Date: 201605
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Toilet Smoke; Overheat; Fire Detectors & Warning
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew; Person Flight Attendant
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Equipment / Tooling; Aircraft
Primary problem: Aircraft

Narrative: After level off at FL340 and approximately 1+30 hours into the flight the Flight Attendant (F/A) in the aft galley called the flight deck and reported to the Captain that the smoke detector alarm in the aircraft left aft lavatory had been activated (was sounding off). Normally; pilot thought where there is smoke there is the possibility of fire. So; the Captain transferred control of the aircraft navigation and radios to me while she continued to question the F/A on the situation; prepared to run the appropriate QRH checklist; talk to all appropriate individuals and agencies; and get the aircraft on the ground safely and expeditiously. Mind you this is all happening simultaneously.During the subsequent questioning of the F/A the Captain asked; 'Did you or do you see fire now or ANY indications of fire?' The F/A response was no. In addition; the Captain asked; 'Is there still smoke; or has it dissipated?' F/A response was the smoke had completely dissipated and the smoke detector alarm is no longer sounding and had been reset.Subsequently; the Captain informed me of the conversation with the F/A and again that there was NO fire; indications of fire; the smoke had completely dissipated; the smoke detector alarm was no longer sounding; and had been reset. Therefore; the Captain made the decision; which I FULLY supported; that no diversion was necessary. However; we complied with the 'two person in the cockpit rule'; and I went to the aft galley/lavatories to check things out as an added precaution.I asked the F/A what happened? The F/A stated; that passenger hot meals were being cooked for the first service. For some unknown reason one of the aft galley oven doors was left open and started smoking. In addition; the aircraft left aft lavatory door was also open. The F/A believed the combination of the oven door left open; the oven smoke; and heat somehow triggered the lavatory smoke detector alarm to sound.I investigated the lavatory in question and found no fire; indications of fire; no smoke at all; or even ANY odor of smoke. In addition; I opened BOTH panels underneath and next to the lavatory sink and the conclusion was the same. In addition; the small black fire extinguishing bottle underneath the lavatory sink had NOT been discharged. Meaning the tips were still black versus silver.I returned to the flight deck; reported my findings to the Captain; and once again we determined no diversion was necessary or warranted. Finally; the Captain and I BOTH determined it was safe to continue and obviously directed the F/A to NOT use that specific oven for the remainder of the flight. The remainder the flight was uneventful.




Synopsis: During a Part 121 flight in cruise; a galley oven produced enough airborne smoke to activate a nearby lavatory smoke detector due to the lavatory door having been left open.