ILS Approach Explained for the INRAT Exam

The ILS is the most common precision approach in Canada and one of the most tested topics on the INRAT. Questions come up in Instrument Approaches, Air Law, and indirectly through approach ban and alternate requirements. If you're fuzzy on how it works, you're leaving marks on the table.

What the ILS actually is

The Instrument Landing System gives you two radio guidance signals simultaneously: the localizer for lateral guidance and the glideslope for vertical guidance. Together they put you on a precise 3-degree descent path to the runway threshold.

The localizer transmits on a frequency between 108.10 and 111.95 MHz — always an odd decimal. It's 700 feet wide at the threshold and widens as you get further out, which matters when you're intercepting it at distance. The glideslope operates on a paired UHF frequency that's selected automatically when you tune the localizer.

The components and what they do

• Localizer (LOC) — lateral guidance aligned with the runway centerline. Full-scale deflection is 2.5° either side.
• Glideslope (GS) — vertical guidance, typically 3° above horizontal. Full-scale deflection is 0.7° either side — much more sensitive than the localizer.
• Outer Marker (OM) — typically 4–7 NM from the threshold. Blue light, continuous dashes on the audio. Marks the point where you should intercept the glideslope at the published altitude.
• Middle Marker (MM) — approximately 3,500 feet from threshold at decision height. Amber light, alternating dots and dashes.
• Decision Height (DH) — the altitude at which you must have visual reference or execute a missed approach. Expressed as a height above touchdown zone elevation.

Decision Height vs Minimum Descent Altitude

This distinction comes up constantly on the INRAT. The ILS is a precision approach, so it uses a Decision Height (DH) — you fly the glideslope down to DH and decide at that point. Non-precision approaches use a Minimum Descent Altitude (MDA) — you descend to MDA and fly level until you either see the runway or reach the missed approach point.

The difference matters procedurally. At DH on an ILS you're still descending; at MDA on a non-precision approach you've levelled off. The exam will test whether you know which applies to which approach type.

ILS categories

Canada uses three ILS categories, each with progressively lower minimums:

• CAT I — DH 200 feet, RVR 1800 feet (or visibility 1/2 SM). Standard at most Canadian airports with ILS.
• CAT II — DH 100 feet, RVR 1200 feet. Requires additional aircraft equipment and crew training.
• CAT III — DH below 100 feet or no DH, RVR as low as 600 feet. Very few airports in Canada have CAT III.

For the INRAT, CAT I numbers come up most often. Know them cold.

What the exam actually tests

ILS questions on the INRAT fall into a few patterns. You'll be asked to identify components, interpret approach plate information, calculate when you should be at specific altitudes on the glideslope, and apply approach ban rules.

The approach ban is a common source of errors. Under CARs, a commercial operator cannot commence or continue an approach below 1,000 feet AGL if the reported RVR or visibility is below the published minimum. The exact conditions under which you can or cannot continue are tested directly.

Glideslope intercept altitude is another one. Questions give you a published crossing altitude at the outer marker and ask whether a specific scenario is correct. The glideslope at the outer marker has a published altitude — fly above it before intercepting, not below.

Missed approach on the ILS

At DH, if you don't have the required visual reference, you fly the published missed approach procedure immediately. No delay. The missed approach point on a precision approach is DH — not the runway threshold, not the middle marker. Fly the missed approach as published; don't improvise.

The INRAT tests missed approach initiation, what constitutes adequate visual reference, and circling approach rules after an ILS. All three are worth reviewing specifically. For comparison, the VOR approach uses an MDA rather than a DH — understanding the operational difference between the two is one of the most frequently tested distinctions in the Instrument Approaches category.