APCH CTLR SHOWED C172 PENETRATED CLASS B AIRSPACE. HOWEVER; ACFT ALTIMETER INDICATED PROPER ARPT ELEVATION DURING PRETKOF ALTIMETER SETTING CHK.

Narrative

AS A CFI; I DEPARTED WITH A STUDENT ON AN INST TRAINING FLT IN VFR CONDITIONS. THE TRAINING WAS TO INCLUDE A VOT CHK AT A NEARBY ARPT THAT HAS A VOT. WE OBTAINED ATIS FOR THE DEP ARPT AND THE VOT ARPT DURING INITIAL RUN-UP. THE ALTIMETER SETTING AT BOTH ARPTS WAS 29.91 AND MY STUDENT SET THIS IN THE ALTIMETER. THE ALTIMETER INDICATED CORRECT FIELD ALT WITHIN A FEW FT. WE THEN DEPARTED THE HOME ARPT FOR THE VOR ARPT AND DID THE VOR CHK. WE THEN DEPARTED THE VOT ARPT DOWNWIND TO THE NW AND MY STUDENT CLBED ON DOWNWIND TO AN INDICATED ALT OF 1500 FT AS I HAD REQUESTED. THE CTLR ADVISED THAT WE WERE AT 1800 FT AND WERE ENTERING THE OVERLYING CLASS B AIRSPACE (FLOOR 1800 FT) AND THAT THE CURRENT ALTIMETER WAS 29.93. I TOOK CTL OF THE ACFT AND DSNDED 100 FT TO 1400 FT INDICATED. I DID NOT WANT TO DSND FURTHER AS WE WERE ABOUT TO OVERFLY A HOSPITAL THAT IS SENSITIVE TO NOISE. WE RESET THE ALTIMETER SETTING AND I ASKED THE CTLR FOR CURRENT RADAR ALT. HE INDICATED WE WERE SHOWING 1700 FT. I CONCLUDED THAT THE XPONDER MAY BE READING 250-300 FT HIGH; AND AFTER THE FLT WE TURNED IN A MAINT RECORD TO HAVE IT CHKED. THE XPONDER WAS LAST CHKED IN FEB/99; AND IS DUE FOR ANOTHER CHK IN ABOUT 120 DAYS. I DO NOT BELIEVE THAT WE ACTUALLY PENETRATED CLASS B AIRSPACE. FURTHER; THERE WAS NO COLLISION ISSUE WITH LARGE JETS PASSING OVERHEAD TO THE CLASS B ARPT. HOWEVER; THE RADAR RECORDS MAY HAVE SHOWN A CLASS B PENETRATION; AND TCASII (IN RESPONSE TO XPONDER ALT) MAY HAVE ISSUED A FALSE TFC ALERT.