Narrative:

As a CFI, I departed with a student on an instrument training flight in VFR conditions. The training was to include a vot check at a nearby airport that has a vot. We obtained ATIS for the departure airport and the vot airport during initial run-up. The altimeter setting at both airports was 29.91 and my student set this in the altimeter. The altimeter indicated correct field altitude within a few ft. We then departed the home airport for the VOR airport and did the VOR check. We then departed the vot airport downwind to the northwest and my student climbed on downwind to an indicated altitude of 1500 ft as I had requested. The controller 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 control of the aircraft and descended 100 ft to 1400 ft indicated. I did not want to descend further as we were about to overfly a hospital that is sensitive to noise. We reset the altimeter setting and I asked the controller for current radar altitude. He indicated we were showing 1700 ft. I concluded that the transponder may be reading 250-300 ft high, and after the flight we turned in a maintenance record to have it checked. The transponder was last checked in feb/99, and is due for another check 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 airport. However, the radar records may have shown a class B penetration, and TCASII (in response to transponder altitude) may have issued a false traffic alert.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.