Understanding ILS Approaches: What Stays the Same and What Varies
- Dan George
- Jul 10
- 4 min read
The Instrument Landing System (ILS) is one of the most common types of instrument approaches used by pilots worldwide. While each ILS procedure has its own unique characteristics based on the airport and runway environment, several aspects are standardized across all ILS approaches. In this guide, we'll explore both the universal features and the key variables that pilots need to understand for safe and accurate ILS execution.
Standard Features of ILS Approaches
1. Localizer Coverage and Function
Chapter 1 of the Aeronautical Information Manual gives the specifications of the ILS. The localizer provides lateral course guidance, transmitting a signal focused along the extended runway centerline. This is unlike a VOR, which emits signals in all directions.
Within 10 NM, the localizer signal is usable in a 35-degree arc on either side of the centerline.
From 10 to 18 NM, the coverage narrows to just 10 degrees either side.
This means that aircraft approaching from the opposite side of the airport might not receive a valid signal, even when close to the airport, until they are within the correct sector.
2. Localizer Antenna Location and Signal Width
The localizer antenna is always located at the departure end of the runway being approached. The signal fans out forward toward incoming aircraft and is designed to be 700 feet wide at the runway threshold (350 feet on either side of the centerline).
This 700-foot standard ensures uniformity across all ILS approaches, but the way this signal is produced varies based on runway length.
3. Glideslope Antenna and Vertical Guidance
Located on the approach side of the runway, the glideslope antenna provides vertical descent guidance. As with the localizer, the glideslope signal becomes more sensitive as the aircraft nears the runway.
Variables in ILS Approaches
1. Runway Length and Localizer Sensitivity
Because the localizer always originates from the departure end and must measure 700 feet wide at the threshold, the length of the runway impacts the signal's divergence:
Shorter runways: The localizer must spread out more quickly.
Longer runways: The signal can remain narrow longer, leading to higher sensitivity further out.
This has a direct effect on how much a given lateral deviation affects needle deflection on the cockpit instruments.
2. Distance from the Localizer Affects Sensitivity
Let’s say you’re 7 NM from the antenna on two different runways. A small lateral movement will result in a full-scale deflection on a longer runway’s localizer but may only cause a partial deflection on a shorter one. That’s because the narrower signal hasn’t fanned out as much.
Understanding Marker Beacons
Many ILS approaches include:
Outer Marker (OM): Located near the glideslope intercept point. It often serves as the Final Approach Fix (FAF)Â if you're flying the localizer-only procedure.
Middle Marker (MM): Typically located at the Decision Altitude (DA)Â point, about 200 feet above the runway threshold.
The OM is at glideslope intercept, while the MM is at decision altitude
How Signal Sensitivity Affects Navigation and Testing
Glideslope Deviation Examples
If the outer marker is at 5.6 NM, and you're:
1 dot above glideslope: You’re roughly 210 feet high.
2 dots above: About 420 feet high.
These numbers can vary depending on where the glideslope intercept occurs.
Localizer Deviation Examples
1 dot right of center: Approximately 775 feet off centerline.
2 dots right: Roughly 1550 feet off.
The degrees of deviation on both the localizer and glideslope depend both on your distance from center as well as your distance from the transmitters
As you approach the runway, both glideslope and localizer become more sensitive, so:
At 1.9 NM (or 500 feet AGL), a one-dot glideslope deviation might mean only 70 feet high, compared to 210 feet at the outer marker.
Similarly, a one-dot localizer deflection might equal 710 feet off instead of 1550.
The FAA tests this concept using charts and instrument mockups, asking you to calculate lateral or vertical deviations at given distances from the runway.
Key Takeaways for Pilots and Students
ILS sensitivity increases closer to the runway.
The localizer signal width is always 700 feet at the threshold, but this is achieved differently depending on runway length.
The location of the outer and middle markers can vary between airports and affect what deflection means in terms of real-world distance.
Q: What is the standard width of an ILS localizer signal?
A: 700 feet at the runway threshold (350 feet on either side of the centerline).
Q: How does ILS sensitivity change as you approach the runway?
A: Both the localizer and glideslope become more sensitive, meaning small deviations result in greater needle deflections.
Q: Where is the localizer antenna located on an ILS approach?
A: The localizer antenna is positioned at the departure end of the runway you're approaching. It transmits a signal outward along the extended centerline, providing lateral guidance to the aircraft.
Q: How far from the runway is the glideslope intercept typically located?
A: The glideslope intercept usually occurs between 5 to 6 nautical miles from the runway threshold. This is often where the outer marker is located and where aircraft begin following vertical guidance down to minimums.
Understanding these subtleties not only helps you on FAA Knowledge Test but also improves your real-world approach precision.
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