CAP PLT ON A CHK FLT IN A C182 INADVERTENTLY ENTERED CLASS B AIRSPACE BY LEVELING OFF AT THE BOTTOM OF CLASS B. THE ACFT WAS THEN SUBJECT TO ASCENDING; WITH ANY MOVEMENT; INTO THE CLASS B AIRSPACE.

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ON NOV/XA/98 I WAS GIVING ACFT CHKOUT INSTRUCTION TO AN INST RATED COMMERCIAL PLT IN A C182. I ASKED PLT TO FLY FROM GILLESPIE FIELD OVER TO SAN VICENTE RESERVOIR AND N TO RAMONA ARPT (LOCATED NE OF SAN DIEGO CITY) AT 3500 FT TO PRACTICE TOUCH-AND-GOES. AT ABOUT XA35; WE TOOK OFF FROM GILLESPIE FIELD. PLT WAS FLYING THE ACFT WHILE I WAS SCANNING FOR TFC. FROM PAST EXPERIENCE; THIS RTE IS ALWAYS BUSY WITH ACFT TRANSITIONING TO AND FROM THE MANY ARPTS IN THE SAN DIEGO AREA. EVERY FEW SECONDS; I WOULD GLANCE AT THE INST PANEL TO CHK ON THE PROGRESS OF THE FLT. AT ABOUT XB45; OVER SAN VICENTE RESERVOIR; I CHKED THE INST PANEL AND NOTICED AN ALTIMETER INDICATION OF 3800 FT. I PROMPTLY REMINDED PLT TO MAINTAIN 3500 FT AND HE QUICKLY DSNDED TO 3500 FT. PLT SAID THAT HE WAS MAINTAINING 3800 FT BECAUSE THAT WAS THE BOTTOM ALT OF CLASS B AIRSPACE. WE WERE AT 3800 FT FOR PROBABLY 10 SECONDS BECAUSE THE LAST TIME I MONITORED THE INST PANEL THE ALTIMETER INDICATED 3400 FT. AFTER LNDG; I ASKED PLT IF HE WENT ABOVE 3800 FT AND HIS RESPONSE WAS 'I DON'T THINK SO.' I AM NOT SURE IF PLT BRIEFLY CLBED ABOVE 3800 FT; AND I HAVE NO REASON TO DISBELIEVE HIM. MOST OF MY FLYING EXPERIENCE HAS BEEN IN CONGESTED AIRSPACE AND MY PERSONAL RULE IS TO OPERATE AT AN ALT OF 300 FT ABOVE OR 300 FT BELOW CLASS B AIRSPACE. MY REASON IS SIMPLE. OPERATING RIGHT AT TOP OR BOTTOM OF CLASS B AIRSPACE DOES NOT ALLOW A PLT ANY SAFETY MARGINS. ENTERING THE WRONG ALTIMETER SETTING OR A FAULTY XPONDER CAN PLACE AN ACFT IN CLASS B AIRSPACE WITHOUT A CLRNC FROM ATC. THAT IS WHY I PREACH MY PERSONAL RULE TO EVERY PLT I FLY WITH -- REGARDLESS OF HIS/HER LEVEL OF EXPERIENCE.