On December 12, 2017, about 0735 EST, Delta flight 2421, Boeing McDonnell Douglas MD-88, N924DL, after being pushed back from the gate, with the towbar still connected, was substantially damaged after the airplane moved forward after engine start and struck the tug at Detroit Metropolitan Airport (DTW), Detroit, Michigan. There were no injuries to the 61 passengers and crew onboard. The flight was operating under 14 Code of Federal Regulations Part 121 as a scheduled domestic passenger flight from DTW to Raleigh-Durham International Airport (RDU), Morrisville, North Carolina. During pushback, the tug driver informed the flight crew they were cleared to start. After the left engine was started, the airplane started to unexpectedly move forward. Both flight crew members were focused inside the cockpit, running checklists, when the tug driver asked if the brake was set on the airplane. The captain responded "no" and the first officer then noticed the position of the tug and told the captain that they needed to stop the airplane. The airplane struck the tug before stopping. After the airplane's brake was set, the flight crew noticed that the engine throttles were not at idle as expected. Ground conditions were reported as slippery. The tug struck the right forward lower fuselage causing damage to the pressure vessel (skin, stringers, and frames).