Quick Answer: An Instrument Landing System (ILS) is a radio-based navigation system that helps pilots line up with a runway and follow the correct descent path, especially during poor visibility.
How does an ILS work?
An ILS provides two main types of guidance:
Localizer (LOC): Tells pilots whether the aircraft is left or right of the runway's approach centreline.
Glide Slope (GS): Tells pilots whether the aircraft is above or below the correct descent path, typically around 3°.
Together, they help pilots follow a precise approach path towards the runway.
A simple example
Imagine an aircraft approaching an airport in fog.
The pilot cannot see the runway clearly, but the aircraft's instruments display guidance from the ILS.
The localizer helps the pilot align with the runway, while the glide slope helps maintain the correct descent path.
The pilot must still follow the published approach procedure, applicable minimums and ATC instructions.
What are CAT I, CAT II and CAT III ILS?
| Category | Simple explanation |
|---|---|
| CAT I | Standard precision approach with specified visibility and decision-height requirements |
| CAT II | Allows operations under lower visibility conditions with additional requirements |
| CAT III | Supports very low-visibility operations using specially approved equipment and procedures |
The actual landing limits depend on the airport, aircraft, crew qualifications and applicable approvals.
Frequently asked questions
Can an ILS land an aircraft automatically?
Not by itself. ILS provides guidance. An automatic landing also requires an aircraft with approved autoland capability and the necessary operational conditions.
Can pilots use ILS during fog?
Yes. ILS is especially useful in reduced visibility, but pilots must still comply with the applicable landing minimums.
Is ILS the same as GPS?
No. Traditional ILS uses ground-based radio signals, while GPS-based approaches use satellite navigation and, where applicable, augmentation systems.
Is ILS available at every airport?
No. Not every airport or runway has an operational ILS.
In summary: The localizer guides the aircraft left and right, while the glide slope guides it up and down. Together, they provide precise runway approach guidance.