Cessna Citation Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Other / Unknown
Aircraft Equipment Problem Less Severe; Ground Event / Encounter Other / Unknown; Ground Excursion Taxiway
General None Reported / Taken
Airport; Weather
Slid off Taxiway A at ZZZ during winter conditions.  Light freezing rain began to fall as we were pre-flighting the plane. We were concerned since we now had to use flaps 7 on takeoff due to having to get Type 4 fluid. Our T/Rs were pinned which meant we had no numbers for a flaps 7; anti-ice on; on a wet runway nor contaminated. I contacted the Ground Control to ask for a runway condition update. They provide no braking advisories and said they were chemically treating the runway. We decided to taxi out; get de-iced/anti-iced and to examine the runway ourselves to see if it was 'dry' prior to taking off. The ramp had patchy ice in some areas; but the PIC noted no problems during a brake check. The PIC made a 90 degree turn onto Taxiway A; with no slipping or problems. Sitting in the right seat; I started the taxi check. I tried my brakes and noticed no problems. I then called for the rudder bias check. The PIC barely cracked the right throttle and the plane began to quickly slide to the left. He closed the throttle. The tiller had no affect and I felt him depress the right rudder in attempt to control the slide. Nothing worked. As we slid towards the grass; a service road was noticed to the left. A suggestion was made to attempt to make it. With the tiller still not responding; the PIC applied left brake.   Just as we were exiting the taxiway in the slide; the left brake grabbed which decelerated us and swung the nose left. Not knowing if we stopped on the service road or not; I contacted the ground and told them we slid into the grass; that we are disabled on Taxiway A; and that braking action was nil on Taxiway A. We exited the aircraft and noticed we stopped perfectly on the service road and at no time did any part of the aircraft leave the pavement. We walked onto the Taxiway A and were barely able to stand and noted about 1/8 of an inch of black ice on the entire taxiway. Later an airport worker noted to us that braking action on Runway 1; our intended runway; went from a brake report of 80 to 30 within 11 minutes; which was the time we were taxing out. This information was not relayed to us during or before taxi. The Airport ended up closing for several hours. Suggestions; It is a possibility that if our T/Rs worked; we may have been able to use them to help stop the plane on the taxiway. It is a suggestion to not dispatch aircraft with disabled items that assist in braking during any and all adverse weather conditions.
Citation flight crew experienced a taxiway excursion event when losing directional control; the reporter indicated black ice was noted after leaving the aircraft.