What was found
AIRCRAFT TOOK OFF . DURING CLIMB TO CRUISE ALTITUDE, CREW AND PILOT VISUALLY AND SMELLED SMOKE OR FUMES, AND A BURNING ODOR. PILOT TURNED AIRCRAFT AROUND AND LANDED AT DEPARTURE. THE ODOR WAS FROM THE HEATER BEING RAN FOR THE FIRST TIME IN 2 YEARS, THE AIRCRAFT IS ACTING AS SPARE FOR PORTALES, NM, BUT NORMALLY OPERATES IN LAS CRUCES. AFTER RAN HEATER AND AIR CONDITIONER FOR A GROUND RUN AND CHECK FLIGHT SMELL CLEARED, NO VISIBLE SMOKE WAS PRESENT DURING GROUND RUN OR CHECK FLIGHT. NO VISUAL SMOKE OR NOXIOUS ODORS WERE DETECTED.