Where There's Smoke...
The smell of smoke or fumes could be the first indication of a fire. History has taught the aviation industry never to ignore the threat of a fire.
Luckily, most incidents of smoke or fumes are not from a fire, but often are from failing components, electrical failures, or bleed air contamination which can still have an impact on the operation of the aircraft or health of the Crew and passengers.
Positive visual identification of the root cause of the smoke or fumes is the only means for allowing corrective action.
Unfortunately, the root cause can not always be accurately verified in-flight by the Flight Crews, and only by Maintenance or Engineers after a thorough investigation of the event. Without positive identification of the source, the Flight Crews have little choice except to divert the aircraft, and in many instances, declare an emergency.
Current FAA data reflects smoke/fume related incidents occur on an average of 3 per day just in the US alone.