Delhi Fog: What is CAT III and how it helps flights land safely in zero visibility Explained. explainer news

Dense fog and mist once again disrupted flight operations at Delhi’s Indira Gandhi International Airport on Monday morning, sharply reducing visibility across the National Capital Region and triggering a wave of delays and cancellations. More than 40 flights were canceled and more than 400 were delayed as airlines adjusted schedules, with officials saying operations are currently being conducted under CAT III conditions.

The Civil Aviation Ministry issued a heavy fog warning for northern India, advising travelers to check flight conditions before traveling and to allow extra time for road traffic, which was slowed to a minimum. While Delhi airport reiterated that poor visibility means operations are taking place under CAT III protocol, airlines like IndiGo and Air India put out a message, asking passengers to closely monitor the updates.

IndiGo said visibility has been significantly reduced due to dense fog and some flights may be proactively canceled to avoid long waits at the airport.

The decline in weather reflects a familiar pattern during northern India’s winter months, when long periods of fog repeatedly test the aviation ecosystem. Although Delhi is equipped with CAT IIIB low-visibility landing system, disruptions continue as landing an aircraft safely in such conditions requires the right combination of airport systems, aircraft equipment and pilot training.

This makes it important to understand how CAT III works and why delays occur even when the system is activated.

Why were flights disrupted due to fog in Delhi?

Fog severely reduces visibility, and even a short period of dense fog can affect aircraft landing, takeoff, and taxiing speeds. Operations slow down as pilots receive fewer visual cues during approach, landing and ground movement. Airlines often delay or cancel flights to maintain safety margins, and runway distances increase as planes require more time between landings.

These decelerations occur even when an airport is operating under CAT III procedures. CAT III is the highest category of instrument landing systems (ILS) used during low visibility conditions. This allows pilots to land even when runway visual range is extremely limited, but does not guarantee that every aircraft or pilot is authorized to use the system. As a result, some flights cannot land until visibility improves, leading to the wave of delays and cancellations that Delhi sees every winter.

What is CAT III and why is it used in winter?

Cat III refers to a category of instrument landing systems that guide aircraft during approach and landing in extremely poor visibility. It is a ground-based radio navigation system that provides information about the aircraft’s alignment with the runway and its vertical descent profile.

Airports use CAT III particularly during winter fog, when pilots cannot rely on visual references such as runway lights or markings.

Delhi airport is equipped with CAT IIIB instrument landing system which allows landing in visibility less than 50 metres. CAT III is intended to keep the airport operational during dense fog, but does not completely eliminate disruptions. Landings may proceed only if an aircraft, its pilots, and the runway are all certified for the applicable category. If any one element does not comply, the flight will have to wait, be diverted or cancelled.

How does the instrument landing system work?

The ILS provides two essential streams of information to the pilot during landing: azimuth guidance and vertical guidance.

Azimuth guidance tells the pilot whether the aircraft is correctly aligned with the centerline of the runway. This comes from the localizer (LOC), whose antenna is located on the far side of the runway. They transmit two narrow radio beams – one slightly to the left of the center line and one to the right. The point at which the beams intersect marks the centerline of the runway. The aircraft’s instruments interpret this information and display how far the aircraft is off the true horizontal path.

Vertical guidance comes from the glideslope (GS), whose antenna is usually located along the middle section of the runway. These antennae also transmit two intersecting beams, one slightly above the ideal vertical profile and one slightly below it. The intersection marks the correct descent path. This lets the pilot know immediately if the aircraft is too high or too low at any point during the approach.

Since pilots cannot rely on vision during dense fog, ILS becomes critical. This replaces visual cues with precise, continuous guidance as to where the aircraft should be horizontally and vertically. In some advanced systems, automated call-outs calculate the distance to touchdown and give signals to deploy flaps and apply brakes.

How are CAT I, CAT II and CAT III different?

ILS ranges are defined by two parameters: decision height (DH) and runway visual range (RVR). DH is the altitude at which the pilot must be able to see the runway environment; If visibility at that point is below limits, the landing should be aborted. RVR measures how far the pilot can see runway lights or markings.

Based on the classification of Airports Authority of India, the categories are:

DH: not less than 60m

RVR: not less than 550 meters

DH: Less than 60m but not less than 30m

RVR: not less than 350 meters

DH: Less than 30m, or no judgment height

RVR: not less than 200 meters

DH: Less than 15m, or no judgment height

RVR: not less than 50 meters

DH: no decision height

RVR: No visibility required (zero visibility allowed)

These limits determine how low a pilot can descend using instruments alone, and how much of the runway must be visible for a safe landing.

CAT III C has the highest capability and allows landing even in zero visibility. Airports such as New York’s John F. Kennedy and London Heathrow support CAT IIIC operations. India does not yet have any runway certified for CAT IIIC.

Which Indian airports support CAT III?

Six Indian airports can support CAT IIIB operations: Delhi, Lucknow, Jaipur, Amritsar, Bengaluru and Kolkata. Delhi was the first country to establish this system. Runway 28 received CAT IIIA in 2001, which was upgraded to CAT IIIB with Advanced Surface Movement Guidance and Control System (ASMGCS) in 2005 at a cost of Rs 55 crore.

Why is there a delay even when CAT III is active?

Even with CAT IIIB, flights can and do get delayed or canceled during dense fog. Some aircraft may not be certified for effective range, and some pilots may not be trained to land using CAT IIIB procedures. If the airport is operating under CAT III and a flight is certified only for CAT I or CAT II, ​​it cannot land until conditions improve.

Ground operations have also slowed down considerably. Taxiing in low visibility requires more space between aircraft and more time between activities on the runway. This reduces how many take-offs and landings can be handled per hour.

Fog also affects the departure order. Aircraft may have to wait longer between takeoffs to maintain a safe distance, and slower ground speeds may cause further backlogs. Within a few hours, this turns into major delays across the network.

What makes winter landings so sensitive?

Landing is the shortest but riskiest phase of flight. Boeing’s research shows that final descent and landing take up only about 4 percent of total flight time, yet 49 percent of fatal accidents occur in this window. Low visibility adds another layer of complexity because pilots cannot use external references to assess alignment or altitude. The ILS compensates for this by providing accurate radio-based indications, but only if the aircraft, crew and runway are all certified for the range of operations.

Fog is persistent in winter in North India and is often so dense that visibility is reduced to less than 200 meters at a time. When this happens, airports must switch to CAT III procedures, but the overall speed of operations inevitably slows down. A single spell of fog can cause delays during the day, affecting flights across the country.