Date: 201409
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Taxi
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown; Ground Excursion Taxiway
Detector: Person Ground Personnel
When detected: Routine Inspection
Result: Flight Crew Became Reoriented; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Human Factors; Procedure
Primary problem: Ambiguous

Narrative: During pushback the left main wheels departed the pavement directly behind the gate. Conditions were dark; headsets in use with a Wing Walker. At completion of the pushback the Tug Driver asked for BRAKES SET.  I complied; the nosepin was removed and I cleared off the Pushback Crew. Prior to disconnect; the tug driver told me to 'Taxi straight out because your left main wheels are very close to the grass.' I asked for clarification and was told the aircraft was on the edge of the grass and to taxi straight out before turning to stay clear (Not exact words). I looked out the left window and could see we appeared very close to the grass and the number one engine seemed close as well; so I asked if the engine was over the grass or not. I was concerned about FOD ingestion during start. I was told it was clear so I acknowledged and cleared off the Crew and called for number one engine start. The F/O started the engine and we completed a normal after start flow and Before Taxi Checklist. I was still concerned about the proximity of the grass to the number one engine so I attempted to taxi out on number two powered up only. We were fairly heavy so I knew it would take a moment to get rolling but we did not seem to be making any progress at all; so I increased power slightly and bumped number one up a little and still no result. After several seconds and still no progress I was sure something was wrong. I shut down number one and called Ops to come and inspect the left side of the aircraft. When the Supervisor arrived on the tug; it was immediately clear things were not normal. At this point we had already restarted the APU so we connected APU electrical and shut down number two and raised the flaps. I indicated I wanted the Ground Crew to come up on headset. He confirmed that the mains were 'On the grass'.  I then notified the FAs via interphone and the Passengers via PA. Then I called my Dispatcher and told her the situation and asked her to notify maintenance so they could have the aircraft inspected if we were able to get back to the gate. About this point the Ground Crew had reconnected the tug and asked me to release brakes so they could 'Pull us off the grass.' Not really sure about the idea but also not having very good information or experience about the situation I reluctantly agreed and the towbar instantly snapped (the A pumps were off and depressurized). Now I was very sure we were well off the pavement and going nowhere. I called Dispatch and got a patch to maintenance to make sure they knew what was happening and advise them of the logbook entry I had made. I also spoke to the Duty Chief. I gave him a thorough description of the events and made sure there were no issues with us continuing our day.  Eventually a plan came together to deplane the Passengers; swap aircraft; and leave this one for an experienced rescue Crew. I spoke with the Passengers from the Flight Attendant station PA; and informed them of the situation and the plan moving forward. The Flight Attendants skillfully handled an orderly and efficient deplaning via the air stairs. The F/O and I inspected the interior and then shut the aircraft all the way down. We went to the next aircraft and continued our trip. For those who are concerned about such things; calling into Dispatch and maintenance and having a Chief Pilot to talk to quickly was very helpful.  I was very grateful for the help and teamwork in; what was for me; a very unusual situation. I  can't be sure from my vantage point what the true causal factors were for the Ground Crew but it seems to me the practice of a single Wing Walker is not covering all our bases too well. Ground Ops may have more input here.   As for me; my only reflective criticism right now is that I should have trusted my instincts and not let them try to move the aircraft at all after the first disconnect. I put too much trust in Ground Crew and Supervisors involved; albeit with little information. I felt like they had a better idea of the situation outside the aircraft but I should have asked more questions. Although I can't say I wouldn't have done the same thing for sure I was leaning that way and maybe more info would have pushed me into waving the Ground Crew off. I knew the aircraft would need to be inspected and had no other intentions but to return to the gate. However; having never really discussed this type of thing in training; or experienced it myself; I didn't immediately understand the full implications or complications in that moment. In reality; we probably didn't risk a great deal allowing them to try just simply because there was no way the tug was going to budge the aircraft but I still would advise others to stop any further action.


Second narrative: During pushback from Gate; the aircraft was pushed too far back. When the aircraft was stopped the left main wheels were off the ramp and on the ground. Although it was dark; lighting was adequate. From the flight deck it's extremely hard to tell the position of the main wheels. Neither the Captain nor I got a sense the aircraft was too far back and we felt nothing abnormal during pushback.


Synopsis: B737-700 flight crew reports being pushed back too far with the left main wheels off the hard surface. The ground crew does not accurately describe the situation but an attempt to taxi out proves fruitless. The engines are shut down and the tug reattached; but the aircraft cannot be moved.