Citation Excel (C560XL)
Flight Crew Air Transport Pilot (ATP)
Other / Unknown
Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Ramp
General Maintenance Action; General Flight Cancelled / Delayed
Weather
While taxiing from the FBO to the parallel taxiway for Runway 20; the aircraft came in contact with severe black ice just prior to the taxiway. The plane began to slide to the right; and brakes from either seat were completely ineffective. The aircraft slid off the ramp onto a snowy area just prior to the taxiway. We immediately shut down the aircraft engines and contacted ATC; advising them of our situation. Our taxi speed prior to the skid was much slower than normal. No reports of black ice were reported on the NOTAMS or over the radios. The aircraft appears to have no damage but has yet to be towed; awaiting [company] maintenance personnel to arrive.The situation would have been avoided if a thorough inspection of the ramp had been made or reports of black ice were relayed to us prior to taxi.
Ramp was covered in thin snow and ice. I was the flying pilot. Once I made the 180 degree turn to exit the ramp we did our taxi brake check and all was good. I proceeded towards the ramp exit to join Taxiway Alpha at a slower than normal taxi speed due to the conditions. The ramp exit; unlike the main ramp; did not have any snow on it and looked only wet in appearance. Approaching in an angle to the exit I started to apply left brake and left rudder to turn left. Plane was not responsive to my inputs and actually felt like it was accelerating. I applied maximum braking with no change. Brakes were functioning; but there was no tactile feel to the tires getting any traction. PIC asked what I was doing and I said I had no brakes. He tried his with no change. The aircraft continued on its path; assisted by the down slope to our right of the taxiway and exited the ramp in to the snow; just short of the Alpha Taxiway. I shut down the engines and ATC asked it we needed any assistance to which we replied yes. After checking that the owner was OK; we proceeded outside the aircraft for a visual inspection. The other Captain; reaching the pavement; almost fell right to the ground. The pavement was covered in black ice and we both had trouble walking around the left side of the plane. When Airport Operations personnel arrived they almost fell and said it was crazy how slippery it was. They then used the airport sand truck to lay down sand at the other ramp to taxiway entrances. I made the call to Assistant Chief Pilot and informed him of the situation. Was instructed to document with pictures and send to company and to not attempt moving the aircraft until our own personnel arrived to monitor. The situation may have been avoided if all ramp areas and taxiway entrances were treated with sand like Taxiway Alpha.
CE560XL flight crew taxiing for takeoff encountered black ice and departed the Ramp.  Lack of sand treatment in the Ramp area was cited as a contributing factor.