What Is an ILS Approach? How the Instrument Landing System Guides You to the Runway

An ILS (Instrument Landing System) approach is a precision instrument approach that uses two radio beams — a localizer for left/right alignment and a glide slope for vertical descent guidance — to guide an aircraft down a stable path to the runway in low visibility, typically to a decision height as low as 200 feet. It is the most accurate ground-based approach a pilot can fly.

The ILS is one of those systems that sounds intimidating the first time you hear it, and then makes complete sense once someone draws you the picture. Two needles. One points you at the centerline, the other points you down the slope. Center them both, and the airplane flies itself onto the pavement. Let’s break it down so it actually clicks, even if you’re still a student pilot flying VFR.

What Is an ILS Approach?

An ILS approach is a precision instrument approach procedure that uses radio signals to guide an aircraft to a runway when the pilot cannot see it due to clouds, fog, or low visibility. It combines two independent beams — one for lateral alignment with the centerline and one for the descent angle — to deliver the most precise ground-based guidance available in the National Airspace System.

The word “precision” is doing real work here. A precision approach gives you vertical guidance, telling you not just whether you’re left or right of centerline but whether you’re high or low on the descent path. A non-precision approach, by contrast, only tells you about lateral position and altitude limits, leaving the descent profile up to you. That vertical guidance is what lets an ILS legally take you much lower toward the runway before you must see it.

You’ll find the ILS published as an Instrument Approach Procedure (IAP) chart for a specific runway. The FAA defines the system and its standards in the Aeronautical Information Manual, paragraph 1-1-9, which is the canonical reference any instructor will point you to.

As a student pilot, you won’t shoot an ILS in actual instrument conditions yet. But the localizer and glide slope needles live on the same instruments you’re already learning, and the mental picture you build now pays off the day you start your instrument rating.

What Are the Components of an ILS?

An ILS is built from two primary guidance components, the localizer and the glide slope, supported by a set of secondary aids such as marker beacons, approach lighting, and sometimes a compass locator or DME for distance. The two guidance beams are what make it a precision system; everything else helps you track your progress and transition to a visual landing.

The localizer antenna array sits at the far (departure) end of the runway and shoots a beam back down the centerline toward you. The glide slope antenna sits off to the side of the runway near the touchdown zone and projects an angled beam upward into the approach path. Together they form a sloping line in space that intersects the runway right where you want to land.

The supporting cast fills in the details. Marker beacons or fixes tell you how far out you are. Approach lights help you spot the runway environment when you break out of the clouds. A compass locator (a low-power NDB) is sometimes co-located with the outer marker to help you find the approach. You don’t need every piece on every approach, but knowing the roles keeps the chart from looking like alphabet soup.

How Does the Localizer Work?

The localizer provides lateral guidance, telling you whether the aircraft is left or right of the runway centerline. It transmits in the VHF band on a frequency between 108.10 and 111.95 MHz — always on odd tenths, which is how you can tell a localizer frequency apart from a VOR frequency sharing the same band.

Picture two overlapping signal lobes radiating back from the far end of the runway, one modulated at 90 Hz and the other at 150 Hz. Right down the centerline the two are equal and your needle centers. Drift to one side and one tone dominates, deflecting the needle. The course is far narrower than a VOR radial — only a few degrees wide — which is exactly why an ILS is so much more precise than VOR navigation.

The rule is the same one you already know from the VOR: fly toward the needle. If the localizer needle sits to your left, the centerline is to your left, so you make a small correction left to recenter it. The closer you get to the runway, the more sensitive that needle becomes, so corrections shrink to gentle nudges on short final.

One thing that surprises new pilots: the localizer course gets tighter as you approach the runway because the lobes converge on the antenna. A correction that barely moved the needle ten miles out can swing it noticeably a mile from touchdown. Smooth and small wins.

How Does the Glide Slope Work?

The glide slope provides vertical guidance, showing whether the aircraft is above or below the optimum descent path to the runway. It transmits on a UHF frequency paired automatically with the localizer — between roughly 329.15 and 335.00 MHz — so you only tune the localizer frequency and the glide slope comes along with it.

A standard glide slope is set to about a 3-degree descent angle, which produces a comfortable, stabilized descent of roughly 300 feet of altitude lost per nautical mile. The glide slope needle is the horizontal one on your instrument. If it rides above center, you’re below the path and need to climb (or reduce your descent); if it drops below center, you’re high and need to increase your descent. Same logic as the localizer — fly toward the needle.

This is the half of the ILS the VOR could never give you. A VOR or a localizer-only approach leaves the descent profile to your own planning and timing. The glide slope hands you a continuous, electronically guided path straight to the touchdown zone, which is what allows decision heights as low as 200 feet.

A word of caution your instructor will hammer later: only trust the glide slope once you’re established inbound on a published segment. Chasing a false glide slope lobe — a higher-angle signal that can appear well above the real path — is a classic trap, and it’s why pilots cross-check published crossing altitudes at the final approach fix.

What Are ILS Marker Beacons and Approach Lights?

Marker beacons are 75 MHz radio transmitters positioned along the approach that announce specific points on the final approach course with a light and a tone in the cockpit. They let you confirm your distance from the runway without a glance at distance-measuring equipment, and historically they defined the key fixes on the ILS.

The outer marker normally sits 4 to 7 nautical miles from the threshold and usually marks the final approach fix, where you begin the final descent. Crossing it triggers a blue light and a low 400 Hz tone of steady dashes. The middle marker sits about half a nautical mile from the threshold near the Category I decision height, flashing an amber light with an alternating dot-dash 1300 Hz tone. An inner marker, used on lower-category approaches, gives a white light and rapid 3000 Hz dots.

Many modern approaches now use GPS or DME fixes instead of physical marker beacons, but the concept lives on, and the lights and tones are still fair game on the FAA written test. Knowing blue-amber-white from outer to inner is the kind of detail that earns easy points.

Approach lighting systems are the visual half of the transition. When you descend to the decision height and look up, the approach lights, runway end identifier lights, and touchdown zone lights help you confirm you actually have the runway environment in sight before you continue to land. Without that visual, you execute the missed approach.

What Are ILS Categories and Decision Heights?

ILS approaches are divided into categories based on how low they let you descend and how little visibility you need. A Category I ILS, the kind tied to private and instrument training, allows a decision height no lower than 200 feet above touchdown and visibility as low as 1,800 to 2,400 feet runway visual range, as described in the FAA Aeronautical Information Manual paragraph 1-1-9.

The decision height (DH) is the altitude on the glide slope at which you must have decided: if the runway environment is in sight, you land; if not, you immediately fly the published missed approach. There’s no “let me look a little longer.” You decide at the DH and act. That discipline is the heart of instrument flying.

Lower categories demand more capable aircraft, more precise ground equipment, and specially trained crews. Category II reduces the decision height to as low as 100 feet, and Category III can take suitably equipped airliners essentially to the surface in extremely low visibility. Those are airline and corporate-jet capabilities, not light-trainer territory.

For your purposes as a student headed toward a private certificate, Category I is the number to lock in: 200 feet and roughly half a mile of visibility. That’s the approach your future instrument training will be built around.

How Do You Fly an ILS Approach?

Flying an ILS comes down to capturing the localizer first, then intercepting the glide slope from below, and finally tracking both needles centered down to the decision height with small, smooth corrections. The whole game is keeping the two needles crossed in the middle while managing a stable, configured descent.

Picture a student on a training flight under the hood with an instructor. Approach control vectors them onto a heading that intercepts the localizer at an angle. As the vertical needle starts to come alive and swing toward center, the student turns to align with the inbound course and lets the localizer settle. Now they’re tracking the centerline, holding altitude, waiting.

Then the horizontal glide slope needle, which had been pinned at the top, begins to descend toward center. As it reaches the middle, the student reduces power, sets a descent of roughly 300 feet per nautical mile, and starts down the slope. From here it’s a constant, gentle dance: a tap of bank to hold the localizer, a small power change to hold the glide slope, cross-checking the final approach fix crossing altitude on the way down.

At the decision height, the student looks up. Runway in sight? Land. Nothing but gray? Power up, pitch for climb, and fly the missed approach exactly as published. That decisive moment — eyes up, decide, act — is the skill the whole approach is built to train. If you want to build that instrument scan and approach picture the right way from day one, that’s exactly what we teach inside the Private Pilot Ground School, and you can start free with the Total Student Pilot course to get your footing first.

The reason we have you visualize it now, long before you’re rated, is simple: the pilots who understand why the needles move never panic when they start moving. They just fly toward them.

PLT Study Guide

These are the FAA Learning Statement Codes (PLT codes) that map to ILS content on the Airman Knowledge Test. Each code below was verified against its official FAA learning-statement wording — the original hint set included a GPS/RNAV code (PLT354) that does not match ILS material, so it was dropped.

PLT357 — Recall radio: ILS. The core ILS code. Know the system at a high level: localizer for lateral guidance, glide slope for vertical guidance, the VHF localizer frequency band, and the fact that the two beams combine to form a precision approach. This is the foundational “how the ILS works” code.

PLT277 — Recall ILS: marker beacon / indicator lights / codes. Be ready to identify the marker beacons by their cockpit indications — outer marker (blue light, 400 Hz dashes), middle marker (amber light, 1300 Hz dot-dash), inner marker (white light, 3000 Hz dots). Expect questions that give you a light color or tone and ask which marker you’re crossing.

PLT083 — Interpret information on an Instrument Approach Procedures (IAP). Know how to read an ILS approach chart: the localizer frequency, the final approach fix, glide slope intercept altitude, decision height, and the missed approach instructions. This is the “read the plate” code.

PLT382 — Recall regulations: approach minima. Understand the minimums that govern a Category I ILS — the 200-foot decision height and the visibility/RVR requirement — and the rule that you may not descend below DH unless the runway environment is in sight. This is the “how low can you go” code.

PLT420 — Recall regulations: instrument approach procedures. Know the regulatory framework for flying an IAP: established on the approach, allowable descent, and the requirement to execute the missed approach when the required visual references aren’t available at the decision height.

Frequently Asked Questions

FROM CHRIS

The ILS looks like a wall of acronyms on day one and a simple two-needle picture by the time it clicks. Lock in the basics — localizer for left and right, glide slope for up and down, fly toward both needles, and decide at the decision height — and you’ll walk into your instrument training already understanding the most important approach in aviation. That head start is exactly the kind of day-one-ready pilot we’re trying to build.

Throttle On!

Chris Palmer

Founder & Chief CFI, Angle of Attack — Two-Time Master Aviation Educator and Gold Seal CFI