B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 208; Flight Crew Type 7100
Confusion; Situational Awareness; Human-Machine Interface
Ground Event / Encounter Vehicle; Ground Event / Encounter Loss Of Aircraft Control
Flight Crew Took Evasive Action; Flight Crew Returned To Gate; General Flight Cancelled / Delayed
Weather
We were pushing back from Gate XX at BWI. The Safety Zone and gate area were clear of ice; but the taxiways were still covered with a layer of ice. Everything was normal up through the start of the second engine. About the time Number 1 engine started to roll back; the tug began to slow at what appeared to be a normal rate and location; but instead of stopping the airplane began to move back forward. I said something to the Tug Driver and; at the same time; the First Officer said the Marshaller was signaling us to stop. I applied the brakes and the airplane stopped. The Tug Driver said that the tug had lost traction and began to slide and he couldn't stop it. The tug had jack-knifed; the tow bar was bent and broken; and the tug's cab had scraped the front of the airplane. We shut the engines down; he called his Supervisor and I called Maintenance. Ground personnel arrived and removed the tow bar and tug; and checked the nose gear for damage. We then started the engines and taxied back to the gate. Maintenance inspected the scratch on the radome and wrote it up. We then pushed again; but waited to start the engines until the push was completed and the brakes were set. The rest of the trip was uneventful.With the icy ramp; we should have waited to start the engines until the push was complete and the brakes were set. Maybe the ramp and taxiways could be cleared better. ACARS Field Conditions report said the taxiways were dry; and the airplane taxied okay; but it was obviously still pretty slick. Also; could the tugs be fitted with chains? That might have prevented the tug from sliding.
[No additional information was included this narrative].
When the B737-700 flight crew started engines during pushback on a slippery ramp the thrust of the engines overcame the coefficient of friction of the push back tug's tires and the jet rolled forward striking the tug and breaking the tow bar. Damage to the aircraft was cosmetic only and the flight departed after separation from the tug and an inspection.