At the conclusion of the air taxi flight, the flight crew were taxiing the small twin-engine airplane to their assigned gate at night. The crew were holding short of the gate area and awaiting clearance to enter. After holding for about 10 minutes with the airplane’s parking brake set, the flight crew heard a loud noise and the airplane lunged forward when it was struck from behind by a tug that was towing an unoccupied airliner. The small airplane sustained substantial damage to its elevator and horizontal stabilizer. The tug’s windshield was damaged during the collision, and the airliner was not damaged. Surveillance video showed that the tug’s headlights were operating as it drove straight along the taxiway toward the small airplane. The small airplane’s wing and tail navigational light were also visible and operating. As the tug approached the rear of the airplane, its headlights brightened, and its speed abruptly slowed just prior to the impact with the airplane’s elevator. According to the company who operated the tug, following the accident, the driver and safety observer in the tug initially reported that its brakes had “failed” but later described that the brakes were not operating as expected. The tug crew also reported that an “oversteer alarm” had been sounding continuously during their operation of the tug, and that they had reported this to a supervisor prior to initiating the tow of the airliner. They were instructed to complete the tow, after which maintenance personnel would evaluate the reason for the alarm. Following the accident, maintenance personnel performed an operational check of the tug’s brakes and noted no anomalies. Further examination revealed that the oversteer alarm was due to a malfunction in the tug’s monitoring system. Given this information it is likely that the tug crew saw the small airplane, but their visual lookout was not sufficient to identify it with sufficient time/distance to stop. While the tug was being operated with an active alarm, this should not have impacted the tug crew’s ability to stop the tug while driving straight on a taxiway.