London - A passenger aircraft and an executive jet came within 100 ft of colliding in mid-air near Heathrow Airport in London, it emerged yesterday. The passenger aircraft was waiting for clearance to land at Heathrow while the Gulfstream business jet, belonging to a US corporation, was heading for Farnborough, Hampshire. According to an air traffic controller: "The radar images of the two planes were seen to merge on the screen, but fortunately they both popped out again the other side."