What do ILS categories I, II and III mean?
In real-world aviation and flight simulation, ILS Categories I, II and III define progressively lower landing minima and the equipment, runway infrastructure, crew qualification and procedures required to use them. CAT I has a decision height no lower than 200 feet; CAT II and CAT III allow operations in progressively poorer visibility.
The category is not simply a label for a stronger signal. If the underlying guidance is unfamiliar, our explanation of ILS localiser and glideslope guidance covers how the system provides lateral and vertical steering.
What are the ILS CAT I, II and III minima?
Under the commonly quoted ICAO framework, each category is defined by decision height and runway visual range, with CAT III divided into IIIA, IIIB and IIIC.
| Category | Decision height | Visibility or RVR | Operating meaning |
|---|---|---|---|
| CAT I | 200 feet or higher | RVR at least 550 metres, or visibility at least 800 metres | Standard precision approach minima |
| CAT II | At least 100 feet but below 200 feet | RVR at least 300 metres | Lower minima requiring additional approval and equipment |
| CAT IIIA | Below 100 feet, or no decision height | RVR at least 175 metres | Very-low-visibility operation |
| CAT IIIB | Below 50 feet, or no decision height | RVR from 50 metres to below 175 metres | The lowest category commonly encountered in service |
| CAT IIIC | No decision height | No RVR limitation | Defined classification, but not normally implemented for routine civil operations |
These are reference definitions, not values to copy blindly into a flight plan. National regulations, special authorisations, runway lighting and operator approvals can produce different or higher published minima. Some countries also publish RVR in feet rather than metres. The approach chart and governing operating rules always take precedence.
Decision height and RVR are different measurements
Decision altitude or decision height is the vertical point at which the required visual reference must normally be established or a missed approach initiated. Decision altitude is expressed relative to mean sea level; decision height is expressed as a height above a runway-related datum and, on lower-category operations, is often associated with radio-altimeter height.
Runway visual range estimates how far a pilot can see runway markings or lights along the runway. It is not the same as general meteorological visibility. Low-visibility procedures may use separate touchdown, midpoint and rollout readings, with the controlling reports determined by the applicable rules.
What changes from CAT I to CAT III?
Each lower category requires more than an ILS receiver and a lower number on the approach chart.
- Runway and ILS: The ground installation must have the required performance monitoring, protected critical and sensitive areas, approach and runway lighting, standby power and suitable RVR reporting.
- Aircraft: The aeroplane needs approved guidance, monitoring and redundancy for the intended minima. CAT III commonly involves an automatic landing system or approved head-up guidance.
- Crew and operator: The operator must hold the relevant approval, and the pilots need the required training, checking, currency and procedures.
- Operating conditions: The ILS, lighting and aircraft systems must be serviceable, the required low-visibility procedures must be active, and reported RVR must meet the applicable minima.
The permitted category is set by the weakest link. A CAT III runway with a CAT II-capable aircraft permits no more than CAT II. A CAT III aircraft using a runway temporarily downgraded to CAT I must use CAT I minima. If the weather is below those resulting minima, the higher category printed elsewhere on the chart does not help.
CAT III systems may also be described as fail-passive or fail-operational. Broadly, fail-passive means a fault should not cause a significant flight-path deviation, but the crew may need to take over or go around. Fail-operational means the remaining system can complete the automatic landing after a specified failure. The aircraft's approved configuration determines which minima are available.
Does CAT III mean a zero-visibility landing?
No. CAT IIIA and CAT IIIB still have RVR limits, even when the operation has no decision height. The aircraft may land without the normal visual reference in an approved no-DH operation, but runway occupancy, rollout guidance and the ability to taxi safely remain limiting factors.
CAT IIIC is the formal no-decision-height, no-RVR-limit classification. It is not a normal practical promise that aircraft can land and taxi in literally zero visibility; ground movement and runway clearance become major obstacles even if the aeroplane can complete the airborne portion automatically.
Do CAT II and CAT III always require autoland?
No universal rule makes every CAT II or CAT III approach an autoland. CAT II may use a coupled approach followed by a manual landing where the aircraft and operator are approved for it. CAT III normally requires an approved automatic landing system or an equivalent approved guidance system, such as certain head-up-display installations.
An autoland performed in good weather is not automatically a CAT III operation. Conversely, pressing APP and capturing the localiser and glideslope does not prove that an aircraft can flare, touch down and track the runway centreline automatically. A mistake we see constantly is treating approach-mode capture as autoland certification.
Using ILS categories in a flight simulator
In a flight simulator, use the published chart minima and the simulated aircraft's documented capability even when the software does not enforce real-world approvals.
- Confirm the procedure type. Check that it is a full ILS rather than a localiser-only procedure. Our comparison of full ILS and localiser-only guidance explains why their vertical guidance and minima differ.
- Apply the weakest-link rule. Use only the category supported by the runway, aircraft configuration and weather. Do not assume that every simulated airliner or every ILS runway supports CAT III.
- Verify the modes. Confirm localiser and glideslope capture and, for an autoland, the aircraft-specific landing-system annunciation. An armed approach mode alone is insufficient.
- Make the decision. At the applicable decision altitude or height, go around if the required visual reference is absent. For a simulated no-DH operation, monitor the system status and go around after a relevant warning or unacceptable deviation.
Many simulators model prevailing visibility more readily than full touchdown, midpoint and rollout RVR, and may not reproduce ILS sensitive-area protection. That makes the exercise useful procedural practice rather than certification-equivalent training. For the interception and mode sequence itself, use our step-by-step MSFS 2024 ILS capture and landing procedure.