B777-300
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Communication Breakdown; Other / Unknown
Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Ground Event / Encounter Loss Of Aircraft Control; Ground Event / Encounter Ground Equipment Issue
Aircraft Equipment Problem Dissipated; Flight Crew Regained Aircraft Control; Flight Crew Overcame Equipment Problem
Human Factors
The First Officer called for pushback and we were cleared to push into the alley tail East a pulled forward to the top of the alley clear of taxiway. The push commenced and we were cleared to start. As we were pushed back the tug driver stopped the push to pull us forward toward the top of the alley and he cleared us to start. Started the left engine and at the top of the alley the tug driver stopped the push. There was no further communication from the tug driver. We were never told to set the parking brake. I felt the tow bar being moved and then heard the interphone disconnect. I called for the tug driver on the interphone and flashed my taxi light but there was no answer from the tug driver. The aircraft was moving forward slowly and I thought we were still under tow.  I looked out the left window and saw a guide man giving me the hand signal that the chocks were in. I brought the aircraft to a stop and set the parking break. I could now see the tug backing out from the nose of the aircraft.This was an extremely dangerous situation. The tug driver did not use any of the standard phraseology or procedures. Someone could have been seriously injured or killed. Also there could have been equipment damage or damage to the aircraft.Cause-  Tug driver not using standard phraseology and procedures. SOP was not followed at all.Suggestion - Making sure all tug drivers follow SOP and use proper communication along with procedures.
Push back instructions were tail East for a XX exit and to have the tug pull up to the top of the alley. CA (Captain) relayed the instructions to the driver. I do not remember hearing the driver's verbiage when talking to the CA regarding push details. The tug driver cleared us to start engines and CA informed him we would start one engine at a time. As the FO (First Officer) started the engines; the driver stopped moving the Aircraft. I did not feel it was quite at the top of the alley which focused my attention on the push operation. We all heard a static 'pop' which sounded like the drivers headset unplugging from the Aircraft. Then we heard noises similar to the tow bar being unhooked from the Aircraft. Once the Aircraft was stopped the CA attempted multiple times to query the driver about whether he wanted the parking brake set. There was no answer and we could see no movement outside the aircraft. I suggested the CA flash the taxi light to get the driver's attention which he did. At that moment the Aircraft began moving forward again at a very slow rate; similar to the tug speed. We all commented that the driver had begun towing the Aircraft again. Seconds later; the CA and I see the tug pulling off to the left side of the Aircraft and the crew chief holding the bypass pin. It was THEN that we realized the Aircraft was moving under it's own power. The CA stopped the Aircraft; set the brake; completed had signals to the crew chief and we completed the checklist while stationary. There was a very brief discussions about what had happened and how dangerous it was. After departing the ramp; I contacted operations and informed them of what had happened and that someone needed to contact the tug driver.Cause - Lack of and non-standard communication from the tug driver. As the Relief Pilot; I was completing other tasks and did not hear the initial communication with tug driver while at the gate. Subsequently; while in cruise; the CA mentioned that the driver's communication was non-standard starting with initial check-in.Suggestion - Strict adherence to the standard communication guidance by the ground crew.
Air Carrier flight crew reported a dangerous situation during gate pushback. Tug crew did not follow SOP nor did they use standard phraseology.