On January 16, 2012, about 0700 eastern standard time, a Boeing DHC-8-102, N839EX, registered to US Airways Express and operated by Piedmont Airlines as flight 4117, sustained substantial damage to the left horizontal stabilizer and elevator when it collided with a deicing truck while taxiing at Tri-State Airport (HTS), Huntington, West Virginia. The scheduled domestic passenger flight was conducted under the provisions of 14 Code of Federal Regulations Part 121. Visual meteorological conditions prevailed and an instrument flight rules flight plan had been filed. The 2 pilots, 1 flight attendant and 33 passengers were not injured. The flight was originating from HTS and was destined for Charlotte/Douglas International Airport, Charlotte, North Carolina. According to the Director of Safety for US Airways, HTS had recently changed from deicing on the gate to off-gate deicing. Prior to the airplane leaving the gate, the Captain spoke with the ramp agent, telling him that he would like to taxi out with one engine, and leave the engine in feather while the airport personnel (vendor) conducted the deicing. The ramp agent (1) then walked to the deice vehicle and conveyed the Captain's request, to which the vendor personnel agreed. The ramp agent (1) then went back to the airplane to confirm with the Captain that his plan was acceptable with the vendor. The ramp agent then said that he told the Captain to taxi out in front of Gate 2, but the Captain said he was told to taxi out in a non-specific manner. After the Captain started the #1 engine, an agent pulled the wheel chocks and then went inside to station operations to man the radio, while another agent marshalled the aircraft out of the Gate 1 parking area. Because of the proximity of another airline’s piece of equipment, the agent doing the marshalling then stood by the wing to ensure sufficient clearance, and gave thumbs up to the flight crew to signal that they were clear of the equipment. After the airplane taxied out and was positioned near gate 2, the Captain stopped the airplane and asked the First Officer (FO) to call station operations and clarify where they were to go for the deicing. The FO spoke with the ramp agent who was working the radio, but the agent could not see the airplane and did not realize the aircraft had left the gate. Assuming that the aircraft was still at the gate, the agent told the crew to taxi 50 feet or so and stop. Without the flight crew’s knowledge, the vendor’s deicing team misunderstood the agent’s thumbs up signal as the signal that deicing could begin. Consequently, the deicing team moved into a position behind the left wing and in front of the left horizontal tail when the aircraft first stopped, and started spraying the rear of the aircraft. Having received guidance from station operations to move 50 feet, the flight crew initiated taxiing and almost immediately felt a bump, then stopped. After seeing the aircraft start to move, the bucket operator yelled to the driver to "back up" and tried to lower the boom, but the aircraft tail had already struck the boom arm.