Date: 201508
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: SF 340B
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
ATC / advisory: Ramp ZZZ
Aircraft component: Airframe
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Object
Detector: Person Flight Crew
When detected: Routine Inspection
Result: Aircraft Aircraft Damaged; General Maintenance Action
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: The aircraft was given a preflight by both the PIC and SIC. We pushed back tail east with the right engine feathered using high pressure air for cooling. All operations appeared to be normal and the flight continued to airport ZZZ.We landed [at ZZZ] and as we taxied to the gate we had to stop for a few minutes for another company aircraft to move out of the gate area. Our aircraft pulled in and the engines were shut down. A few minutes later the FO went outside to do the post flight. The lead ramp supervisor asked the FO to come over to the cargo door area to look at something. As the FO walked past the left side of the cockpit; he called me to come out and look at the bottom of the cargo door area.The damage below the cargo door was a large 2.5 meter by 10 cm scrape with multiple depressions in the exterior skin. Primer paint and bare aluminum was exposed. The depressions had raised high spots over the bulkhead and stringers. The damage shows strong blunt force impact of an object striking the lower fuselage and continuing aft.The ground crews had spotted this damage as the aircraft taxied in. At least three direct witnesses saw the damage and surveillance cameras showed the aircraft taxi in before ground crews approached the aircraft. The lead supervisor even waited to push the loading stairs any closer until the FO and myself saw the damage.Photos of the damage and log book write up were taken and emailed to maintenance control. Dispatch and the Chief Pilot were notified. Contract maintenance would not sign off on the open write up. More inspections were needed below the cargo floor boards before the ferry permit was completed. The aircraft was then ferried for major repair work.There is strong evidence that shows the damage occurred on the ground [at our departure airport.] There was no damage noted during the preflight and the damage was noted first by the ZZZ ground crews and then by the flight crew. The impact force of the dents made by a bag cart or a GPU would be very hard to miss if it was being pulled away by a tractor. The individual responsible for this must have seen or felt something. There was no notification given to the flight crew after the damage occurred. The flight continued to ZZZ as an unairworthy Part 121 aircraft. The damage was only noted after the fact. My recommendations are very simple for avoiding a recurrence of this event. If you break or damage something; admit it and don't put innocent lives at stake with a damaged flying airplane. Some gates are very crowded and more work space is needed.


Second narrative: [Report Narrative Contained No Additional Information.]


Synopsis: After landing; evidence of damage below the cargo door of the SF340 was discovered by the ground crew.