An instrument approach consists of four segments. They are the initial approach segment, intermediate segment, final approach, and the missed approach. The initial approach begins at the initial approach fix, IAF. From there, the pilot might proceed to an intermediate approach fix or to the final approach fix, depending on the procedure. In some procedures, a final approach point exists instead of a final approach fix. Finally, the missed approach segment begins at the missed approach point, which might be a time, an altitude, a crossing, or a distance. These segments and their associated terms will be discussed in more detail as we go through specific types of approaches.
A common precision approach is the instrument landing system, or ILS. The ILS provides guidance information, range information, and visual information to the pilot.
Guidance is provided by two highly directional transmitters, the localizer and glide slope. The localizer provides lateral guidance, and the glide slope provides vertical guidance down the final approach. Tracking the localizer and glide slope is somewhat similar to tracking a VOR radial. The pilot tunes and identifies the ILS frequency. ILS identifiers consist of a three letter identifier preceded by the letter "I" for ILS. The course deviation indicator then indicates left and right lateral deviation, while the glide slope needle indicates vertical deviation. The pilot adjusts heading and descent rate to fly the airplane down the center of the desired final approach trajectory.
Unlike a VHF Omni directional Range, the localizer and glide slope are both directional. They provide usable signals only along the final approach course. Localizer signals are usable only when at or below 4,500 above the landing field elevation, within 10 degrees of the final approach course, and within 18 nm from the localizer transmitter. When within 10 nm of the localizer transmitter, a usable signal can be received 35 degrees to either side of the final approach course. Outside of this area, localizer signals are not reliable.
Glide slope signals are normally usable to a distance of 10 nm. You should intercept the glide slope from beneath by leveling off at an intercept altitude first, then capturing the glide slope. Should you be high on an approach, it is possible you may receive false glide slope signals. A normal glide slope is broadcast at a 3 degree angle. Copies of the glide slope are broadcast above the real glide slope that might be much steeper. If you come in high, you might start receiving what appears to be a valid glide slope signal telling you to descent at an unusually steep angle. Or, the glide slope may exhibit some other invalid behavior such as working in reverse.
As you approach the runway, the localizer and glide slope become more and more precise. This means they will also become more and more sensitive. Aircraft are funneled from a relatively large intercept area, where pilots initially receive the ILS signals, down to a very small area right near the runway. At the end of an ILS approach, the entire localizer course is just 700 feet wide.