Indian budget airline IndiGo has signed an agreement to buy 180 Airbus A320 aircraft in a record sale worth 16.4 billion dollars. The European plane maker Airbus said in a statement it is the largest single firm order number for large jets in commercial aviation history. The value of the deal was not announced, but according to Airbus' catalogue price the sale is worth 16.4 billion dollars.The low-cost airline has signed a Memorandum of Understanding to acquire 150 single-aisle A320 neo jets while the other 30 planes are to be standard versions of the A320. IndiGo is the first client for this new version of the aircraft, which Airbus only unveiled last month and plans to start delivering in early 2016.