On August 9, 2014, about 0735 central daylight time (CDT), a Bombardier CL600 airplane, N943LR, collided with a tug during pushback from the gate at San Antonio International Airport (SAT) San Antonio, Texas. The airplane sustained substantial damage to the fuselage structure and internal damage to nose landing gear. The airplane was registered to and operated by Mesa Airlines Inc. as US Airways flight 2763. The four flight crewmembers and passengers on-board were not injured. Visual meteorological conditions prevailed at the time of the accident and the flight operated on a instrument flight rules flight plan. The scheduled, domestic passenger flight was operated under the provisions of 14 Code of Federal Regulations Part 121. The flight was destined for Phoenix Sky Harbor International Airport (PHX), Phoenix, Arizona. The first officer reported the tug driver did not have an operable headset and the pushback was initiated using hand signals. The airplane was positioned on the ramp at a 90-degree angle to the gate. The tug driver stated after turning the airplane onto the taxiway he "reversed back," pulling the airplane forward. The tug stopped perpendicular to the left nose of the airplane, but the airplane continued to roll forward while still attached to the tow bar. The airplane rolled into the tug impacting the left side of the fuselage. Examination revealed the tow bar shear pin had failed, but the investigation could not determine if the shear pin failed prior to or during the pushback process. In accordance with company policies and procedures, a postaccident drug test of the tug driver was administered about 9 hours after the accident, which was positive for marijuana. According to 49 CFR Part 40 Section 40.87, the initial test cut off is 50 ng/ml, but a positive marijuana test can be reported if the confirmatory test identifies 15 ng/ml or more of marijuana metabolite (tetrahydrocannabinol carboxylic acid, or THC-COOH) in urine. According to the NTSB Medical Officer, about 30% of THC is eventually excreted in urine, primarily as THC-COOH. However, its presence in urine only indicates prior THC exposure. After smoking marijuana, it can take as long as four hours for THC-COOH to appear in the urine at concentrations above the initial reporting cut off of 50 ng/ml. Positive urine test results generally indicate use within hours to a few days; however, the detection window can be significantly longer following chronic, heavy use.