Patna, Dec. 3: Fog continues to affect flight operations at Jayaprakash Narayan International Airport with the onset of winter.

How fog has disrupted the flights schedule can be gauged from the fact that no flight is landing at Patna airport before 11am nor are scheduled flights taking off before that.

The fog has forced different carriers - Air India, GoAir and IndiGo - to withdraw some flights operating from Patna till February 15. Reason: poor visibility in the early hours, which further delays other flights later in the day. As the whole of northern India, particularly New Delhi and eastern Uttar Pradesh, are engulfed in dense fog in the morning, Patna-Delhi and Patna-Lucknow routes appear to be the worst affected.

While IndiGo has withdrawn three flights, GoAir and Air India too have withdrawn their flights on the Patna-New Delhi route. This has brought the total number of flights operating from Patna airport from 25 to 20, cutting down options for fliers.

The flights withdrawn by IndiGo airlines are 6E494 (Patna-Ranchi-Delhi), 6E339 (Bangalore-Calcutta-Patna-Lucknow-Mumbai-Vadodara) and 6E634 (Delhi-Lucknow-Patna-Calcutta-Goa). The two other flights to be withdrawn are GoAir's G8 124 flight (New Delhi-Patna) and Air India's AI 415/416 on the New Delhi-Patna-New Delhi route.

Flight operations have been adversely hit as visibility has reduced to less than 100m in the morning hours, an official of a private airline said today, adding: "As against the required visibility of at least 1,200m, the visibility has reduced to such a level that it has almost become impossible to stick to scheduled timings, forcing us to cancel and re-schedule flights."

Airport director Rajendra Singh Lahauria said: "We have taken a few steps to improve flight operations under poor visibility conditions. Additional tarmac lights or approach lighting system (ALS) have been installed on an additional stretch of 220m ahead of the runway to improve flight operations under poor visibility conditions."

ALS comprises high-intensity lights, which helps pilots establish visual contact with the runway while landing a plane.

However, the shorter runway at Patna airport is one of the major factors responsible for delayed flight operations. Patna airport is marked as one of the most critical airports in the country because of its short runway and obstacles in its approach funnel. Against the total runway length of 2,286m (7,500 feet), only 1,954m (6,409 feet) remains available at the time of take-off because of the airport's location constraint.

This is the reason why even after installation of ALS, flights are getting delayed beyond schedule, said an official of the Airports Authority of India (AAI) at Patna airport.

The other factor responsible for flights getting delayed in foggy condition is that Patna airport is not equipped with CAT III Instrument Landing System (ILS) which enables pilots to land aircraft even in the midst of considerably reduced visibility.

CAT III ILS enables the aircraft to land and take off even in 50m visibility and also during heavy rainfall.