###While we are not given any complete indication, it is a safe bet that Nick Fury did not send in attack aircraft because they were more of a threat to the civilian population than they would be an asset to the Avengers. Conventional military air support has a great deal of collateral damage involved, something Nick Fury is hoping to prevent.

 - We see the chaos with Thor, Hulk and Iron Man fighting in the lead against the Chitauri with Captain America, Hawkeye and Black Widow fighting a ground battle. Air-to-air combat using fighter jets would have meant air-to-air missiles (whose effectiveness could only be guessed at) and .50 caliber rounds filling the air, penetrating buildings and causing mass casualties among civilians. 

 - In theory, this is why the Avengers were called. As living weapons, SHIELD expects the Avengers to do their best to both limit casualties and direct their intelligence onto finding a way to stop the enemy without inflicting more casualties than the military would using conventional weaponry.