EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Workload; Troubleshooting; Physiological - Other
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Flight Crew Returned To Gate; Flight Crew Took Evasive Action; General Declared Emergency; General Maintenance Action; General Evacuated
Aircraft
We pushed on time from the gate. We started both engines for the short taxi to the runway. After turning onto Taxiway X from XX; the First Officer smelled electrical smoke in the flight deck. The Captain; then also smelled what he believed to be smoke. Immediately then the Flight Attendant called the flight deck; also smelling electrical smoke. The Flight Attendant indicated the smell was strongest in the rear of the aircraft. We asked Ground Control if we can pull over and look at a problem; Ground asked if we needed assistance. We stated no and will advise.The smoke smell became very strong; making us believe that we were on fire. The First Officer stated that we immediately return to the gate and we were making a 180 on the taxiway. Operations were still at the gate from the push. We then told Ground to roll the trucks. We have no visual indications or aural indications of a fire; however; the cabin was filling with electrical smoke. We parked at Gate X; deplaned all passengers immediately through the main cabin door; leaving all passenger items behind. ARFF examined the cabin and flight deck; they also smelled smoke and began a thermal scan of the flight deck and baggage compartment. ARFF also had no visual indications of a fire; but could smell it. Contract Maintenance then arrived. ARFF needed the aircraft to be pushed away from the gate. The Mechanic and the flight crew then started the process of re-powering the aircraft one system at a time to see if we could determine the cause of the electrical smoke. We then came to starting the APU; after the BIT test the First Officer began the APU start cycle. The airplane started 'clicking' loudly and there was '0' rotation on the APU. I then exited the aircraft and did a visual inspection of the APU; I then noticed all scorching and fire damage to the right side of the aircraft. I took photos and texted them to Maintenance Control.  Cause was APU FAILURE.
During taxi out; both EMB-145 pilots and the Flight Attendant detected a strong cabin odor; so the aircraft returned to the gate and was evacuated. During troubleshooting; a failed APU was found.