What was found
PILOT & CREW REPORTED STRONG BURNING RUBBER SMELL IN COCKPIT AT ENG RUN UP TO IDLE. PILOT ABORTED FLIGHT & CALLED MECHANIC. CHECKED ACFT SYS, FINDING NO OBVIOUS DEFECTS. HAD PILOT RUN ENG AT IDLE & FULL POWER WHILE CHECKING VARIOUS SYSTEMS INCLUDING BLEED AIR DEMISTER SYS & AIR CONDITIONING SYS. NO INDICATION WARNING LIGHTS WERE ILLUMINATED & NO DEFECTS WERE NOTED. SMELL WAS NEVER REPRODUCED EVEN FAINTLY & ACFT WAS RETURNED TO SERVICE. MECHANIC BELIEVES SMELL MAY HAVE BEEN FROM PORTABLE ELEC HEATER THAT SITS ON LITTER & KEEPS CABIN WARM IN THE WINTER. SOMETIMES THE WARM AIR ENHANCES THE SMELL OF THE RUBBER & PLASTIC TYPE LITTER MATTRESS PAD COVERING, AS THIS ALSO IS NOTICEABLE IN THE SUMMERTIME, IF THE SUNSHADE IS NOT INSTALLED ON WINDSCREEN.