INSTRUCTOR PLT OF A STUDENT TRAINING FLT TOOK OVER ACFT CTL FROM THE TRAINEE TO ASSURE THAT THE CLASS C AIRSPACE WAS NOT PENETRATED DURING CLB OUT FROM AN UNDERLYING UNCTLED ARPT. WITH THE LATEST ALTIMETER SETTING; THE PENETRATION WAS ONLY MISSED BY 50 FT.

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THE DEP ARPT; 3I7; LIES BENEATH THE SHELF OF DAY CLASS C AIRSPACE WHICH BEGINS AT 2400 FT MSL OVER 3I7. MY STUDENT AND I SET THE ALTIMETER TO FIELD ELEVATION (1031 FT MSL) PRIOR TO DEPARTING 3I7. ONCE AIRBORNE AND CLBING SBOUND; BUT STILL BENEATH DAY CLASS C; I INSTRUCTED THE STUDENT TO LEVEL AT 2000 FT MSL TO PROVIDE PLENTY OF CLRNC BTWN US AND CLASS C UNTIL I CALLED DAY. THE STUDENT WAS USED TO FLYING A C150. AS A RESULT; HE WAS NOT FAMILIAR WITH THE MORE NOSE DOWN PITCH ATTITUDE OR STRONGER STICK FORCE REQUIRED TO LEVEL THE ACFT AT 2000 FT MSL. THE C172 IS ALSO MORE DEMANDING OF PITCH TRIM THAN THE C150. SO WE DRIFTED SLOWLY UPWARD PAST 2000 FT MSL TO 2150 FT MSL WHERE I FIRST SPOKE UP; TELLING THE STUDENT TO WATCH HIS ALT. THE CLB STOPPED MOMENTARILY AND I DIVERTED MY ATTN TO THE RADIOS TO CALL DAY APCH FOR FLT FOLLOWING TO I69. APCH DID NOT REPLY WITH OUR CALL SIGN; SO COMS HAD NOT YET BEEN ESTABLISHED. KNOWING WE WERE NOT YET ALLOWED INTO CLASS C; I LOOKED BACK OVER AT THE ALTIMETER WHICH WAS NOW APCHING 2300 FT MSL. I TOOK CTL OF THE ACFT AND LEVELED AT 2300 FT MSL JUST AS DAY AND I ESTABLISHED RADIO CONTACT. A CLB TO OUR REQUESTED ALT OF 5500 FT MSL WAS APPROVED AND WE INITIATED THE CLB. DAY ISSUED AN ALTIMETER SETTING WHICH WE SET INTO THE ALTIMETER. ACCORDING TO THE NEW SETTING; OUR ALTIMETER WAS READING 50 FT LOWER THAN ACTUAL AND CORRECT. WE THEREFORE NEVER REACHED THE FLOOR OF CLASS C (2400 FT MSL); BUT 2350 FT MSL MAY WELL HAVE BEEN CLOSE ENOUGH FOR OUR MODE C TO READ 2400 FT ON HIS RADAR SCOPE; THEREBY MAKING US AN INTRUDER. IN THE FUTURE; I WILL ALWAYS STAY AT LEAST 300 FT BELOW OR AWAY FROM THE BOUNDARY OF ANY AIRSPACE THAT I HAVE NOT YET BEEN CLRED INTO IN ORDER TO ACCOUNT FOR POSSIBLE MODE C INACCURACIES OR ALTIMETER SETTING AND INST INACCURACIES. I WILL ALSO BE MORE ASSERTIVE CONCERNING STUDENT DEVS FROM MY CLRNC INSTRUCTIONS AND WILL TAKE PHYSICAL CTL OF THE ACFT SOONER IF DEVS ARE NOT BEING CORRECTED IN A TIMELY MANNER.