Date: 202108
Local time of day: 1801-2400
Aircraft: B777 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Reporter organization: Air Carrier
Reporter function: Captain; Check Pilot; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Communication Breakdown
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Ground Event / Encounter Other / Unknown
Detector: Person Flight Crew; Person Flight Attendant
When detected: Taxi
Result: Flight Crew Overcame Equipment Problem; Flight Crew Returned To Gate; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I began this trip with my flying First Officer (FO) starting leg 3 of his transition operating experience from the Airbus; one International Relief Officer (IRO) relatively new to the fleet and still not yet consolidated; while the other IRO was a seasoned FO on the 777 fleet.We pushed back from gate.  After the left engine start was complete we noticed a strange and unfamiliar odor that wasn't readily identifiable.  A brief discussion took place on the flight deck about the possibilities of the odor origin and all agreed it was not an electrical smell so we continued with the right engine start procedure.  Within seconds of the right engine start completion; smoke was noticeable in the cockpit along with a very strong and identifiable electrical burning odor.  We noticed flashing switch lighting on the overhead panel; specifically the window heat switches but no EICAS warnings had appeared.  Thoughts of Swiss Air flight 111 many years ago instantly entered my mind and rather than execute a lengthy non-normal procedure; I made the decision based on being on the ground that we would be better off in the long run to remove power immediately from the aircraft.  I elected to shutdown both engines at that point and have one of my IROs open the door and check in the back with the purser and check if the cabin had smoke as we were assessing whether to initiate an evacuation or not at this point.  There was a lot of ground traffic around us in all directions; both automotive and aircraft; and I was wanting information as to just how bad things were in the cabin before committing to an evacuation. XXXXXX reacted above and beyond any expectations I had and promptly reported back with the following information:Door 1R Flight Attendant (FA) had seen smoke coming from the floor area around the jumpseat as well as the seat 1L floor area.  The customer in seat X was relocated away from the smoke area.  I was told there was some smoke in the galley and forward seats but that was it and that it appeared to cease escalating and that the floor was not hot. XXXXXX had also opened up the electronics (EE) compartment door and performed a visual inspection that revealed a scorched panel with no visible fire.  He mentioned; that while the scorched panel was warm; it was not hot and felt certain that whatever had precipitated the event had stabilized and all we had was smoke at this point with electrical odor. With that firsthand information and no further comments from the FAs; I made a PA informing the customers and inflight crew of the situation; realizing there probably were flashing lights along with some smoke in the cabin we were certain there was no further reason to suspect a fire and that we would be returning by tow back into the gate.  I would later find out from the FA and Passengers through sharing of information in the boarding lounge that it was the initial PA that was responsible for de-escalating anxiety and growing concern that would later facilitate a timely and orderly deplaning of the aircraft at the gate as well as all but 2 of our original customers re-boarding the new jet. It was  XXXXXX's 'eyes on' first hand assessment along with the purser and 1R door FA assessment that the smoke had ceased to proliferate that led to de-escalating my thoughts of evacuation onto a busy tarmac and into that of a tow back to the gate and expedited deplaning.We contacted ramp and explained our situation... 'XXX on taxiway has smoke in the cockpit with all engines shut down and requested that they facilitate a tow back to the gate right away'.  The ramp agent working the frequency appeared agitated with us and told us we would need to contact Maintenance to coordinate.  I repeated with urgency; our situation as unable to taxi and needing their help with coordination... I received no answer.  XXXXXX; my other IRO assumed the responsibilities of communicating with ramp trying to facilitate help and continued to receive a comment that 'if wecould just taxi back it sure would make things a lot easier'.  I continued to make PA to let folks know our progress and what the plan was when we got to the gate.  We had shut down all fans and other electrical items such as cabin services and entertainment.  Later we would discover that the APU had actually auto-shutdown and we were on battery only.  The smoke had gotten pretty thick in the cockpit but was now beginning to subside while we were being towed back to gate.  We requested Airport Rescue Firefighting (ARFF) be standing by should things deteriorate and instructed operations to let Ramp know that we did not want any cargo doors open for fear of possibly fueling a bad situation with large amounts of fresh air.  Once at the gate; we coordinated with airport personnel that we had no electrical power to enter a log writeup  and subsequently completed the additional paperwork for Maintenance with regard to smoke.  I was able to inspect the cabin and had noticed quite a bit of smoke during the deplaning.  I would later find out from the flight attendants; that the smoke did not actually start appearing in the aft sections of the cabin until the door was opened and passengers started deplaning.  Once off the jet; I debriefed my crew using company CRM skills; spoke with and briefed Chief Pilot as well as submitted the required incident report before making my way to the next aircraft.  I met with our inflight crew at the new aircraft and spoke with each of them to thank them for their calm and professional approach to what had potential to have been a very different outcome.  Everyone assured me that [they] were fit and ready to continue with our new jet into ZZZZ.  I did speak with one FA who waited for me outside the flight planning room.  She wanted to tell me she had experienced a headache and was removing herself from the trip. I made a PA in the boarding lounge thanking everyone for remaining calm and following their crew member instructions and proceeded to offer a very high level explanation of the event while praising the highly experienced crew I had to work with who were quick to identify and de-escalate the situation allowing us to deplane orderly and continue to ZZZZ with this new jet. We were fortunate that this happened on the ground and that we had the option of simply and quickly removing the power from the aircraft.  We all felt certain that after seeing the pictures; that had this happened in flight; we would have never had time to run a multi-page procedure hoping to isolate before something unrecoverable may have occurred within the electrical system ultimately affecting a successful outcome.A few things to note after our cockpit crew debrief:Of interest is that we elected to open the cockpit windows to vent the smoke.  I suspect that because we were on the ground; no one ever mentioned the memory item of donning the O2 mask or the thought that we might exacerbate the problem by opening the windows.  We did however consider this idea as it pertained to opening the cargo doors as we were being towed back. We train 'smoke in the cockpit/cabin' in the simulator and it can become somewhat of a spring loaded evacuation solution as we don't have any outside cues from which to assess.  In real time with real visual feedback in the moment; one gets an overwhelming reality check as to just how dangerous an evacuation of 250+ people can be on a busy tarmac if there are other options.  It may pay dividends to assess the risks and current information prior to committing to an evacuation. In retrospect; after seeing the scorched panel; it is believed that the initial odor was that of burnt dust and paint before the onset of obvious electrical smoldering.  It may also be worth a precautionary discontinuance of starting with any odor that is not readily identifiable. With both engines shut down; we were surprised to later discover the APU had also auto-shutdown contributing to some confusion as we were on battery only. Moving forward; I will now add to my before pushback ritual; the review of a telephone number for operations as we all felt strongly that we spent precious minutes trying to solicit help on the radio from ramp when we could have probably made a phone call directly to Ops to help coordinate all of the services.




Synopsis: B777 Captain reported a fume event of unidentifiable source after gate pushback resulting in a gate return and deplaning of passengers.