EMB ERJ 190/195 ER/LR
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Flight Crew Returned To Gate
Aircraft
We were pushing from gate and shortly after the Captain asked me to start the Number 1 engine we heard a loud explosion and the aircraft went electrically dark. Power came back online almost immediately most likely as a result of the engine driven generation coming online. As we were attempting to assess the cause of the explosion; rampers began yelling that they saw flames and smoke coming from the tail of the aircraft. While the Captain was trying gather information from the rampers; I referenced the QRC for APU fire and found the appropriate page in my QRH. The information we were gathering from outside the aircraft made it sound like there was no present fire; but they did tell us there were parts of the APU on the ground. The APU had automatically shut itself down (or failed) and there were no fire indications displayed in the cockpit. The Captain spoke with our Number 2 Flight Attendant and assessed no flames; fumes or smoke in the cabin. I quickly asked ramp if we could take gate as it was unoccupied and we are pointed directly at it. I then needed to confirm the request with ground and told them we would need emergency personnel and firetrucks to assess the exterior of the aircraft. Emergency responders were quick to arrive and report no flames or smoke outside the aircraft. We arrived safely at gate and deplaned.
During pushback from gate; the ground crew gave clearance to start the engines. I directed the First Officer to start the engines and he began the start sequence for engine 1. In the midst of the start sequence after engine light off; there was a noticeable thud throughout the plane and the plane lost normal electrical power and the EICAS was filling with page upon page of cautions. It appeared at first that the APU generator had come off line; so I reset it to no avail as the APU had apparently failed. Power was restored shortly thereafter because engine 1 was able to complete its start sequence. We were then notified by both the ramp personnel and a pilot on the ramp frequency that the APU had shot a plume of flames and parts were scattered about the ramp. After asking both our ground crew and ramp about fire; they all indicated that it was just smoking a little. Out of caution; we asked for the [Aircraft Rescue and Fire Fighting] to come verify that we weren't on fire (there was no fire indication). In the interim; I checked with the flight attendants to be sure there was no smoke or fumes in the cabin and they confirmed there were not. I made two announcements to the passengers during the event informing them that we would be returning to the gate and to not be alarmed by the presence of fire trucks. Once the fireman confirmed that we were not on fire; we were towed to gate and deplaned the passengers normally.According to the Maintenance Personnel; the rear turbine completely failed.This could likely not be avoided; though it did show me that things are not always so clean and clear in an abnormal situation. We spent some time as a crew and with our cockpit jump seater trying to determine if we had an actual emergency. There is no procedure for this type of thing and no checklist since there was no fire. Not being able to see what's happening also didn't help. There was also a bit of information overload when ramp control; other pilots; the pushback crew on the inter phone; and flight attendants are all trying to communicate at the same time.
EMB-190 flight crew reported they experienced Auxiliary Power Unit failure during pushback.