Fog enshrouding East Midlands airport prevented tower controllers from realising that a parking stand was occupied by a Jet2 Boeing 737-300, before the jet was struck by a Ryanair 737-800 taxiing past it.
The Ryanair aircraft, arriving from London Stansted on 30 April, had been cleared to follow the shortest path to stand S22.
This meant passing behind the Jet2 aircraft on stand S24. This aircraft did not show up on the surface-movement radar display.
Low-visibility procedures were in place at the time, with runway visual range down to just 300-325m, and the fog made the Jet2 aircraft "invisible" to air traffic controllers, says the UK's Air Accidents Investigation Branch.
"Had visibility been better, [controllers] would have had an opportunity to visually acquire the obstruction and offer a different route [to the Ryanair crew]," it adds.
Although the Jet2 aircraft was correctly parked, the short 737-300 was occupying a position normally intended for a 737-800 – a larger aircraft, which might have made the lack of clearance more obvious to the Ryanair crew.
The inquiry says aeronautical information charts did not indicate that clearance could be compromised, and even though the Ryanair crew was aware of the narrow margin and kept to the taxiway guidance line, the 737-800's right winglet sliced into the Jet2 aircraft's horizontal stabiliser.
"Commercial flight crew routinely operate on airfields where following established taxiway markings generates safe separation," says the inquiry. "Repeated achievement of safe outcomes through compliance builds confidence and trust that airfield markings are safe to follow."
Investigators also point out that previous probes have highlighted the limitations of the human eye in judging relative distance at ranges above 10m. This perception problem is increased, it says, on aircraft equipped with blended, rather than angular, wing-tips.
East Midlands airport's operator subsequently closed stand S22 pending a safety review and surveyed other parking stands to identify similar separation risks. Both carriers involved also alerted crews to the hazard of reduced separations with certain stands at the airport.