Narrative:

Approximately 5 to 10 minutes after reaching cruising altitude of 15;000ft; the captain (the non-flying pilot) said 'smells like something is burning.' I (the first officer/flying pilot) did not recognize the smell. But after 2 to 3 minutes; I smelled something and said 'smells like an oven toaster.' but there was neither a master warning nor caution lights on and all gauges indicated normal conditions. We noticed faint smoke in the cockpit but couldn't tell where the smoke came from. Then; we heard the smoke detector in the lavatory and the captain asked the flight attendant if she saw any smoke. She replied that there was faint smoke in the cabin also. We put on the oxygen masks; did the emergency check list; declared the emergency to ATC and descended to 10;000ft. We decided to return to our departure airport since the initial smoke was faint and had completely disappeared after we turned off the bleed air (depressurization flight). We determined that since there was no further smoke or smell; the smoke detector alarm had ceased; there was no master warning or any other caution lights; all gauges indicated normal operation and the aircraft appeared to be operating normally that there was no need for special assistance for landing and no need to evacuate the aircraft upon our return. We landed at our departure airport. The next day I spoke with the mechanic who fixed the airplane which we were on. He found that one of the compressor blades of the air cycle machine was broken; causing a small oil leak. The oil was burning and the air to inside the airplane contained that smoke.

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Original NASA ASRS Text

Title: A DHC8-200's Air Cycle Machine compressor blade failed allowing hot oil into the air conditioning system. The crew detected smoke and not knowing the source executed the emergency procedures and returned to the departure airport. The smoke dissipated when the air conditioning was turned off.

Narrative: Approximately 5 to 10 minutes after reaching cruising altitude of 15;000ft; the Captain (the non-flying pilot) said 'smells like something is burning.' I (the First Officer/flying pilot) did not recognize the smell. But after 2 to 3 minutes; I smelled something and said 'smells like an oven toaster.' But there was neither a master warning nor caution lights on and all gauges indicated normal conditions. We noticed faint smoke in the cockpit but couldn't tell where the smoke came from. Then; we heard the smoke detector in the lavatory and the Captain asked the Flight Attendant if she saw any smoke. She replied that there was faint smoke in the cabin also. We put on the oxygen masks; did the emergency check list; declared the emergency to ATC and descended to 10;000ft. We decided to return to our departure airport since the initial smoke was faint and had completely disappeared after we turned off the bleed air (depressurization flight). We determined that since there was no further smoke or smell; the smoke detector alarm had ceased; there was no master warning or any other caution lights; all gauges indicated normal operation and the aircraft appeared to be operating normally that there was no need for special assistance for landing and no need to evacuate the aircraft upon our return. We landed at our departure airport. The next day I spoke with the mechanic who fixed the airplane which we were on. He found that one of the compressor blades of the Air Cycle Machine was broken; causing a small oil leak. The oil was burning and the air to inside the airplane contained that smoke.

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.