Understanding ILS

• 7 min read

Disclaimer: This post may contain affiliate links or advertisements. This means I earn a small commision if you decide to make a purchase through the links, at no extra cost to you.


An Instrument Landing System (ILS) enables pilots to shoot precision instrument approaches to a runway.

Before digging into the nuts and bolts of ILS, let's review what an instrument approach, and more specifically, a precision-instrument approach is.

You may already know that Instrument approaches are IFR procedures designed to guide the aircraft from the en-route part of the flight to a position from which it can make a safe landing.

A precision approach provides lateral (left and right) and vertical (up and down) course guidance on the final approach leg.

The ILS is one of the oldest yet most widely used instrument approach procedures still in service. It is still, by far, the most commonly used precision instrument approach procedure.

ILS relies on multiple ground-based and aircraft equipment that provides:

  • Lateral guidance,
  • Vertical guidance,
  • Range information, and
  • Visual transition to the runway

Localizer

Localizer M39

The primary component of the ILS is the localizer, which provides lateral course guidance. It can serve as part of an ILS approach or in a stand-alone localizer-only procedure.

VORs and localizers share the same navigation radio and display equipment in the flight deck.

Navigation with localizers and VORs is very similar. In both, the pilot stays on the desired track by keeping the CDI (Course Deviation Indicator) centered. Unlike VORs, which facilitate navigation on any bearing around them (from 0º to 359º), localizers only support a single, specific direction.

Unlike in VOR navigation, turning the OBS knob when a localizer is in-use does not affect the CDI displacement. However, it is still a good idea to set the OBS to the inbound course of the published approach for better situational awareness.

Note about reverse sensing

Typically, we want the CDI to deflect to the side of the desired track.

With reverse sensing, the needle veers to the wrong side.

This phenomenon may cause disorientation and lead the pilot to navigate away from the course instead of toward it.

For localizer navigation, a simple, old-style VOR display has an advantage over an HSI when it comes to reverse sensing. Since the OBS selection doesn't move the needle, a regular VOR display is not affected by reverse sensing when using a localizer.

In contrast, HSI-type displays, which eliminate reverse sensing with VOR stations, do not eliminate it with a localizer.

To avoid this problem with an HSI and have better situational awareness regardless of the dial type, always set the OBS to the inbound course.

Localizer specs

Localizers operate on VHF frequencies between 108.1 and 111.95 MHz. You should note that the VOR frequency range is broader (108.0 to 117.95 MHz) and overlaps this range. Therefore, localizers use only odd tenths decimals (e.g., 108.1, 110.7 MHz) while VOR stations in this range use even tenths (e.g., 108.0, 110.0 MHz).

Localizer course widths range between 3º and 6º. The specific angle chosen puts the course width at the runway threshold at about 700'. Typically, this is a 5° total width (or 2.5º full deflection to each side), four times more sensitive than a VOR.

Outside the full-width deflection range, the aircraft receiver should still sense the localizer signal within a given coverage range. The coverage area goes up to an altitude of 4,500'. It covers 35º to each side of the centerline up to 10 NM and 10º up to 18 NM from the antenna.

The localizer transmits a Morse-code ID that typically starts with an "I," followed by a three-letter code. Before trusting the localizer for navigation, you must identify that the correct station is in use. Identifying the station is done by listening to the Morse code over the navigation radio. Modern avionics, such as the Garmin 1000, can automatically decode the morse code and display it to you, eliminating the need to listen and decipher it yourself.

Localizer plan view

How does the localizer work?

The localizer antenna array is located at the departure end of the runway. It uses Amplitude Modulation (AM), like in your old car radio, to modulate two signals on the single VHF frequency — one at 90 Hz and the other at 150 Hz. In a narrow pattern, these signals are emitted to each side, left and right, of the localizer centerline.

The aircraft receives the 90 Hz signal more intensely when it is on the center's left side and the 150 Hz more strongly when on the right side. Both emissions are at equal amplitude on the localizer's centerline.

The aircraft's navigation equipment interprets these two signals' ratio and deflects the CDI needle accordingly.

Glideslope

The glideslope provides vertical course guidance. Its angle varies with terrain requirements at the specific runway but is typically 3 degrees.

Glideslope specs

Glideslope frequencies range between 329.3 and 335 MHz (UHF).

Each one of the 40 available localizer frequencies has an associated glideslope frequency. The glideslope receiver automatically tunes to the correct frequency when the pilot sets the localizer.

You may never need to worry about the glideslope frequency when flying, but if curious, you can find the localizer-glideslope frequency pairs in the FAA's Aeronautical Information Publication (AIM).

The glideslope guidance's width is 1.4º so that full needle deflection is 0.7º in either direction. Typically the glideslope signal transmits as far as 10 NM from the antenna.

How does the glideslope work?

The glideslope operates very similarly to a localizer but on a vertical plane. The glideslope UHF frequency carries two signals, one at 90 Hz and the other at 150 Hz. These signals are directionally transmitted above and below the desired approach path, accordingly. The aircraft GS equipment interprets both signals' amplitude to display the aircraft's vertical location above or below the path.

Glideslope errors

The glideslope is subject to false signal errors. False glide slopes may be present above the desired path. These false signals are at a higher angle than it and may cause the pilot to fly a steeper approach than expected.

To prevent following the wrong slope, always intercept the glideslope from the published altitudes on the approach fixes.

Glideslope profile view

Range information

Range, or distance, information helps the pilot identify the aircraft's position on the instrument approach.

Several types of equipment can provide the pilot with range information.

Marker Beacons

Each marker type implies a specific range from the runway and is indicated in the cockpit by a color light and Morse code.

  • Outer Marker (OM): located 4-7 miles from the runway threshold. It indicates the position at which the aircraft should intercept the GS at the appropriate interception altitude ±50ft. A blue flashing light and a series of audible dashes ("---") at 400 Hz identify the OM on the marker beacon receiver in the cockpit.
    Outer Marker, Denelson83, Public domain, via Wikimedia Commons
Outer Marker dash-dash-dash
  • Middle Marker (MM): Placed about 3500ft from the runway. Indicates the approximate point where the GS meets the decision height, usually at 200ft above the touchdown zone elevation. Identified by an amber light and an audible pattern of dot-dash-dot-dash (". - . -") at 1,300 Hz.
    Middle Marker, Denelson83, Public domain, via Wikimedia Commons
Middle Marker dot-dash-dot-dash
  • Inner Marker (IM): placed between the MM and runway threshold. It indicates the point where the glide slope meets the DH on a CAT II ILS approach — identified by a flashing white light and an audible series of dots ( "...") at 3,000 Hz.
    Middle Marker, Denelson83, Public domain, via Wikimedia Commons
Inner Marker dot-dot-dot
  • Back Course marker (BC): Indicates the FAF on selected back course approaches. This marker shares the same light indication in the cockpit as the inner marker. Back course markers are not part of an ILS approach. They are identified by the white, inner marker's light and an audible series of double dots at 3,000 Hz (".. ..").
BC Marker dot-dot, dot-dot

Marker Beacons

As part of the ILS equipment, marker beacons are the traditional means for range information. However, thanks to DME and RNAV, many ILS installations omit some or even all marker beacons.

Compass Locator Markers

Some marker beacons have an associated compass locator co-installed with them.

Compass locators are low powered NDB stations that add another way to identify the OM or MM and navigate to it.

Many ILS approach configurations have a compass locater placed at the OM. This compass locator is called a Locator Outer Marker (LOM). With it, the pilot can directly navigate to the OM and intercept the inbound course, without the need for RNAV or vectoring by ATC.

Compass locators typically transmit at 25 Watts or less to a range of 15 miles and are available on frequencies between 190 and 535 kHz.

A two-letter Morse ID audibly identifies the compass locator.

The following can substitute for the OM

  • Compass locator
  • Precision Approach Radar (PAR)
  • Airport Surveillance Radar (ASR)
  • Distance Measuring Equipment (DME), VOR, or NDB authorized in the instrument approach procedure.
  • RNAV with a GPS capable of fix identification

Approach Light System

An Approach Light System (ALS) is a pattern of lights installed at the runway's approach end. It provides the pilot with visible means to transition between instrument-guided flight into a visual approach to landing.

Approach Light Systems

The ALS extends from the landing threshold into the approach area up to:

  • 2,400 - 3,000 feet for precision instrument runways, and
  • 1,400 - 1,500 feet for non-precision instrument runways.

Some Approach Light Systems include sequenced flashing lights (SF), often nicknamed "The Rabbit." These appear to the pilot as a ball of light traveling towards the runway twice a second.

The pilot can estimate the available flight visibility according to the ALS configuration's visible parts by being familiar with the pattern of different Approach Light Systems.

ILS Approach categories

ILS CategoryVisibilityDH
CAT I2,400' or 1,800'200'
CAT II1,200'100'
CAT IIIa> 700'< 100' or no DH
CAT IIIb150'-700'<50' or no DH
CAT IIIc0'No DH

Flying an ILS approach

ILS Indications By Aircraft Position

To avoid catching a false glideslope signal, the pilot should use published altitudes and intercept the glide path from below.

While on the final approach course, maintain the desired flight path by keeping the localizer and glideslope indicators centered. The needles' location represents the relation of the vertical and lateral tracks to the aircraft.

When the needle is right of center, the pilot should turn right to intercept the course. Likewise, when the indicator is on the left, the pilot should turn left.

Similarly, the glideslope needle indicates the position above and below the desired path. When the glideslope indicator is above the center, the aircraft is too low; when it is under the center, the plane is too high.

A Decision Height (DH) on the glideslope marks the missed approach point at which the pilot should decide whether to land or to go around.

When can you descend from the Decision Height / Altitude?

At the Decision Height, you must perform a go-around unless you meet the following three conditions:

  1. The aircraft is continuously in a position where the pilot can make a safe landing at the intended runway using normal descent rates and normal maneuvers.

  2. The flight visibility at least as prescribed for the procedure, and at least one of the following visual references are distinctly visible at the runway of intended landing:

  3. The Approach Light System (Except for CAT II and III procedures). ALS with red sidebars or red terminating bars (such as ALSF-I and ALSF-II) allow you to descend below the DH, down to 100' above the touchdown zone height, when these red bars are insight.

  • The threshold.
  • The threshold markings.
  • The threshold lights.
  • The visual glideslope indicator. (i.e., PAPI, VASI)
  • The touchdown zone or touchdown zone markings
  • The touchdown zone lights.
  • The runway or runway markings.
  • The runway lights.

Recommended Reading