Pilots landing a Virgin Atlantic Airbus A340-300 at night in Nairobi were forced to execute a late go-around after the jet entered an unexpected fog bank on touchdown, and strayed almost completely off the side of the runway.

Investigators from the UK are trying to understand why visibility information transmitted to the crew during the 27 April approach apparently differed greatly from the conditions recorded by automated sensors.

Although both pilots had good visual contact with runway 06's approach lights at the 200ft decision height, the aircraft entered fog while still at 20ft and the first officer, flying the jet, lost sight of the right-hand runway edge lights.

As the A340 touched down it remained on its two main landing-gear bogies, drifting to the left. "The commander became aware of the left runway edge lights moving rapidly closer to him before he lost the lights completely and was only aware of their position by the glow of the lights illuminating the fog," says the Air Accidents Investigation Branch.

Although the commander immediately ordered a go-around, the aircraft's left gear came off the side of the runway, some 960m (3,150ft) from the threshold, travelling off the surface for 180m. The right gear left the runway surface but managed just to stay within the shoulder boundary.

The A340 stayed on the ground for only about five seconds neither its nose-gear nor its centre-mounted main gear contacted the surface. It became airborne and diverted to Mombasa, where it landed without further incident. None of the 122 occupants was injured and the jet, G-VAIR, suffered only minor abrasion damage to its fuselage.

During the approach to Nairobi, air traffic controllers relayed to the crew that a preceding aircraft had reported landing visibility of 3,000m and a cloudbase of 300ft. But an automated weather station recorded a minimum runway visual range of 550m.

"The investigation will continue towards establishing the runway surface condition, the visibility of the markings, and condition of the lighting to quantify what, if any, contribution they may have made to this incident," says the AAIB.

"Further enquiries will be made regarding the difference between the [runway visual range] recordedand that passed to the crew of [the A340]."

The inquiry will also examine whether light luminescence from the edge lighting affected the runway visual range.

Nairobi's runway 06 is 4,117m long and 60m wide, of which the outer 7.5m on either side is a paved shoulder. It has no centreline lighting - none is required - but the investigators point out that the runway edge lights are 7.5m from the declared strip, rather than the ICAO-standard maximum distance of 3m.

Source: Air Transport Intelligence news