ILS Approaches

Instrument Landing System (ILS) is a precision approach nav. aid which provides accurate course, glide slope, and distance guidance to a given runway. An ILS is like using a VOR for navigation, only it has vertical guidance. You must either have a HSI or a VOR indicator to be able to us an ILS. There are four parts to an ILS, they are the localizer, glide slope, marker beacons, and the approach lighting system. The localizer aligns you with the centerline of the runway. The width of the localizer varies between 3 and 6 degrees, and the service volume of a localizer is10 nautical miles within 35 degrees, and 18 nautical miles within 10 degrees of course heading. The glide slope is a transmitter offset from the runway centerline. This provides vertical guidance to the runway. Full-scale deviation is 0.7 degrees above or below the center of the glide slope. The marker beacons provide range information. There are usually two marker beacons for an ILS they are the outer marker, and the middle marker. The outer marker can be anywhere between 4 and 7 miles from the runway, and the middle marker is usually located 3,500 feet from the threshold. (where you land) Some airports which have a category II and III ILS have an inner marker, and that indicates the decision height (DA) which is where you must go around if you do not see the runway environment. There is much more to know before trying an ILS I would never be able to cover everything in this blog, I just explained what it was and the different components of an ILS. If you would like more information on this topic look in chapter 8 page 21 (8-21) of the Jeppesen Guided flight discovery instrument commercial book.