Clearing the fog: Why flights were able to land but not take off at Delhi airport

The fall in temperature in the presence of moisture leads to the formation of fog. When the dewpoint and temperature are the same, the air is saturated and helps condense the water vapor into water droplets, which float around, and this is what we know as fog. Dust and other pollutants in the air add to the thickness of this fog. Winds help move the fog bands, and larger fog bands will be around for a couple of days while smaller ones will go away quickly if the winds are strong.

Fog is not a phenomenon restricted to India and there have been large-scale operational disruptions in Europe and the US in the past, with the airports in London, Brussels, Frankfurt, and Amsterdam being impacted at the same time. Additionally, airports in the EU also face low visibility due to snowstorms, among other weather-related issues. Like Delhi, they, too, face disruptions and cancellations when engulfed in fog.

How do airports deal with such disruptions and what happened at Delhi?

Delhi, like many other airports, has the CAT III B ILS system installed. The Instrument Landing System allows aircraft to land by giving vertical and horizontal guidance.

The various categories ranging from Cat I to Cat IIIB help planes land in reduced visibility. For example, the Cat IIIB ILS allows landing in visibility up to 50 metres. However, the visibility needed for take-off is 125 metres. This is under the LVTO procedure, which stands for Low Visibility Take-Off. The Runway Visual Range (RVR) is what is measured and taken into consideration. As visibility remained above 50 metres and below 125 metres, the airport could handle landings but could not clear takeoffs. In terms of technology, this is the best that is available and has been working well except in rare circumstances.

The situation thus compounded and led to a problem where aircraft were coming in but did not have bays to park since the planes meant to fly out had not been able to take off due to the lack of minimum visibility.

This led to longer wait times in planes, both for arriving passengers and those waiting to depart. With aircraft stuck in Delhi, there was a cascading effect, which impacted operations across the country.

What has the government done?

The minister of Civil Aviation intervened and immediately directed officials to make Runway 29L operational for Cat III operations. The main runway, which has been closed for resurfacing since September, is yet to get operational and has seen a couple of extensions of its closure.

Apart from focusing on the infrastructure side, instructions have gone out leading to the setting up of a central control room to handle issues. Additionally, airports have been asked to apprise stakeholders, including the government, regularly.

Will this happen again?

If and when visibility drops to zero or near zero, all take-offs will come to a halt and that would mean a repeat. With the operationalisation of the closed runway, and additional ILS certification, which was suspended earlier, things should smoothen out in terms of operations.

Starting January 19th, Delhi’s airspace and the airport will be closed from 1020 until 1245 hours for the Republic Day flypast practice and the flypast during the main event. Heavy fog on any of these days would mean additional pressure on the system. What matters is how the airport and airline treat passengers. A humane approach will go a long way in ensuring that nobody comes to blows in a similar situation.