Date: 201202
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Icing
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Situational Awareness
Anomaly: Ground Excursion Taxiway; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Returned To Gate; General Flight Cancelled / Delayed
Contributing factors: Airport; Human Factors; Weather
Primary problem: Ambiguous

Narrative: After deicing was complete and we copied all pertinent deicing data. We then started the second engine. While stationary; we performed our before taxi and takeoff checks prior to leaving the ramp area. This was to minimize distractions on the taxi out to the runway. We informed Ground that we were ready to taxi. They cleared us to taxi and that 'braking action was reported as good'. Approaching the first turn; I had been following the tracks of a previous aircraft in the freshly laid snow. On this taxiway braking was now good to fair. I brought the aircraft to a stop. I made a right turn heading south and continued to taxi. Upon entering the next intersection I stopped the aircraft. Braking action was fair. I found that I was now aligned too close to the left of the taxiway. I was concerned that the left turn would require too wide of a radius; due to traction; in order to make the left turn. My First Officer suggested a right 270 because there was plenty of room to negotiate the turn. I assessed the situation and agreed that was a better plan. We asked permission to make a right 270. Tower granted permission. While the aircraft was stopped; I turned the nose wheel to the right. I released the brakes and began to let the aircraft move. I then added power. The aircraft moved forward a little to initiate momentum. Then the aircraft began to turn right. Suddenly; the aircraft slid forward moving left to right across the taxiway heading south. It appeared that we were also now on a slight downward slope. Simultaneously I went to idle and straightened the nose wheel and began applying brakes. We were now moving at approximately 5 MPH. Nothing would stop us. I believe that I may have moved the nose wheel to the left in a vain attempt to steer us on a more centered course on the taxiway we were on. The aircraft 'gently' came to rest in the grass at the right hand corner of the intersection of the two taxiways. The nose wheel was turned to the left. There was virtually no forward travel between the pavement and the nose wheel entering the mud. I had the First Officer inform Ground that we would require assistance. Within ten minutes or so the Airport Operations officials where at our side. One of the Airport vehicles; an SUV; moved from left to right in front of our aircraft. He was traveling at about 5-10 MPH and applied brakes and slid at least six feet or more with the tires locked. He demonstrated this twice in front of the cockpit. I had an Airport Official take a picture of the nose gear which I then forwarded to Maintenance Control. They towed us back approximately six feet. I was informed by the towing foreman; that my right main gear was now six feet away from the grassy area. They unhitched the aircraft and gave us permission to start and taxi out. There were several inches of new snow and the aircraft was still on a decline facing south on the taxiway at the same 35 degree angle. I declined to taxi and requested a tug to return the aircraft to the gate. Snow was falling; de-ice operations in effect and active runway clearing operations [were] being conducted. Reports of good braking action or less. If I should find myself in a similar situation; particularly at a smaller city airport where personnel and funds are in short supply; I would request braking reports and ask for the conditions of the taxiways. I would also make the request that the taxiways be cleared of snow with a braking report being made. If I found that the situation appeared to be worse than was being reported by the Tower; I would coordinate with Dispatch to have the airport ensure taxiway safety or begin plowing operations.




Synopsis: An Air Carrier had a taxiway excursion while taxiing for takeoff in snowy conditions with the braking action reported as good.