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.
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.