A Turkish Airlines plane bound for Switzerland has been called back to Istanbul after a bomb threat, reports have said on Friday.Turkish Airlines flights have had to take emergency measures against bomb threats three times in less than a month.The Boeing 738 aircraft was diverted back to Istanbul while over Bulgarian, sources have reported. It was carrying 151 passengers, according to an airline spokeswoman.The flight has landed safely in Istanbul after bomb threat. A paper with 'there is a bomb' written on it was found in the plane. Bomb experts investigated the plane, which was parked in a remote area in the airport.After a detailed investigation, the bomb threat has been found to be a hoax alert. Passengers will be taken to Basel on another plane.Recently, a Boeing 777-300 ER plane traveling from Istanbul to Sao Paulo made an emergency landing in Casablanca and a flight from Istanbul to Portugal returned back to Istanbul on April first in two threats which were found to be hoax alerts.