Narrative:

At rotation we experienced a failure of the left AC electrical bus. The first officer continued to fly as we cleaned up the aircraft. We both noticed a dramatic increase in fog in the cockpit seeming to come from everywhere; including the overhead panel; to the extent that it could have been smoke. I donned oxygen and established communication. Knowing that returning to the departure airport would be the quickest place to land if indeed we had smoke; I declared an emergency and requested vectors back there. After initial vector; ATC reported weather at 300-1; so I changed the plan to a nearby airport. As we hadn't smelled an acrid type smell and fog dissipated somewhat; we determined over time that the smoke was more than likely a/C fog. I felt comfortable restoring the lost power so took the aircraft and the radios and directed the first officer to accomplish the AC bus off checklist. All systems returned to normal. An overweight landing was accomplished uneventfully. Since all systems appeared stable; I elected to taxi to the gate to deplane. Interested parties were thoroughly debriefed. My initial thought process was that if we did indeed have smoke; to leave left AC bus off and land as soon as possible. Even after determining that the fog was not smoke; I still focused on getting the aircraft on the ground as soon as possible just in case there really had been some smoke that could recur. With that in mind; I advised my flight attendants of my intentions. (They had also noticed increase in AC type fog toward the aft of cabin). I elected not to contact dispatch as by then we were established on final. First officer and I discussed inability to get landing weights; I elected to land anyway due to my sense of urgency; and having no doubt that we could land safely. Flight time was eleven minutes. With the flap gauge inoperative throughout much of the flight; we had no confirmation of actual flap position. As a precaution; we made a possible flap overspeed entry in the log for the worst case overspeed. This probably never happened if the flaps were operating as advertised with the flap handle. Our scenario was probably confusing to approach but they did a great job handling us. [They provided] timely information while at the same time not pestering us with questions while we were busy.

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Original NASA ASRS Text

Title: B757 Captain experiences a left AC bus failure at rotation. A great deal of air conditioning fog is present and initially concerning but begins to dissipate. An emergency is declared; the AC bus is restored and the crew diverts to a suitable airport.

Narrative: At rotation we experienced a failure of the left AC Electrical bus. The First Officer continued to fly as we cleaned up the aircraft. We both noticed a dramatic increase in fog in the cockpit seeming to come from everywhere; including the overhead panel; to the extent that it could have been smoke. I donned oxygen and established communication. Knowing that returning to the departure airport would be the quickest place to land if indeed we had smoke; I declared an emergency and requested vectors back there. After initial vector; ATC reported weather at 300-1; so I changed the plan to a nearby airport. As we hadn't smelled an acrid type smell and fog dissipated somewhat; we determined over time that the smoke was more than likely A/C fog. I felt comfortable restoring the lost power so took the aircraft and the radios and directed the First Officer to accomplish the AC Bus Off Checklist. All systems returned to normal. An overweight landing was accomplished uneventfully. Since all systems appeared stable; I elected to taxi to the gate to deplane. Interested parties were thoroughly debriefed. My initial thought process was that if we did indeed have smoke; to leave left AC bus off and land ASAP. Even after determining that the fog was not smoke; I still focused on getting the aircraft on the ground ASAP just in case there really had been some smoke that could recur. With that in mind; I advised my flight attendants of my intentions. (They had also noticed increase in AC type fog toward the aft of cabin). I elected not to contact Dispatch as by then we were established on final. First Officer and I discussed inability to get landing weights; I elected to land anyway due to my sense of urgency; and having no doubt that we could land safely. Flight time was eleven minutes. With the flap gauge inoperative throughout much of the flight; we had no confirmation of actual flap position. As a precaution; we made a possible flap overspeed entry in the log for the worst case overspeed. This probably never happened if the flaps were operating as advertised with the flap handle. Our scenario was probably confusing to Approach but they did a great job handling us. [They provided] timely information while at the same time not pestering us with questions while we were busy.

Data retrieved from NASA's ASRS site as of July 2013 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.