B777 Undifferentiated or Other Model
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 232; Flight Crew Total 28793; Flight Crew Type 2870
Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Inflight Event / Encounter Other / Unknown
Air Traffic Control Provided Assistance; Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem; General Maintenance Action
Environment - Non Weather Related
While in cruise at FL350 we received [a] Sigmet at XA:32 from Dispatch for volcanic ash. Estimated ash top FL200. At XA:48 we received Sigmet 2 from ZZZ Center; for same volcanic ash; also from surface to FL200. Approximately 100 [miles] east of waypoint at XB:15 some of the Flight Attendants started to complain of an acrid odor in the aft galley. We ran the Volcanic Ash Non-Normal procedure. It appeared that we did not fly into actual volcanic ash after referencing the checklist. It is possible we did enter a sulfur dioxide gas cloud; or less likely; in my opinion; into an ozone layer. Out of an abundance of caution; I turned off both recirculation fans and requested a climb to FL370 which ATC granted. I notified ATC; and then contacted Dispatch on the Sat phone. I also directed the Purser to wake one of the Relief Pilots; and had her inspect the cabin for odor and fumes. It was decided that continuing on was prudent; the acrid smell was not increasing in intensity; and lasted for approximately 45 minutes. None of the passengers complained of any discomfort; and apparently were unaware of any odor or fumes. After landing I briefed a Technician and a Flight Manager that my best guess was it was probably a sulfur dioxide gas cloud associated with the volcano eruption.
B777 Captain reported acrid cabin odor during cruise most likely due to sulfur dioxide gas from a volcano eruption in the vicinity.