Airwork Hermes G-AKFP was on a flight from Blackbushe to Singapore with intermediate stops at Karachi, Delhi, and Calcutta. Approaching Calcutta, the Hermes was cleared for a runway 19L ILS approach 04:41. At break-off height shower passed and the crew were not able to see the runway; an overshoot was carried out. At 05:01 ATC offered the captain a radar assisted approach to runway 01R; he was no. 2 to land at that moment. Radar Control then guided the aircraft and cleared it for a visual landing at 05:29. At that moment the aircraft was one mile from the threshold and to the left of the runway 01R centreline. After breaking through clouds the captain observed the runway. R/T was turned down and the captain continued visually. In fact the Hermes was approaching runway 01L. An Indian Airlines DC-3 has just been cleared to line up and hold on runway 01L. On landing, the Hermes struck the DC-3. PROBABLE CAUSE: “An error on the part of the Commander of the Hermes aircraft in turning down the R/T during the final stage of the radar assisted approach and in deciding to continue the approach under conditions which did not enable him to identify positively the correct runway.”