A NEAR-COLLISION between a Suckling Airways Dornier 228 and a Business Air Saab 340 near the UK's Trent VOR navigation beacon was the closest the investigators "...had ever had to consider", according to an Aircraft Proximity Hazard Panel report.
The two aircraft were on reciprocal headings on Airway A2. The 228 was at 8,000ft (2,400m) when the Saab 340 was cleared down to 9,000ft. The 340 captain acknowledged 9,000ft, but set 8,000ft on the autopilot.
The air-traffic sector coordinator warned the controller that the altitude of the 340 showed 8,500ft and she immediately warned the captain. The aircraft were already passing each other, however, with a separation of "less than 0.25nm [0.45km] horizontally/0ft vertically," says the report.
Source: Flight International
