The
ILS (Instrument Landing) System
- What is ILS?
What Is ILS?
ILS...a confusing, intimidating enemy, until we have two quarts of fuel left in the tanks and can't see the runway. With a little luck, maybe we can tame this Lion into becoming a docile friend.
ILS stands for Instrument Landing System, and is comprised of two highly accurate and directional radio beams that serve two completely different purposes, but share a common goal; allow us to fly accurately and exactly to the approach-end of an ILS designated runway, every single time, regardless of weather. In order to use an ILS system, the aircraft must have the ability to receive an ILS Localizer signal and an ILS Glideslope signal. In the early days of ILS, these were two discrete instruments, but since the 1950's these functions have been consolidated into a single gauge we now call the Horizontal Situation Indicator, or simply HSI, which also serves as a standard VOR-1 radio. [See HSI training].
A good place to start is by defining what ILS does, in basic terms: ILS radio beams are extremely directional and accurate, and the signals are short range...under 30 miles. ILS assumes you're already in the air at a reasonable approach altitude, generally under 5000' MSL and within 30 miles of the airport. But not too close to the airport; you wouldn't setup for an ILS approach at, say, 5 miles out. It's important to note here that ILS itself doesn't DO anything to or with your aircraft; ILS simply provides the radio beams...you (or Autopilot) have to make the aircraft follow them. The purpose of ILS is to establish a tightly-controlled horizontal and vertical "corridor" that will take us inbound to the approach-end of an ILS runway, which is accomplished by following two radio beams generated by the ILS system...the Localizer and the Glideslope. Let's discuss them.
The Localizer radio beam extends outward from the approach-end of an ILS runway, and is horizontally aligned with the runway centerline; flying left or right of the Localizer is reflected in the HSI, as shown below:
The Glideslope radio beam extends outward from the approach-end of an ILS runway, and is elevated at a 3-degree upward pitch. The purpose of the Glideslope is to give us an exact descending glide-path to the end of the runway.
The HSI gauge has Glideslope Indicators that move up and down to show our vertical positioning with respect to the Glideslope radio beam...or seen another way, the gauge tells you where the beam is, with respect to your aircraft.
In order to maintain the ILS glide-path on an ILS approach, it's imperative that we follow the Glideslope exactly, which means to keep the Glideslope Indicators centered vertically in the HSI. The picture below shows HSI response as related to our vertical positioning. Again note that the Glideslope Indicators only tell us where the beam is...it's our job to keep the indicators properly aligned in the HSI in order to stay ON Glideslope. Since airspeed is critical for managing altitude, it follows that managing airspeed is critical for maintaining a good Glideslope.
