PARIS: London Heathrow remained the busiest aviation hub in Europe last year but Istanbul Airport was nearly neck-and-neck and is likely to overtake it soon, an industry group said Thursday. With 84.48 million passengers, Heathrow had a 0.7 percent increase in traffic last year, ACI Europe reported, citing the use of larger planes by airlines at “the capacity-constrained British hub.” British authorities say a third runway will be added at Heathrow but it is not expected to be ready before 2035.