ILS Approaches Explained (for non-pilots)

ILS stands for Instrument Landing System, the most widely used and accurate system for guiding aircraft to airports during low visibility and/or low cloud ceilings.

Photo by Caleb Woods on Unsplash

Types of ILS Approaches

The most common type of ILS is Category I (CAT I), which typically brings aircraft to about 200 feet above the ground. However, local geographic restrictions may prevent an ILS from achieving this altitude. Less common are Category II (CAT II) and Category III (CAT III) approaches, which guide aircraft down to 100 feet and 50 feet above the ground, respectively.

In Canada, any pilot with an Instrument Flight Rules (IFR) rating can fly a certified IFR aircraft on a CAT I approach. However, CAT II and CAT III approaches require specific training, certification, and additional aircraft equipment.

How ILS Works

Regardless of the type of ILS approach, the concept remains the same. Ground-based equipment sends out two radio beams:

  • Localizer: Provides horizontal guidance (left and right).
  • Glide Path: Provides vertical guidance (up and down).

In the flight deck, pilots use two needles to interpret the signals representing the glide path and localizer:

  • The center of the instrument represents the aircraft's position.
  • If the glide path needle moves up, the aircraft is below the glide path, and the pilot needs to reduce the descent rate to return to the correct path.
  • If the localizer needle moves left, the aircraft is right of the intended course, and the pilot must correct accordingly.

As the aircraft approaches the runway, the guidance signals become more sensitive, requiring only minor adjustments to stay aligned with the localizer and glide path.

Created by “Bob the Oyster”. The original image can be found here: http://commons.wikimedia.org/wiki/File:ILS_gauge.svg. This image is used under this license: http://creativecommons.org/licenses/by-sa/4.0/. The original image was modified to include two red arrows pointing from two descriptions. 

Decision Height and Missed Approaches

Every ILS has a decision height, which is the altitude at which pilots must determine if they can safely land. If the runway is not visible by this altitude, the pilots must execute a go-around, also known as a missed approach. During a go-around, the pilots discontinue the landing attempt and climb away from the ground to reattempt or divert to an alternate airport.

Limitations of the ILS

While the ILS is the most widely used and accurate instrument approach system, it is also expensive to install and maintain. As a result, only airports with significant traffic and frequent adverse weather conditions invest in these systems.

The ILS remains a cornerstone of aviation safety, guiding aircraft safely to their destinations even in challenging weather conditions.

This article is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. 

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