The Instrument Landing System (ILS) is a radio navigation system that provides precision guidance to aircraft approaching a runway.
ILS approaches allow most general aviation pilots to land in as little as 1/2 statute mile visibility and as low as 200-foot cloud ceilings.
There are several components that make up an ILS system:
The localizer antenna is used for horizontal guidance, and it's positioned on the far end of the runway. The localizer transmits signals on 108.1 MHz, up to and including 111.95 MHz (odd tenths only). Localizers have an adjusted course width so the course is 700 feet wide at the runway threshold (full scale fly-left to a full scale fly-right).
Two signals are transmitted laterally: one at 90 Hz and one at 150 Hz. Where the two frequencies intersect is usually aligned with the extended runway centerline, and is shown as "on-course" when viewing cockpit instrumentation. The ILS 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 down the middle of the course.
While you might receive localizer signals outside of the service volume, the localizer 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.
The glideslope provides vertical guidance, and the antenna is typically located 750 to 1250 feet down the runway, and 400 to 600 feet from the side of a runway's centerline. You can usually find the glideslope shed next to the runway's aim point markers.
The glideslope works the same as a localizer, but just turned on its side. The equipment transmits 90 Hz and 150 Hz lobes, which are interpreted by the ILS receiver.
The beam is 1.4 degrees thick, with .7 degrees of glidepath projected on either side of the beam. A typical glideslope will take the airplane down toward the runway at a 3-degree angle.
Objects below 5,000 feet AGL have a tendency to reflect glideslope signals. This can create false glideslopes, which are often at 9-degree and 12-degree angles to the runway. Pilots are taught to intercept the glideslope from below to ensure they don't capture a "false" glideslope.
If you were to actually capture a false glideslope, you would fly a much steeper descent angle to the runway.
The approach light system (ALS) helps pilots identify the runway environment in low-visibility. It's designed to help pilots transition from instrument flying to visual flying, and also to aid with identifying the runway's centerline.
The ILS was originally developed before DME was widely accessible. Because of that, marker beacons are sometimes included in an ILS approach. Each beacon designates a specific position on the approach, with an audible tone and/or visual light that illuminates in the cockpit.
Sometimes there may be one or two, but not necessarily all three kinds of marker beacons established on a specific approach:
To fly an ILS, you first align your aircraft with the runway, using the localizer as guidance. This is typically done by radar vectors from ATC, or with a procedure turn when flying a full procedure approach. As you fly toward the runway following the localizer in level flight, you intercept the glideslope the final approach fix (The lightning bolt symbol in the image below). After you intercept the glideslope, you start a gradual descent. The glideslope typically provides a 3-degree descent to the runway.
There are several different ways that the localizer and glideslope can be represented on flight instruments, but in most glass-panel aircraft, they're represented as a green line or triangle for the localizer, and a green diamond or triangle for the glideslope. The localizer and glideslope indications represent the center of the localizer course and the glideslope course. If you get off course, either left/right or high/low, you "fly toward the needle" to get back on course.
As you get close to the runway, the localizer and glideslope signals become more sensitive, because the course width of both decreases the closer you get to the runway. Using small corrections, and avoiding "chasing the needle", is essential to fly an ILS all the way to minimums.
The ILS has been used for nearly 100 years, and it's one of the few instrument approaches that can get aircraft to the runway in near-zero visibility and ceilings. What's the lowest weather you've flown an ILS in? Tell us about it in the comments below.
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