Date: 201705
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
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: Electrical Wiring & Connectors
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: As we boarded the aircraft for preflight; I saw 5 mechanics around seat 20A inspecting an inbound write up; regarding reported excessive heat at the sidewall panel. There was also an accompanying mark on the spot that appeared to be a scuff mark; (surface only); rather than a deterioration of the composite material. The chief mechanic explained that by the emergency exit windows there were heat strips in the panel that would get warm; but should not produce excessive heat; as the write up suggested. I encouraged them to find the person who wrote up the aircraft; but they were unable to contact him. [Company maintenance] and local maintenance concurred that the aircraft could be released; finding no related damage; and my crew and I felt comfortable with that decision. All flight attendants were briefed and vigilant to this write up. At 2+20 into flight; a passenger reported the sidewall was very hot; at the same spot; and she was unable to put her head against it; due to the heat. The International Reserve Officer (IRO) was coming off of break; so the First Officer (FO) went back to observe and assess; and returned; reporting it was hot; but he could place his hand on it (describing it as coffee pot hot). After consulting with Dispatch and Maintenance; to discern if we could control the heaters with any circuit breakers (inaccessible in flight); I decided we would [advise ATC of emergency situation]; and if possible; return to ZZZ; although diversion enroute might occur if the heat at the hot spot continued to increase significantly. The FO coordinated with ATC via HF; and announcements were made as we turned off track. We received an initial clearance to ZZZ via Controller Pilot Data Link Communications (CPDLC); with a load prompt; but that point was 3000 miles in the opposite direction of ZZZ. After a few messages; we were cleared to ZZZ; FL370. The IRO ran the diversion checklist; we obtained the weather for all possible enroute diversion airports; and kept a close eye on the nearest alternate choices; in case of deteriorating conditions. I briefed the lead flight attendant of our plan and of the possibility of divert if conditions deteriorated. A second hot spot; 6 inches below the first; was reported by the flight attendant stationed at that seat. As we descended through FL180; the flight attendant reported that the hot spot was getting hotter. There was no visible smoke or odor throughout the flight. We landed uneventfully; flaps 30; and taxied to the gate.The coordination of clearances via CPDLC was a distraction. Upon coast out; before the emergency; we were given a new route clearance with a load prompt direct to 40W. We queried; and the next clearance eliminated the 30W position in the load route prompt. Querying again; we received the full route clearance as expected. We were careful during the coordination for the turn off the track to make sure we received the correct clearance; after the previous confusion.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B757 flight crew reported a return to departure airport when a hot spot developed at a sidewall panel that was an inbound write up.