An unused air traffic control tower caught fire at Charlotte-Douglas International Airport during demolition Thursday, when investigators say molten slag from ongoing work inside the tower dropped onto piled foam insulation. 

Four construction workers were briefly trapped inside the tower after it caught fire about 8:10 a.m., and all four were able to escape the building under their own power and did not require medical treatment, according to posts from the Charlotte Fire Dept. The fire went to second alarm but was controlled in 10 minutes after firefighters arrived to see heavy smoke coming from the tower. 

The tower, owned by the Federal Aviation Authority, was being demolished following the construction and commissioning of a new tower, constructed as the airport pursues its $3.1-billion capital expansion program. 

The new, 370-ft-tall tower began operations in 2022, and was constructed by Archer Western, which, according to an FAA announcement from 2016, was also responsible for the old tower’s demolition as well as equipment, installation and training costs.