Date: 201807
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: A320
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: Drinkable/Waste Water Syst
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While in cruise the aft flight attendants called the flight deck to report a possible faint electrical smell coming from the aft airplane left lavatory. All cockpit indications were normal. The non-flying pilot reviewed the QRH to look for an applicable procedure. The non-flying pilot reviewed the maintenance logbook for previous related write ups and found none. The non-flying pilot made frequent checks with the aft flight attendants every couple minutes to determine if the smell was getting stronger or dissipating and the flight attendants advised everything was the same; the faint smell was not improving or worsening. During the descent; the flight attendants called to report there was visible smoke coming from under the lavatory sink and they had switched off the water heater. Immediately; the non-flying pilot applied the SMOKE/FUMES/AVNCS Smoke Procedure; leading the pilots to turn off the COMM push button to deactivate the power to the lavatory and galleys; [and] continued to check with the aft flight attendants on the status of the smoke. They advised the smoke was not getting any worse. We continued to landing. At the gate; Maintenance determined the water heater to be faulty and placed the item on MEL. This was a faulty piece of equipment.


Second narrative: During cruise the aft flight attendants called to say that they smelled something unusual in the aft left lavatory. They said it smelled more like electrical than anything else. I asked if it was just in the lavatory or if it could be smelled anywhere else. I asked if they saw any smoke and if it appeared to be getting better or worse. They said it was strongest in the lavatory; but could slightly be smelled in the aft galley and they did not see any smoke. I asked them to monitor and to immediately let me know if it got worse. I called every few minutes to ask if it was better or worse. Each time they said that it was about the same and that they did not see any smoke. They reconfirmed that the smell was like electrical.I sent a message to Dispatch via ACARS to let them know that Maintenance should be notified as we would want to have it checked out [upon arrival]. We referred to the QRH to be prepared to react as things developed. I called again and explained that if there was visible smoke that I would be turning the emergency exit lights on and turning off all power to the galleys; lavatories; etc. I tried to get them to be more descriptive on the smell and how strong it was so that I could determine if I should start removing power.A few minutes later; during descent to land; they called back to say that they opened the panel under the sink and saw smoke. They turned off the power switch to the water heater and called me. I turned on the emergency exit lights and turned off the commercial button per the QRH. We were about 15 minutes from landing at this point; so the nearest suitable airport was [our destination]. I [advised ATC] and requested crash/fire/rescue to be notified. Throughout the whole process; the First Officer was flying and had control of the airplane. He did a great job and managed the flying and ATC while I was off com 1.CFR was lined up and down the runway. As we cleared; we asked if they could see any smoke. They did not. We had already coordinated with ATC that if there was no visible smoke on the outside and no more smoke on the inside; that we would plan to taxi to the gate to deplane. The flight attendants reported that there was no more smoke witnessed. CFR followed us to the gate and 2 firefighters went to the lavatory as soon as the main door was opened. They found only residual smoke odor under the lavatory sink and no hot spots.


Synopsis: Air carrier flight crew reported smoke and fumes from the aft lavatory due to a faulty water heater.