London - Two airliners came dangerously close to colliding after an air traffic controller instructed the wrong pilot to descend, said a report yesterday. The pilot of the higher of the two Heathrow-bound Boeing 757s spotted the danger and climbed to safety as the aircraft came within 400 ft of each other. In a report on the incident, on November 22nd last year, the Joint Airprox Working Group - which studies airmisses - said it had been a "very serious".