Date: 201412
Local time of day: 0601-1200
Location: BWI.Airport, MD
Flight conditions: VMC
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground BWI
Aircraft component: Interphone System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Communication Breakdown
Second reporter function: Captain
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown; Ground Incursion Taxiway
Detector: Person Flight Crew
When detected: Other Pushback
Result: Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Aircraft

Narrative: We were ready to pushback with a normal pre-departure scramble to get pushed on time. Captain had attempted to contact Ground Crew and there were problems with the Tug Driver's headset. As I recall; the Tug Driver got the Captain's attention to open the side window and stated that we could hear him; but he could not hear us. I was finishing my last minute cockpit prep and the Captain and Tug Driver discussed hand signals. Soon after; a Ground Crewman appeared on my side of the aircraft and was signaling to release brakes (we later realized this was the Tug Driver and not an additional Ground Crewman). I directed him to go to the other side of the aircraft to confer with the Captain. Captain gave a released brakes signal; and within a few seconds; the aircraft started moving backwards before receiving clearance. I immediately tried to communicate with Ground Control that we had lost communication with the Tug Driver and were pushing back. The first attempt failed due to being blocked by another aircraft talking to Ground Control. By the time I was able to communicate with Ground Control; we were already onto the taxiway and they advised that there was another aircraft inbound and to give way to them. I advised them that we couldn't stop the pushback since we had lost communication with the tug. The other aircraft was far enough away that they saw what was happening and stopped well before there would have been a collision hazard.Hand signals are a tricky issue. There needs to always be an additional Crewman to relay signals at stations where the driver cannot be seen by the Flight Crew. Also; we had forgotten that the Tug Driver supposedly could hear us so we may have been able to stop by using the flight interphone. Instead of remembering this; we focused on trying to alert Ground and other aircraft that we were on the move.


Second narrative: I failed in communicating with my Tug Driver. To prevent this from happening; I will be a better and more deliberate communicator.


Synopsis: When the flight crew and the tug driver failed to fully brief one another on a pushback without two way communication; the flight crew found themselves being pushed without a clearance to do so and unable to communicate with the sole member of the pushback crew. They were unaware there would be no additional ground crew with which to exchange hand signals.