Date: 201904
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Commercial; Flight Crew Instrument
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Ground Event / Encounter Other / Unknown
Detector: Person Flight Attendant; Person Flight Crew
When detected: Aircraft In Service At Gate
Result: General Flight Cancelled / Delayed; General Maintenance Action; General Evacuated
Contributing factors: ATC Equipment / Nav Facility / Buildings; Aircraft
Primary problem: Ambiguous

Narrative: A321 parked at gate [at] ZZZ. At near passenger boarding completion; with PCA (Precondition Air Unit) attached to aircraft and operating; Flight Attendant [reported] uncomfortably high cabin temperature. We then started the APU. Within 2 to 3 minutes; both First Officer and I smelled a strong burning rubber odor; followed immediately by a call from the Flight Attendant stationed in the aircraft aft that the cabin was filling up rapidly with smoke. After visual confirmation; I shut down the APU and initiated an emergency deplaning and requested ARFF (Air Rescue and Firefighting). I then switched off all power to the aircraft except the utility bus. The First Officer did a quick exterior inspection and requested ground personnel to disconnect PCA. No external smoke or fire observed. We completed an AML (Aircraft Maintenance Log) entry and smoke and odor form. We then proceeded to another assigned aircraft. No passengers or crew members reported medical conditions associated with the smoke and odor event. Unknown cause at the time we left aircraft to fly spare aircraft.


Second narrative: We were preflighting our aircraft at [the] gate in ZZZ with external ground air connected. During passenger boarding with the main cabin door still open; we started to smell burning rubber. Shortly thereafter; flight attendants reported visible smoke in the cabin and we noticed it on the flight deck about the same time. Captain shut down the APU and initiated an emergency deplaning. He requested through Company Operations that they [advise the airport] and request ARFF (Air Rescue and Firefighting) for us. Captain then switched off all power to the aircraft except the utility bus. I went outside the aircraft looking for signs of smoke and notified baggage loaders to get out of the cargo areas. I did not see any source of smoke. Captain completed an AML (Aircraft Maintenance Log) entry and smoke and odor form. We then proceeded to another assigned aircraft. No passengers or crew members reported medical conditions associated with the smoke and odor event.We are not sure if the smoke was coming from the aircraft APU or if it was coming from the jetway air conditioning system. Maintenance was still unsure of the source of smoke at the time we changed aircraft and departed.


Synopsis: A321 flight crew reported deplaning and requesting ARFF due to strong burning rubber odor and smoke onboard.