Date: 201702
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; Captain
Anomaly: Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Vehicle
Detector: Person Flight Crew
When detected: Taxi
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: As we approached the [gate] there was one ramp agent with lighted wands directing us in. There was no tug at gate and he was standing on the line with an unobstructed view of our aircraft. There was a de-ice truck parked parallel to the safety zone. I had commented that they must have been de-icing that day because there was glycol on the ramp surface which obstructed our view of safety zone markings; but since the truck was parallel and not on an angle or perpendicular; it did not appear to be in a position that would encroach on zone. The Captain slowly taxied into gate and I noticed he was constantly checking his wing and the truck position.I could not see our wing on that side so I kept my eyes on ramp agent to call out if he crossed wands. The Captain was giving extra room and was favoring the line opposite the truck position and the ramp agent kept trying to correct him towards the truck side. We then felt impact. By then there were other ramp agents standing around and were all waiting for us to pull closer. The marshalling ramp agent kept directing us forward; but we were stopped with brake set to figure out what happened. I yelled down to a ramp agent that we thought we hit the truck and he shook his head and told me 'we were good'. He then walked around nose and came back and gestured with hands the amount of room we had to spare from truck. We had parking brake set and did not move so the ramp agents then walked all the way to truck and came back to confirm it was struck. The agent maneuvered the jetway out to meet us in and we deplaned normally. Inspection showed the left outer most slat had a dent.Additional Note: As we were deplaning we felt the aircraft shake back and forth. Someone had moved the deice truck farther away from wing.Equipment and/or vehicles should never be left unmanned in a safety zone. There should be more use of wing walkers with a better view of close quarters when aircraft are directed to parking; especially when it is dark and the normal visual cues that are used to judge distance are not present. Finally; there was no confusion between myself and the Captain in regards to the tuck. It looked close; but in our operation it always looks close. The lighting and irregular position of the gate were factors; but after all is said and done we are the flight crew and it is our aircraft. We could have just stopped.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 crew reported they hit a deice truck as they were being marshaled into the gate.