London Heathrow Airport's operator and its air traffic service are to assess whether wing-tip clearance warnings to taxiing aircraft can be improved, after analysis of ground collisions revealed circumstantial similarities.

During an inquiry into a wing-tip clash between a SriLankan Airlines Airbus A340 and a British Airways Boeing 747-400, the Air Accidents Investigation Branch identified nine other ground collisions that had taken place at Heathrow since 1975.

"In all cases the collisions were the result of a widebody aircraft attempting to pass a stationary aircraft waiting at a holding position," it says.

Various contributing factors led to the incidents, it adds, including the difficulty of judging wing-tip clearance from the cockpit of a large aircraft, and the belief of some pilots that following a centreline track ensured adequate separation.

In its newly-released report into the SriLankan collision the AAIB says the airport operator should "improve the effectiveness" of warnings issued to manoeuvring aircraft. These warnings, it adds, should clarify to pilots that separation is not assured, regardless of taxi clearances given by air traffic controllers.

The SriLankan A340 had been attempting to pass the BA 747 on the night of 15 October 2007.

Although the A340 crew had heard an automatic departure message before taxiing, which included a caution over wing-tip clearance, the AAIB says the pilots had misinterpreted its meaning as a disclaimer.

While the 747 was stationary at the A1 holding point, near the threshold of runway 27R, the A340 was cleared to take the adjacent taxiway on the left and hold at A2.

The A340 crew understood that the taxi clearance "meant that there was nothing to impede their progress" towards A2, says the AAIB, but the pilots nevertheless discussed their separation from the 747 and even steered the A340 to the left of the taxiway centreline.

None of the passengers and crew members, nearly 650 in total, was injured in the subsequent collision but the 747's left winglet was severed, 1.7m (ft) from its tip, and the A340 sustained winglet and navigation light damage on its right wing.

Air traffic service NATS, which has responsibility for Heathrow ground control, points out that it is "incumbent" on pilots to ensure adequate clearance while taxiing but states that it is willing to work with Heathrow's operator to explore options for improvement.

Source: Air Transport Intelligence news