Date: 201911
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737-900
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: TRACON ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport; Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On takeoff as we were setting power; there was a slight noticeable odor as we pushed the thrust levers up. While I did notice it; it was not particularly concerning to me as I was aware the engine had work done recently and the packs had been off as the aircraft was in MX all night prior to us accepting it. Climbing out of ZZZ prior to flap retraction we received an inter phone call from the rear FA XX_11 XX_16 stating that there was a fire and the cabin had a burning smell that was worsening and there was a little smoke or haziness. My FO (First Officer) confirmed the FA (Flight Attendant) said there was a fire and I immediately let ATC know that we would need to return to ZZZ ASAP. I did not specifically declare an emergency; but I did exercise my emergency authority. We were immediately given vectors for a downwind for 26. We continued to clean up the aircraft and I transferred control and ATC to the FO and I called the cabin again. I questioned the FA further who then indicated that there was no fire; nothing was hot; but there was an extremely strong odor with a burning like smell. With the urgency slightly reduced; I elected to call MX (Maintenance) and ask if the work they did may have created the odor and if it should clear up. MX indicated the work accomplished should not have created the odor; but that if it cleared up; we would probably be okay as long as we confirmed there was no fire. I again called the cabin and was told the smell was dissipating slightly but was still quite strong. I elected to return to ZZZ. We were vectored for a visual/ILS to Rwy XY and landed uneventfully underweight and taxied to the gate. AARF (Airport Rescue Firefighting) followed us to the gate and met the aircraft. After all pax (passengers) deplaned; AARF went to the aft cabin and confirmed no fire or heat but they did smell the odor. No passengers showed any ill medical effects and all deplaned normally. Nice job by the entire crew!


Second narrative: While climbing after departing from Runway XXR at ZZZ; at about 1000 feet; the aft Flight Attendant called the flight deck and informed us that she thought something was on fire in the aft galley/lav area; that there was an odor and some smoke; and it was '...getting worse.' The Captain and I had noticed a slight odor as we set takeoff power; but we had already discussed that the #2 engine had been bore-scoped recently and had been in Maintenance all night before we accepted the aircraft; and the little odor we noticed didn't seem concerning. However; upon hearing that there seemed to be smoke and it was getting worse in the back we immediately began discussing the need to return to ZZZ and informed ATC. We were given vectors for a left downwind to land on Runway XY. The Captain transferred control of the aircraft and the radios to me as he called the aft Flight Attendant to get more information. It seemed from that conversation that there was doubt as to whether there was a fire after all; as nothing seemed hot; but there was still a strong burning odor. I informed ATC that we wanted a longer downwind vector as we troubleshot and I loaded the ILS approach to XY. The Captain then called Maintenance and asked if the work they had done on engine #2 would cause the odor; and they didn't think it would. After further discussion with the Flight Attendants the Captain elected to return to ZZZ. We received vectors to a visual approach to XY and landed uneventfully. Fire trucks met us at the runway and followed us to gate XX. Passengers deplaned and none seemed to show any illness. After passengers had left the aircraft; fire personnel and maintenance personnel came aboard to investigate. Post flight discussions indicated there did not seem to be any fire or heat damage but there was still a burning odor in the cabin.


Synopsis: B737 flight crew reported Flight Attendant notification of burning odor; smoke and possible fire in the passenger cabin shortly after takeoff. Flight crew executed an air return.