Date: 202108
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: Dash 8 Series Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Air Conditioning Distribution Ducting; Clamps; Connectors
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Second reporter function: Flight Attendant In Charge
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew; Person Flight Attendant
When detected: Aircraft In Service At Gate
Result: Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: When we arrived at Aircraft X at spot XX; the APU was running and the other crew was deplaning. My crew; boarded the plane at gate and started to do our preflight inspections to get the flight ready. While I was doing my originating flow; the APU auto shutdown and only displayed a 'fail' light. I grabbed the E&A [Engine & APU] and ran the checklist; which only asked if there were any other lights illuminated. The answer was no so the checklist was complete. At this point it started to get warm on the plane; and we were just starting to board our specials. I allowed the APU to stay off for a few minutes; and then opted to turn the APU on again since there were no other indications of an issue. I started the APU again and it started fine; and the generator also came online fine. When I pressed the bleed air switch; the cabin and cockpit instantly started filling with heavy smoke. I instantly shut the APU down; and turned around to Flight Attendant A and told them to get the passengers off. By then; we only had about 5 passengers or so on the aircraft. After deplaning the passengers; I asked Ops to call the Fire Department to come and make sure there was nothing still burning on the airplane. They came and verified that there were no hot spots in the airplane. Maintenance was notified and the crew did a short debrief to insure we were still safe to fly. We continued our flight about an hour later.


Second narrative: We had a plane swap; exiting crew said everything was fine; we got on. Captain was starting the APU then a [circuit] breaker blew and cabin lights went out... Captain did his checks... restarted APU... by this time we had our specials and people who needed extra time about 10 total on board. I was helping a women to her seat when I looked up and saw the end of cabin had smoke at that exact time Captain said to stop boarding. I immediately got on PA and instructed everyone to deplane.   Captain called Fire Department and Maintenance they found no hot spots. We got our bags off then went to another plane to continue into ZZZ1.  Captain was great and asked if we were both OK.   Very thankful this happened while on the ground.


Synopsis: Air Carrier Captain and Flight Attendant reported a fume event during passenger boarding. Passengers were deplaned and Maintenance was requested.