SOFIA (Bulgaria), February 25 (SeeNews) - Bulgaria's government aviation unit Aviootryad 28, which provides air transport services to high-ranking state officials, said that it has opened a tender for two four-year contracts for maintenance of its Airbus A 319 aircraft and supply of spare parts and consumables, worth a total of 4.2 million levs ($2.4 million/2.1 million euro), VAT excluded.