Narrative:

Upon takeoff; we smelled some unknown fumes. It was not a burning smell; but nonetheless we could not identify what it might be. We continued the departure upon my request and as we climbed; it dissipated. A short while later; it returned (the smell); and then went away. As we leveled off at FL260; it returned and we all agreed to declare an emergency and return to departure airport. As there were no indications otherwise of any problem with system and the so said there was no unusual smell in the cabin. I told ATC we were doing this as precautionary. After landing; we taxied off the runway and had the flight director infrared camera; the cabin and gear [checked] per my request. Nothing found. We taxied to our ramp and debriefed maintenance.

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Original NASA ASRS Text

Title: A B727 FLT CREW NOTICED FUMES OF UNKNOWN ORIGIN AFTER TKOF AND DURING CLB. AFTER REACHING CRUISE ALT; THEY DECLARED AN EMER AND RETURNED TO DEP STATION.

Narrative: UPON TKOF; WE SMELLED SOME UNKNOWN FUMES. IT WAS NOT A BURNING SMELL; BUT NONETHELESS WE COULD NOT IDENT WHAT IT MIGHT BE. WE CONTINUED THE DEP UPON MY REQUEST AND AS WE CLBED; IT DISSIPATED. A SHORT WHILE LATER; IT RETURNED (THE SMELL); AND THEN WENT AWAY. AS WE LEVELED OFF AT FL260; IT RETURNED AND WE ALL AGREED TO DECLARE AN EMER AND RETURN TO DEP ARPT. AS THERE WERE NO INDICATIONS OTHERWISE OF ANY PROB WITH SYS AND THE SO SAID THERE WAS NO UNUSUAL SMELL IN THE CABIN. I TOLD ATC WE WERE DOING THIS AS PRECAUTIONARY. AFTER LNDG; WE TAXIED OFF THE RWY AND HAD THE FLT DIRECTOR INFRARED CAMERA; THE CABIN AND GEAR [CHKED] PER MY REQUEST. NOTHING FOUND. WE TAXIED TO OUR RAMP AND DEBRIEFED MAINT.

Data retrieved from NASA's ASRS site as of January 2009 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.