A GA ACFT FROM A FLYING CLUB HAS A MODE C ALT READOUT PROB WITH SEVERAL ATC FACILITIES WHILE USING A HANDHELD GPS NEAR CGT; IL.

Narrative

ON THE EVENING OF SEP/FRI/01; GRISSOM APCH RPTED MODE C WAS READING 400 FT LOW (WHILE LEVEL AT 4000 FT MODE C SHOWED 3600 FT). THIS OCCURRED JUST MINS AFTER TURNING ON THE HEATER FOR THE FIRST TIME THIS SEASON. RTE OF FLT WAS MAO-MIE-CGT-MDW. ON THE EVENING OF SEP/SAT/01; WE DEPARTED MDW ASSUMING THAT MODE C WOULD BE ACCURATE IF WE LEFT THE HEATER OFF. THIS WAS AFTER REVIEW OF THE CLUB SQUAWK BOOK AND SEEMS NO PRIOR MODE C ERRORS. ORD DEP RPTED MODE C INDICATED 6500 FT; WHILE I WAS LEVEL AT 5000 FT. THEY ASKED IF I HAD PRIOR MODE C PROBS. I REPLIED YES; BUT THOUGHT THE PROB WAS RESOLVED. CYCLING PWR DID NOT HELP. FLT CONTINUED WITHOUT MODE C. NEAR SBU VOR; I TURNED OFF A HANDHELD GPS I WAS USING TO BACK UP THE VORS AND ASKED IND APCH TO VERIFY MODE C. WHILE LEVEL AT 5050 FT THEY INDICATED 5100 FT. THE MODE C ERROR APPEARS TO BE CAUSED BY THE GPS. STEPS TO PREVENT FUTURE MODE C ERRORS: 1) ALERTED THE NEXT MEMBER TO USE THE PLANE WHAT HAPPENED. 2) ALERTED CLUB PLANNING OFFICER. HE SAID THE AVIONICS SHOP WILL TAKE A LOOK. 3) LOGGED ERROR IN THE CLUB SQUAWK BOOK. FLT CONTINUED ON TO HAO WITHOUT FURTHER MODE C ERRORS WHILE LEAVING GPS OFF. ALSO NOTE ON SEP/FRI/01 CVG AND DAY APCH DID NOT INDICATE ANY ERRORS.