Airports - Fogged Out

While flying at all North Indian airports is affected during the ‘fog season’ to some extent or the other, disruption at the Indira Gandhi International Airport (IGIA), New Delhi has a cascading effect on flight operations at other airports as well

Year after year, with certitude and regularity, the monsoon and winter seasons give operational function-aries of airlines flying in India some anxious and sleepless nights. Winter adversely affects the flying operations in North India for a period of around two months. Nominally, the “foggy period” is taken by the India Meteorological Department (IMD) to commence on December 8 and end on February 15 every year, whereas the Directorate General of Civil Aviation (DGCA) regards this period as between December 10 to February 10. While flying at all North Indian airports is affected during the “fog season” to some extent or the other, disruption at the Indira Gandhi International Airport (IGIA), New Delhi has a cascading effect on flight operations at other airports as well. This is so because New Delhi is the aviation hub in the North and a major base for all airlines except Paramount Airways. Despite the preventive measures, flight schedules during the fog season continues to be frequently disrupted. However, in recent times, there has been a small but perceptible downward trend in the number of such episodes.

Statistically, during the fog season, visibility could be expected to drop to below 50 m for nearly up to 150 hours at IGIA. Flight operations are impossible under these conditions. The atmospheric visibility along the runway in use in the direction of arrival and departure of aircraft is referred to as the runway visual range (RVR), and it is electronically measured with a high degree of accuracy.

Low RVR affects flight operations in two critical phases of flight—landing and take-off. While arrivals are regulated by landing minima, a stipulated minimum value of RVR based on the landing aids available at the airport, departures are dependent on the low visibility take off (LVTO) procedures put into place by the DGCA. An essential requirement of these procedures is availability of another airport with visibility conditions permitting a landing, within about an hour of flying time from the aerodrome of departure. The two interrelated but distinctly different yardsticks that define the limitations for landing and take-off separately, occasionally lead to confusion. The confusion is sometimes compounded because of the fact that the arrival and departure vary with the aircraft type. The existing criteria for precision approaches which were revised by the DGCA prior to the onset of the last fog season are as follows:

Category II Precision Approach Runway: An instrument runway served by ILS and/or MLS and visual aids intended for operations with a decision height lower than 60 m (200 ft) but not lower than 30 m (100 ft), and RVR not less than 300 m.

Category III Precision Approach Runway: An instrument runway served by ILS and/or MLS to and along the surface of the runway and:

• Intended for operations with a decision height lower than 30 m (100 ft) or no decision height and a runway visual range not less than 175 m.
• Intended for operations with a decision height lower than 15 m (50 ft), or no decision height and a runway visual range less than 175 m but not less than 50 m.

It would be evident from the above that arrival is not possible if the RVR is less than 50 m. The minimum RVR for departure under LVTO conditions is 150 m as stipulated by the DGCA. DGCA is in the process of revising the minima to 125 metres as a result of a recent consultation meeting with the airlines. In fact, New Delhi is the only Indian airport to have Cat III ILS installed on two of its three runways and so the impressively low minima outlined above under Cat III A and Cat III B, in fact, relate only to New Delhi. At other non-Cat III airports, minima in respect of RVR and decision height would be relatively higher.