Date: 202108
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
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: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Communication Breakdown
Second reporter function: Relief Pilot; Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Ground Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: Other pretaxi engine start
Result: Air Traffic Control Provided Assistance; Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Gate; Flight Crew Requested ATC Assistance / Clarification; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While parked at spot XX on the ramp at ZZZ we were cleared to start the engines. After starting the left engine we noticed a strange smell. There were 4 crew members on the flight deck so we discussed the situation. After asking the Lead Flight Attendant if they could smell anything strange; to which they could not; we thought it could be something from the outside being sucked into the ventilation system. We decided to start the right engine and see if things cleared up. After a normal start we heard a grinding/thumping sound (hard to tell exactly; but definitely not normal). We immediately shut both engines down only to find out that the APU  had also shut down. The plane was now dark. I tried to restart the APU only to find out that nothing worked. One of the Relief pilots went down into the amp bay to see if he could find the source of the smell and now smoke. The smoke was not very thick so we had no reason to think there was a threat of fire. The FO found that a box had shorted out and burned several wire bundles. There was no further evidence of a fire so the Captain decided to have the plane towed back to the gate and the passengers removed. I included 2 threats for Flight Attendant and crew communication because after the incident we had no PA  system to communicate with the flight attendants in case an evacuation was needed. This would have slowed down the process.


Second narrative: After push back and disconnect from the tug; the left (#1) engine was started. Upon commencement of the right (#2) engine start a very faint electrical odor was sensed. We deemed it prudent to call door 1 to inquire if the flight attendants also noticed any smell or smoke; to which they responded negative. as the right engine became stabilized after start the smell became worse and a shuddering or thumping sound was heard. No flight deck warnings occurred but there appeared to be some electrical malfunctions on the overhead panel (window heat lights flicking on and off). We decided as a crew that shutting down both engines promptly was the best move; to remediate the situation as quickly as possible. After both engines shut down; the APU failed leaving us with battery power only. At this time we noticed a small amount of smoke in the cockpit; but decided it was not dangerous enough to issue an emergency evacuation of the aircraft. The second Relief Pilot went into the cabin and stated there was smoke there as well; and he could see what appeared to be a burn mark in the amp compartment. With only one operable radio we had to switch between communication with Ramp Tower and Operations. This caused some confusion as to who we should be talking to; in order to get an expeditious return to the gate. The Ramp Tower controller did not seem to think the situation was urgent and this may have led to a delay in having a tug and ground crew called to return us to the gate. Flight attendants and passengers were briefed and there were no adverse events in the cabin. We returned to the departure gate uneventfully. Fire and police were called to be on hand and inspect the aircraft. Upon parking an exterior walk around was performed and no exterior damage noted.


Synopsis: B777 flight crew reported a fume event during engine start  after push back.  Both engines were shut down due to a persistent faint fume; abnormal noise; and simultaneous APU failure.  A safe ground return was completed.