Directional Aid (LDA) Approach -

These are approach systems which include at least a localizer facility, but which are not aligned with the runway center-line. Straight-in minimums will be published if the offset is less than or equal to 30 degrees from the runway centerline; circling-only minimums otherwise. An LDA may or may not have a glideslope component. Unlike an ILS, don't expect to have marker beacons. The LDA/DME RWY 19 is an LDA with glideslope, and is perhaps the most difficult approach on the east coast of the US. Try it sometime when a crosswind is blowing!


	KDCA LDA/DME RWY 19. Click to enlarge.