A PART 135 TURBOPROP USED ITS GPS FOR AN IFR OP WHEN IT WAS NOT CERTIFIED FOR THAT TYPE OP. THE OBSERVING ACI POINTED OUT THE ERROR.

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WE HAD AN FAA INSPECTOR ONBOARD (LINCOLN; NE; FSDO) OBSERVING. AFTER WE LANDED AT OUR FIRST STOP; MAXWELL AFB; HE INFORMED US OUR GARMIN GPS 155 WAS NOT APPROVED FOR IFR FLT (VFR ONLY). HE HAD CHKED THE AFM WHEN HE SAW NO PLACARD INDICATING ITS LIMITATIONS; IF ANY. THIS WAS THE SECOND DAY I HAD BEEN FLYING THIS PARTICULAR ACFT AND THE CAPT AND I BOTH BELIEVED IT WAS IFR CERTIFIED AS IT WAS LISTED AS SUCH IN OUR COMPANY'S OPS SPECS. THE DIRECTOR OF OPS OF OUR COMPANY SAID HE FLEW THIS ACFT UNDER AN IFR FLT PLAN FOR THE PLANE'S OPS CERTIFICATION AND HE BELIEVED THE GPS WAS CERTIFIED FOR IFR USE AS A RESULT. WE CONTINUED THE REST OF OUR RTE USING VORS ONLY ONCE THE FAA INSPECTOR BROUGHT THE MATTER TO OUR ATTN. SUPPLEMENTAL INFO FROM ACN 396969: I BELIEVED IFR OPS WERE APPROVED BECAUSE ALL THE OTHER PLANES AT THE COMPANY ARE AND THE PLACARD IN THE COCKPIT WAS UNCLR. THE PLACARD SAID GPS NOT SOLE SOURCE OF NAV. WE WERE ALSO USING VORS TO FIX OUR POS. CORRECTIVE ACTIONS: LOOK IN EACH ACFT AFM AND CHK; DON'T ASSUME.