An explosive three-alarm fire engulfed two hangars at Brown Field Tuesday, sending a thick plume of black smoke over the airport and forcing a temporary closure of its runways. The blaze in the 1400 block of Continental Street erupted for unknown reasons shortly after noon, according to the San Diego Fire-Rescue Department. It caused about $100,000 in damage to the structure and about $600,000 in damage to the contents inside the buildings, including two light planes inside the structures. A third hangar was badly damaged. The fire's cause is under investigation.