COMMERCIAL PLT INST TRAINEE UNDER IFR HOOD IN VMC ON VECTORS FROM PIC SAFETY PLT ENTERS DEN CLASS B AIRSPACE.

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I WAS FLYING 'UNDER THE HOOD' ON AN ILS APCH FOR RWY 26 AT FTG. THE SAFETY PLT; WHO MEETS THE INSURANCE MINIMUMS FOR THE RENTAL ACFT; HAD ASSUMED THE POS AS PIC. I WAS FLYING ON AN INBOUND COURSE 8.9 DME FROM RWY 26 AND ON A 265 DEG RADIAL FROM BYERS VOR. THE VISIBILITY WAS GREATER THAN 10 SM AND THE CEILING WAS GREATER THAN 10000 FT. THE SAFETY PLT AND I KNOW THE AREA WELL AND IDENTED ALL NAVAIDS ALONG OUR RTE FROM GOODLAND; KS. THE SAFETY PLT CONTINUED TO GIVE ME RADAR VECTORS TO FTG. I BEGAN TO QUERY THE SAFETY PLT ASKING IF HE COULD SEE FTG AT 16 DME FROM DENVER VOR. AT THIS TIME I NOTICED THAT OUR NAVAIDS WERE NOT OPERATING PROPERLY. I QUERIED THE SAFETY PLT ONCE MORE; 'DO YOU SEE THE ARPT?' HE GAVE ME NO INDICATION OF BEING OFF COURSE. WHEN HE AGAIN REPLIED NO; I DECIDED TO COME OUT FROM UNDER THE HOOD. WHEN I LOOKED UP AND OUTSIDE; I WAS AWARE THAT WE HAD INADVERTENTLY ENTERED THE TCA OF CLASS B AT DENVER INTL ARPT. I INFORMED THE SAFETY PLT THAT WE WERE INSIDE THE TCA. AT THAT TIME THE SAFETY PLT TOOK OVER THE CTLS AND INFORMED ME THAT HE WAS NOW PIC. I RELINQUISHED ALL CTLS AND BEGAN SCANNING THE AREA FOR TFC. THE PIC IMMEDIATELY PROCEEDED TO DSND AND TURN L TO AN EASTERLY HDG. WE LANDED AT FTG WITHIN 5 MINS. THE 3 CONTRIBUTING FACTORS THAT LED TO THIS INCIDENT ARE: 1) THE MISCOM BTWN THE SAFETY PLT AND I ENRTE. 2) THE SAFETY PLT MISIDENTING VISUAL REFS AND BEING UNSURE OF OUR POS AFTER BYERS VOR; WHICH THREW US OFF COURSE. 3) AND FINALLY; THE POORLY MAINTAINED AVIONICS OF THE RENTAL ACFT THAT GAVE US POOR RECEPTION OF THE ILS AND DENVER VOR. FURTHERMORE; THE ADF BECAME INOP AND WAS UNUSABLE.