Approach lights to relax visibility criterion
♦ Approach lighting system (ALS): A set of light series of lightbars, strobe lights, or a combination of the two installed on the approach end of an airport runway. It allows the pilot to visually identify the runway environment and align the aircraft with the runway upon arriving at a prescribed point on an approach
♦ Number of new lights: One on Pir Ali Path (airport road) and two inside Patna zoo (south-east corner)

Visibility breather
♦ Visibility criterion till 2012: 1600m 
♦ Visibility criterion for 2013 winter: 1200m 
♦ New visibility criterion after additional ALS become functional: 800m

Visibility on foggy days
♦ 1600m: 2-3pm
♦ 1200m: 11am-12 noon
♦ 800m: 9-10am

The minimum visibility criterion for aircraft at Jayaprakash Narayan International Airport would be revised from the existing 1,200 metres to 800 metres shortly after the installation of three additional approach lighting systems.

The criterion means the visibility at any given time should be at least at the minimum level to allow landing or taking off of aircraft at the airport.

The lesser the minimum visibility criterion the better it is considered in aviation parlance because it paves way for smoother operation of flights irrespective of weather factors like downpour and fog, which reduce actual visibility and hit flight operations.

Though the benefit of the reduced level of minimum visibility would not be felt immediately but during winter months when Patna generally remains enveloped in fog during most part of the day, this factor would become important and would allow better conditions for flight operations at the airport.

The reduction is visibility level at Patna airport is in sight because of the ongoing installation of three additional approach lighting system (ALS) - one on Airport Road and two inside Patna zoo. It is happening almost a year behind the scheduled installation.

An ALS is a set of high intensity lights, which guide the pilot to establish visual contact with the runway while landing an aircraft.

The pillar for installing the ALS, commonly known as tarmac lights, has been constructed on the Airport Road and officials in Airports Authority of India (AAI) claimed that the lights on all three spots are expected to be functional within a month.

When functional, these lights would reduce the existing minimum visibility criterion for flight operation to 800 metres at the airport.

Minimum visibility criterion is also known as runway visual range (RVR) in aviation parlance. Explaining how the additional tarmac lights would reduce the RVR for flight operation at Patna airport, city-based aviation expert Atul Singh said: "Ideally, the minimum length for installation of ALS is 420 metres before the runway, which allows the flights to land with visibility at 800 metres or less. However, the ALS at Patna airport are installed on a stretch of 210 metres prior to the runway. This is due to the fact that Airport Road comes immediately after the runway, followed by Patna zoo. Installation of additional approach lights beyond the boundary wall of the airport would however solve this issue to an extent."

On January 2 last year, the then chief secretary Ashok Kumar Sinha in a meeting held between the officials of state government and the AAI had given in-principle approval to the latter's proposal to install the three additional tarmac lights.

Deliberating on the progress made in installation of the additional tarmac lights, a senior official in AAI said: "We have constructed the pillar on Airport Road and two similar pillars inside Patna zoo premises would also be constructed in next few days. The tarmac lights are being installed along the central alignment in the aircraft approach funnel and the distance between each of them is 30 metres. The only challenging task for us is to do underground cabling to connect the three ALS. We would do it using tunnel cabling method, which does not require digging of the ground. Accordingly, we intend to make the three ALS operational within a month."

Apparently, Jayaprakash Narayan International Airport is marked as one of the most critical airports of the country due to its short runway and obstacles in its approach funnel. Against the total runway length of 2,286m (7,500 feet), only 1,954m (6409 feet) remains available at the time of take-off because of its location constraint.

The flying experience from the city airport has remained quite bad during winter months in the recent years due to shorter length of runway and the tarmac lights. Taking into consideration the frequent disruption of flight operations at Patna airport owing to foggy conditions during peak winter season, the authority reduced the minimum visibility requirement for flight operation from 1,600m to 1,200m in 2013.

Despite the relaxation in visibility criterion, none of the flights was still able to land here before 12 noon in the peak winter season.

Accordingly, the AAI in December 2013 submitted an application to the state government, environment and forest department and road construction department to install more ALS on the approach funnel.