Narrative:

We were at cruise just prior to our equal time point when we both noticed an acrid smell in the cockpit. I called the a flight attendant on the interphone to ask if something might be burning in an oven. As I started to talk to the a flight attendant; the other flight attendants came up on other cabin handsets. They were all saying they were seeing and smelling a lot of smoke in the cabin. I instructed them that we were turning around for ZZZ and to keep me informed.we donned our O2 masks and initiated our air turn back to our ETOPS alternate of ZZZ. An announcement was made to other aircraft on our common frequency and an emergency was declared with center on HF. As we were in our right turn the flight attendants reported that all of the lavatory smoke detector alarms were going offand our D flight attendant had donned his pbe and was checking lavatories and overhead compartments with the assistance of the other flight attendants.we performed the smoke/fire/fumes QRH checklist; turning off the right pack and waiting for the required 2 minutes. The smoke started to dissipate in the flight deck and cabin. We were now offset 15 NM and ATC then cleared us to a block altitude of 360 to 370 and direct ZZZ. We proceeded with no smoke noticed in the flight deck or cabin and all engine indications appearing normal.when we were a little over an hour out ATC cleared us from FL370 to FL350; and while descending smoke again appeared in the cabin and flight deck only to dissipate again with our level off. The first officer and I again discussed whether it could be engine related but noticed no abnormal engine indications. In preparation for our descent I briefed that if we encountered smoke again we would continue with the balance of the smoke /fire/fumes checklist and descend rapidly to below 10;000 ft and depressurize the aircraft to try to get rid of any smoke picked up during the descent. I briefed the flight attendants and passengers that smoke might increase on our descent and what our plan was if that were to happen.after starting our descent into ZZZ our flight attendants reported that smoke was getting thick again in the cabin and we noticed smoke again in the flight deck. ATC cleared us to expedite our descent to 9;000 ft at which point we depressurized the aircraft and all smoke and fumes disappeared for the remainder of the flight.while on vectors for a modified downwind with approach control I noticed that the left engine oil quantity was low with all other indications remaining normal. At this point we were only a couple of minutes from the FAF for our runway. After discussing the oil indication with the first officer as a precautionary measure I then elected to set the left engine at idle power and perform the landing using the single engine procedures and checklists. This was accomplished prior to the FAF.we had requested fire equipment to stand by and after landing we proceeded to the gate under our own power. After arriving at the gate; we accomplished our aircraft shutdown; opened both cockpit windows and instructed the flight attendants to open both forward and both aft doors; for ventilation. Company personnel were waiting for us as well as our company mechanics. The crew deplaned after all of the passengers were off the aircraft.it appears that our left engine experienced an internal failure leaking oil/allowing smoke into the cabin pressurization system.

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Original NASA ASRS Text

Title: An overwater B757-200 flight encountered smoke throughout the aircraft and diverted to their ETOPS alternate under emergency conditions. Ultimately determined source of smoke to have been engine oil ingested into the air conditioning system.

Narrative: We were at cruise just prior to our equal time point when we both noticed an acrid smell in the cockpit. I called the A Flight Attendant on the interphone to ask if something might be burning in an oven. As I started to talk to the A Flight Attendant; the other Flight Attendants came up on other cabin handsets. They were all saying they were seeing and smelling a lot of smoke in the cabin. I instructed them that we were turning around for ZZZ and to keep me informed.We donned our O2 masks and initiated our air turn back to our ETOPS alternate of ZZZ. An announcement was made to other aircraft on our common frequency and an emergency was declared with Center on HF. As we were in our right turn the Flight Attendants reported that all of the Lavatory smoke detector alarms were going offand our D Flight Attendant had donned his PBE and was checking lavatories and overhead compartments with the assistance of the other flight attendants.We performed the smoke/fire/fumes QRH checklist; turning off the right pack and waiting for the required 2 minutes. The smoke started to dissipate in the flight deck and cabin. We were now offset 15 NM and ATC then cleared us to a block altitude of 360 to 370 and direct ZZZ. We proceeded with no smoke noticed in the flight deck or cabin and all engine indications appearing normal.When we were a little over an hour out ATC cleared us from FL370 to FL350; and while descending smoke again appeared in the cabin and Flight deck only to dissipate again with our level off. The First Officer and I again discussed whether it could be engine related but noticed no abnormal engine indications. In preparation for our descent I briefed that if we encountered smoke again we would continue with the balance of the smoke /fire/fumes checklist and descend rapidly to below 10;000 FT and depressurize the aircraft to try to get rid of any smoke picked up during the descent. I briefed the Flight Attendants and passengers that smoke might increase on our descent and what our plan was if that were to happen.After starting our descent into ZZZ our Flight Attendants reported that smoke was getting thick again in the cabin and we noticed smoke again in the flight deck. ATC cleared us to expedite our descent to 9;000 FT at which point we depressurized the aircraft and all smoke and fumes disappeared for the remainder of the flight.While on vectors for a modified downwind with Approach Control I noticed that the left engine oil quantity was low with all other indications remaining normal. At this point we were only a couple of minutes from the FAF for our runway. After discussing the oil indication with the First Officer as a precautionary measure I then elected to set the left engine at idle power and perform the landing using the single engine procedures and checklists. This was accomplished prior to the FAF.We had requested fire equipment to stand by and after landing we proceeded to the gate under our own power. After arriving at the gate; we accomplished our aircraft shutdown; opened both cockpit windows and instructed the Flight Attendants to open both forward and both aft doors; for ventilation. Company personnel were waiting for us as well as our company mechanics. The crew deplaned after all of the passengers were off the aircraft.It appears that our left engine experienced an internal failure leaking oil/allowing smoke into the cabin pressurization system.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.