An Instrument Landing System, or ILS, guides an aircraft to a runway when clouds, fog, rain, or darkness make visual flying difficult. ILS approach categories describe how low the pilot can descend and how little runway visibility is allowed before landing. The lower the category minimums, the more precise the ground equipment, aircraft systems, crew training, and airport lighting must be.
These categories matter because they determine whether a flight can safely land or must go around or divert.
Understanding Aviation: ILS Categories
The signals must be interpreted with care because they are most useful only in the correct part of the approach. The localizer becomes more sensitive as the aircraft gets closer to the runway. A small needle movement near touchdown can represent a small sideways error, so large steering corrections can make the approach unstable.
The glideslope can produce misleading lobes above the intended path. Pilots intercept it from below, following published procedures, to avoid tracking a false signal. Airport buildings, vehicles, snow, and aircraft can disturb signals if they enter protected areas near the antennas.
A decision height is an operational stopping point, not merely a number on an instrument. At that height, the crew must have enough of the required visual references to continue. These may include approach lights, runway edge lights, the threshold, or runway markings.
Seeing a vague glow is not enough. The crew must be able to judge that the aircraft is correctly positioned for landing.
If the required reference is missing, the procedure requires a go around. In the lowest category operations, the aircraft may continue using approved automatic landing equipment until it reaches the flare and rollout stages.
Runway visual range measures what a pilot is likely to see along the runway, which is more useful for landing than a general weather visibility report. Sensors beside the runway estimate how far runway lights or markings can be seen through fog, rain, snow, or haze. A long runway can have different reported values near the touchdown zone, midpoint, and far end.
The touchdown value is especially important because it describes the first part of the runway that the aircraft will use. Crews compare these reports with the limits for their aircraft approval and the approach being flown.
Low visibility operations depend on a complete chain of equipment and people. The aircraft needs properly working receivers, radio altimeters, flight controls, warning systems, and often multiple autopilot channels. The crew needs specific training and recent practice.
The airport must maintain suitable lighting, protected signal areas, and accurate monitoring of the ILS. Some automatic landing systems are fail passive. After a failure, they disconnect or give a warning, leaving the crew to take control.
Others are fail operational and can continue safely after certain failures. These distinctions affect which approach categories an aircraft may use.
When studying an ILS approach, separate guidance from performance. The needles show whether the aircraft is on the desired path, but they do not guarantee a safe descent rate or landing speed. Vertical speed equals groundspeed times descent gradient.
Groundspeed changes with wind, while airspeed can stay nearly constant. A strong tailwind therefore needs a higher descent rate to remain on the same path.
Pilots monitor speed, descent rate, power, configuration, and deviations together. A stable approach makes it easier to recognize a problem early and carry out a go around before the situation becomes unsafe.
Key Facts
- ILS guidance has two main signals: localizer for left and right alignment, and glideslope for vertical path.
- Typical ILS glideslope angle is about 3 degrees, giving a descent gradient near 318 ft per nautical mile.
- CAT I minimums are typically DH = 200 ft and RVR at least 550 m or visibility about 800 m.
- CAT II minimums are typically DH = 100 ft to below 200 ft and RVR at least 300 m.
- CAT III approaches allow DH below 100 ft or no decision height, with very low RVR depending on the subcategory.
- Descent rate estimate: vertical speed = groundspeed x descent gradient, so at 140 kt on a 3 degree glideslope, vertical speed is about 740 ft/min.
Vocabulary
- Instrument Landing System
- An Instrument Landing System is a radio navigation system that provides precise lateral and vertical guidance to a runway.
- Decision Height
- Decision height is the height above the runway where the pilot must decide to continue the landing or begin a missed approach.
- Runway Visual Range
- Runway visual range is the distance a pilot can see along the runway using lights, markings, and instruments.
- Localizer
- The localizer is the ILS component that tells the aircraft whether it is left or right of the runway centerline.
- Glideslope
- The glideslope is the ILS component that gives the aircraft a safe vertical descent path toward the runway.
Common Mistakes to Avoid
- Treating ILS category as only a weather label is wrong because it also depends on runway lighting, ground equipment, aircraft certification, and crew qualifications.
- Confusing decision height with runway visual range is wrong because decision height is a vertical altitude limit while RVR is a horizontal visibility distance.
- Assuming CAT III always means fully automatic landing is wrong because CAT III operations may still require specific monitoring, procedures, and aircraft capabilities.
- Using CAT I minimums for every ILS approach is wrong because CAT II and CAT III can permit lower minimums only when the airport, aircraft, and crew are approved for them.
Practice Questions
- 1 An aircraft is on a 3 degree ILS glideslope with a descent gradient of 318 ft per nautical mile. How high above the runway should it be 5 nautical miles from touchdown?
- 2 A jet flies an ILS at a groundspeed of 150 kt. Using a 3 degree glideslope descent gradient of 318 ft per nautical mile, estimate the required vertical speed in ft/min.
- 3 A runway reports fog with RVR 350 m and a cloud base near 120 ft above the runway. Explain why a CAT I approach would not be enough, and identify which ILS category might be required if the aircraft and crew are approved.