SMOKE IN COCKPIT IN A NIGHT OP. RETURN LAND.

Narrative

AFTER LEVELING OFF AT 4000 FT ENRTE TO BTR; I NOTICED A STRONG BURNING SMELL. I CLOSED THE AIR VENTS AND THEN NOTICED A LOT OF SMOKE COMING FROM THE DASHBOARD. I THEN NOTIFIED ZHU OF THE PROB AND REQUESTED A HDG BACK TO AEX. THE CTLR GAVE A HDG AND ASKED ME IF I NEEDED ANY ASSISTANCE. I SAID I WOULD LIKE TO HAVE ANYTHING THEY HAD SINCE I DID NOT KNOW HOW BAD THE PROB WAS. I THEN SHUT THE ELECTRICAL SYS OFF AND THIS IMMEDIATELY SOLVED THE PROB. TURNED THE MASTER SWITCH BACK ON TO LOWER LNDG GEAR AND LANDED WITHOUT ANY OTHER PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR HAD BEEN FLING AN ACFT THAT WAS USED THE DAY BEFORE ON A PIPELINE INSPECTION FLT AND THE PLT THAT HAD USED IT REPLACED ONE OF THE BULBS IN THE INST PANEL WHICH HAD BURNED OUT. THAT ACTION WAS NOTED IN THE LOG BOOK SO MIGHT BE CONSIDERED ACFT MAINT ACTION OUTSIDE OF THE LEGAL VIEWPOINT; SINCE THE PLT WAS NOT AN ACFT MECH. IN DOING SO; HE PLACED AN OVERSIZED BULB IN THE SOCKET WHICH TOUCHED THE PLASTIC HOUSING ON THE INST CASING AND; AS OUR FRIEND THE RPTR TOOK OFF ON HIS NIGHT JOURNEY; THE BULB STARTED TO BURN THE COVER; PRODUCING ALL OF THE SMOKE AS RPTED. BEING DARK; THE RPTR COULD NOT SEE WHERE THE SMOKE CAME FROM AND THEREFORE JUST SHUT OFF THE ELECTRICAL PWR.