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How An ILS Works

Did you know that Instrument Landing System (ILS) technology has been around since the 1930s? It's the most common approach flown in bad weather, and it even helps aircraft auto-land on a runways during zero-zero weather conditions. Here's how it works...

This 1930s Technology Hasn't Changed Much

On January 26th, 1938, a Pennsylvania Central Airlines Boeing 247D landed using an ILS approach during a snowstorm in Pittsburgh. It became the first passenger airliner in the USA to fly an approach and land using an ILS as the only navigation aid. The same vertical and lateral guidance systems haven't changed much in the 90 years since.

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Localizer (Horizontal Guidance)

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 that they're 700 feet wide at the runway threshold (full scale fly-left to a full scale fly-right).

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Two signals are transmitted laterally, one that's 90 Hz and one that's 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 perfectly 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.

Glideslope (Vertical Guidance)

Glideslope equipment is usually 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 still 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 towards the runway at a 3 degree angle.

False Glideslopes

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. This is why pilots are always taught to intercept the glideslope from below, to ensure they don't capture a "false" glideslope.

If it were to actually occur in an airplane, it's difficult to miss the extremely steep descent angle of these false glideslopes.

Marker Beacons

The ILS was originally developed before DME was widely accessible. Because of that, marker beacons are typically included on 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:

  • Outer Marker: Identifies glideslope intercept or the Final Approach Fix (light flashes blue)
  • Middle Marker: Identifies decision height (light flashes amber)
  • Inner Marker: Identifies decision height for a CAT II ILS (light flashes white - we'll talk about what CAT II means below)

Approach Lighting

Approach light systems (ALS) support the ILS to enhance low-visibility minimums. They're designed to help pilots transition from instrument flying to visual flying, and also to aid with identifying the runway's centerline. Increasing intensity of ALS capabilities can noticeably reduce visibility minimums, allowing for aircraft to arrive at the airport during poor conditions.

Live from the Flight Deck

ILS Categories And Approach Minimum Weather

There are just a few categories of ILS approaches under ICAO and FAA standards. For most general aviation pilots, CAT I approach weather minimums are the only ones they can fly. But corporate aircraft and airlines, with additional equipment and pilot training, can fly CAT II or CAT III approaches.

  • CAT I: 200 foot DH, 1/2 mile visibility (may be 1,800ft, 1,210ft, or 2,600ft depending on airport and number of crew)
  • CAT II: DH between 100-100 feet, 1,000 feet of visbiity
  • CAT IIIa: DH between 50-100 feet, 600 feet of visibility
  • CAT IIIb: DH less than 50 feet or none published, 150 feet of visibility
  • CAT IIIc: no limitations (must be able to taxi with zero visiblity, not yet available at any airport worldwide)

Flying The ILS

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.

You then fly toward the runway and intercept the glideslope from underneath, so you don't intercept a false glideslope. After you intercept the glideslope, you start a gradual, (typically) 3 degree descent toward 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.

As you intercept the glideslope and start descending toward the runway, localizer/glideslope indications move if you get off course, indicating that you need to fly left/right to stay on course, or increase/decrease descent rate to stay on glideslope. To correct for both, you "fly to the needle".

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 Precision Approach For Almost 100 Years

The ILS is one of the few instrument approaches that can get aircraft to the runway in near-zero visibility and ceilings. No wonder it's been used for nearly 100 years.


What's the lowest weather you've flown an ILS in? Tell us about it in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and commercial pilot for Mokulele Airlines. He holds multi-engine and instrument ratings, and is an aviation student at the University of North Dakota. He's the author of the articles, quizzes and lists you love to read every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures at http://www.swaynemartin.com.

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