Date: 201906
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: IMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: APU
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Flight Crew
When detected: Pre-flight
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Environment - Non Weather Related; Aircraft
Primary problem: Aircraft

Narrative: At approximately; XA:40 local; the Purser informed me one of the aft flight attendants reported a burning plastic smell and was investigating. We had been in the boarding process for some time and had almost boarded all the passengers. Several minutes later; after the First Officer returned from doing the exterior inspection; the Purser reported the aft flight attendants were reporting smoke in the cabin. The First Officer and I agreed he should go check it out. Several minutes later he returned and reported there was visible smoke coming from over one of the overhead bins. We decided to deplane the aircraft and call fire and rescue. While he coordinated with ZZZ operations on the radio to have fire and rescue sent to the aircraft; I made a PA telling the passengers to exit the aircraft because of the smoke. At the time; not knowing how bad the smoke was; I told them to take their belongings with them. Something I regret now. I then called ZZZ Maintenance on the radio to request a Mechanic come investigate the source of the smoke. While I was discussing this issue with them; the smoke became more pronounced. The First Officer was standing in the flight deck door and with consultation from the Purser agreed they needed to get everyone off as quick as possible. They began urging the passengers to hurry off the aircraft as fast as they could and to leave their belongings. While they did this; I began to consult the QRH for a smoke in the cabin checklist. As soon as the passengers had left the aircraft; the First Officer; went to the back of the cabin to inspect the area where the smoke was coming from. He yelled to me in the flight deck it had a burnt electrical smell and suggested we de-power the aircraft. I closed the packs; the APU bleed; shutdown the APU; then shutoff the battery switch off. Almost immediately; the smoke began to dissipate. Maintenance arrived several minutes later and began investigating the source of the electrical problem above the overhead bin. Fire and Rescue may have arrived; but was told by station personnel the problem was resolved and they were not needed. Feeling it was safe to stay on the aircraft; the flight crew with the assistance of an in-flight Supervisor who had arrived carried the passenger's belongings off the aircraft and lined them up in the jet bridge. Station operations escorted the passengers down to retrieve their belongings.




Synopsis: B737 Captain reported 'burning plastic' odor during passenger boarding. Passengers were deplaned and Maintenance was requested.