The Instrument Landing System (ILS) is the gold standard of precision approaches — and the only approach in U.S. civil aviation that can legally bring an aircraft to the runway in essentially zero visibility. As of 2026, the FAA recognizes five distinct ILS approach categories: CAT I, CAT II, CAT IIIa, CAT IIIb, and CAT IIIc. Each lowers the minimum decision height and required visibility further than the last, with CAT IIIc theoretically permitting a landing with no decision height and no runway visual range minimum at all.
Most GA pilots will only ever fly CAT I approaches. But understanding how the categories work — and what makes the lower minimums possible — is one of the most useful pieces of IFR knowledge a pilot can carry.
What Defines an ILS Approach Category?
ILS categories are defined by two numbers: decision height (DH) and runway visual range (RVR).
Decision height is the altitude above the runway touchdown zone at which the pilot must have the required visual references in sight — approach lights, runway threshold, touchdown zone markings — or execute a missed approach. It’s measured using a radio altimeter (radalt) for CAT II and CAT III operations, not a barometric altimeter, because radalt accuracy is essential at the lower altitudes involved.
Runway visual range is the horizontal distance a pilot can see down the runway from the approach end, measured by transmissometer equipment at the runway. It’s expressed in feet or meters and reported in 100-foot increments. The lower the RVR, the more restrictive the operation.
Together, DH and RVR define how close to zero-zero conditions an approach can legally proceed.
The Five ILS Categories
Here’s how the FAA’s authorized minimums break down, as published in 14 CFR and FAA Order 8400.13:
Category I (CAT I). Decision height of 200 feet and RVR of 2,400 feet (or 1,800 feet with touchdown zone and centerline lighting). This is the most common ILS approach and the only one most GA pilots will ever fly. CAT I is flown by single pilots, uses standard barometric altimetry, and requires no special aircraft certification beyond a working ILS receiver.
Category II (CAT II). Decision height of 100 feet and RVR of 1,200 feet (with newer authorizations allowing as low as 1,000 feet RVR). CAT II requires specialized crew training, specific aircraft equipment including dual ILS receivers and a coupled autopilot, and the airport must have CAT II-rated ground equipment and lighting. The decision height is measured by radio altimeter, not barometric.
Category IIIa (CAT IIIa). No decision height, or a decision height below 100 feet. RVR not less than 700 feet. Most CAT III operations performed worldwide are CAT IIIa.
Category IIIb (CAT IIIb). No decision height, or a decision height below 50 feet. RVR less than 700 feet but not less than 150 feet. CAT IIIb operations typically require autoland capability — the aircraft lands itself, with the crew monitoring.
Category IIIc (CAT IIIc). No decision height and no RVR limitation. Theoretically permits landing in zero visibility. While CAT IIIc is technically defined in the FAA’s framework, it is not used operationally because pilots and ground crews still need some visibility to taxi off the runway safely after landing. No U.S. airport is currently authorized for CAT IIIc operations.
What Makes Lower Minimums Possible?
Going from CAT I to CAT III isn’t simply a matter of flying lower. Each step down requires significantly more equipment, training, and infrastructure on three sides: the aircraft, the airport, and the crew.
Aircraft requirements. CAT II and CAT III aircraft need redundant systems — typically dual ILS receivers, dual autopilots, dual flight directors, and radio altimeters. CAT IIIb autoland aircraft also require fail-passive or fail-operational autopilot architectures, meaning a single autopilot failure cannot cause an unsafe landing. Aircraft must be type-certified for the specific category and individually inspected to confirm the systems meet performance standards.
Airport requirements. CAT II/III runways need more capable ILS ground equipment, including stricter signal-monitoring tolerances. They also require enhanced lighting — high-intensity approach lights, touchdown zone lighting, centerline lighting, and runway edge lighting that meets specific standards. ILS critical area protections become much more stringent: ground vehicles and aircraft must hold farther back from the antenna during CAT II/III operations to avoid signal distortion. Airports often have separate “ILS hold short” lines that activate only when CAT II/III operations are in progress.
Crew requirements. CAT II authorization requires specific ground and flight training, including simulator-based low-visibility operations practice. CAT III crews must also be trained in autoland procedures, low-visibility taxi techniques, and the specific failure modes associated with autoland systems. Both pilots typically need to be qualified for CAT II/III — single-pilot CAT III is generally not permitted.
Operator authorization. The operator (airline or Part 135 charter company) must hold an FAA letter of authorization for each category. CAT II and CAT III operations are typically limited to Part 121 air carriers and Part 135 operators with the necessary fleet and training programs. CAT I is available to any IFR-rated pilot in an appropriately equipped aircraft.
Why CAT III Matters for the Airline System
CAT III operations exist because fog and low-visibility weather routinely cause significant disruptions at major airports. Without CAT III capability, airports in fog-prone regions — London Heathrow, Paris Charles de Gaulle, Seattle-Tacoma, San Francisco, Boston Logan — would close for hours or days during certain weather patterns.
The economic impact is substantial. A major hub closing for six hours due to fog cancels hundreds of flights and disrupts tens of thousands of passenger itineraries. CAT III capability allows operations to continue when CAT I and CAT II minimums would otherwise shut down the airfield.
For airlines, CAT III equipage and training is a competitive advantage. The aircraft that can land in 700-foot RVR keeps flying when others divert. The crew certified for autoland can complete missions that less-equipped crews must abort.
What Do GA Pilots Need to Know?
If you fly IFR in a typical GA single or light twin, you’re flying CAT I approaches. Your decision height is 200 feet above the runway. Your visibility minimum is half a statute mile (2,400 feet RVR) — or, on better-equipped runways, three-eighths of a mile (1,800 feet RVR) with touchdown zone and centerline lighting.
Here’s what’s worth understanding even if you’ll never fly a CAT III approach:
The 200-foot DH is conservative for a reason. It assumes a typical GA aircraft, normal pilot skills, standard barometric altimetry, and no special equipment. CAT II and CAT III pilots fly lower minimums because they have additional systems and training — not because the weather conditions allow more aggressive flying.
RVR is more accurate than ground visibility. When you have access to RVR data on an approach, use it. RVR is measured at the runway, in real time. Ground visibility is reported from an observation point that may not reflect actual conditions on final.
Approach lighting matters. The reason CAT I minimums drop from 2,400 to 1,800 RVR with touchdown zone and centerline lighting is that those lights give you reliable visual references closer to the runway. If you’re flying into an unfamiliar airport with limited lighting, plan for higher minimums.
The missed approach is always an option. Reaching DH without the required visual references means you go missed — every time. Continuing below DH without seeing the required references is a violation and a major safety risk.
ILS critical area protection affects you, too. Even at GA airports, if CAT II or CAT III approaches are in progress, you may be held farther back from the runway during taxi to prevent signal interference. Watch for the special ILS hold short signs.
The Bottom Line
ILS categories are a layered system that pushes the lowest visible decision point closer and closer to the runway as equipment, training, and infrastructure capability increase. CAT I serves the vast majority of pilots and approaches. CAT II adds specialized capability for airlines and corporate flight departments. CAT III enables autoland operations down to functionally zero visibility for properly equipped aircraft and trained crews.
Understanding where your aircraft and authorization sit in this framework is one of the foundational pieces of IFR knowledge. The next time you fly an ILS to 200 and a half, you’ll know exactly what the system is doing — and what the next steps down would require.
Frequently Asked Questions
What is the difference between ILS CAT I, CAT II, and CAT III? ILS Category I has a decision height of 200 feet and RVR of 2,400 feet (or 1,800 feet with enhanced lighting). Category II has a decision height of 100 feet and RVR of 1,200 feet. Category III has no decision height or a DH below 100 feet, with RVR as low as 150 feet (CAT IIIb) or unlimited (CAT IIIc, theoretical only). Each lower category requires more aircraft equipment, crew training, and airport infrastructure.
What is decision height (DH) on an ILS approach? Decision height is the altitude above the runway touchdown zone at which the pilot must have the required visual references (approach lights, runway threshold, touchdown zone markings) in sight — or execute a missed approach. For CAT I, DH is 200 feet, measured by barometric altimeter. For CAT II and CAT III, DH is lower and measured by radio altimeter for greater precision.
What is runway visual range (RVR)? Runway visual range is the horizontal distance a pilot can see down the runway from the approach end, measured by transmissometer equipment installed at the runway. RVR is reported in 100-foot increments in feet (or in meters internationally) and is more accurate than reported ground visibility because it’s measured at the runway itself. ILS approach minimums use RVR as the visibility standard for most operations.
What is the lowest legal ILS minimum? The lowest authorized ILS minimums in the U.S. are Category IIIb: no decision height or DH below 50 feet, with RVR not less than 150 feet. Category IIIc theoretically allows zero DH and zero RVR but is not used operationally because pilots and ground crews still need visibility to taxi safely after landing.
Can GA pilots fly CAT II or CAT III approaches? Generally, no. CAT II and CAT III approaches require specific aircraft certification (dual ILS receivers, autopilot coupling, radio altimeter, sometimes autoland), specialized crew training, and an operator letter of authorization from the FAA. These authorizations are typically held by Part 121 air carriers and well-equipped Part 135 operators, not by GA pilots flying personal aircraft. Most GA pilots fly only CAT I approaches.
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