PLT OF A C310 PENETRATED CLASS B AIRSPACE DUE TO ATTEMPTING TO FLY AT AN APPROPRIATE ALT FOR THE DIRECTION OF FLT DURING A MULTI-ENG INSTRUCTOR PLT TRAINING FLT.

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WE DEPARTED CENTENNIAL ARPT'S RWY 35R AND WE WERE CLRED FOR A R DOWNWIND DEP. THE ARPT ELEVATION IS 5883 FT. ANOTHER FLT INSTRUCTOR WAS IN THE R SEAT AND A THIRD INSTRUCTOR RODE IN THE BACK R SEAT. I SUGGESTED; AFTER THE INITIAL TURN TO THE S AFTER TKOF; WE HEAD BACK TO COLORADO SPRINGS AT 8500 FT. AFTER ABOUT 1 MIN HE SUGGESTED WE FLY BACK AT 9500 FT. WE TOOK AN INITIAL HDG OF 150 DEGS SO; 9500 FT SOUNDED BETTER. I HAD SUGGESTED WE FLY AT 8500 FT BECAUSE THE FLOOR OF THE CLASS B AIRSPACE IN THAT AREA IS 9000 FT. I FORGOT ABOUT THAT WHEN THE PNF SUGGESTED 9500 FT FOR PROPER VFR CRUISE ALT. THE TERRAIN IS 6500 FT RISING FAIRLY SHARPLY TO NEAR 8000 FT. AFTER REACHING 9500 FT AND FLYING THERE FOR ABOUT 3-5 MINS; I REALIZED WE WERE PROBABLY INSIDE THE DENVER CLASS B. I IMMEDIATELY DSNDED TO 8700 FT. I THINK THE INCURSION STARTED ABOUT 5 MI S OF CENTENNIAL ARPT AND ENDED 5 MI LATER WHICH IS THE END OF THE AIRSPACE. BEFORE TKOF THE INSTRUCTOR AND I AGREED THAT I WOULD FLY AND HE WOULD NAV AND WORK THE COMS. WHEN I REALIZED WE WERE IN THE CLASS B; HE SEEMED TO BE CONFUSED ABOUT WHERE THE LINE SEPARATING THE 10000 FT FLOOR AND THE 9000 FT FLOOR EXISTED. FURTHER; HE WAS USING THE DENVER SECTIONAL INSTEAD OF THE VFR TERMINAL AREA CHART. I SHOULD HAVE REMEMBERED WHERE THE FLOOR WAS. ALSO; THE DENVER CLASS B EXTENDS BEYOND THE 30 NM MODE C VEIL. THIS IS CONFUSING BECAUSE THE FLOOR CEILING (120 OVER 90) SYMBOL IS RIGHT ON THE MODE C VEIL LINE AND TO THE E; THE 120 OVER 100 IS BTWN THE OUTER EDGE OF CLASS B AND THE MODE C VEIL LINE. I TRUSTED THE OTHER PLT/INSTRUCTOR WITH THE NAV (VERT AS WELL AS HORIZ) BUT I WAS PIC AND SHOULD HAVE PAID CLOSER ATTN TO OUR POS. I DON'T KNOW WHY THE CLASS B FLOOR IS 9000 FT IN THAT AREA AS IT IS BTWN 2 AREAS WHOSE FLOOR IS 10000 FT AND IT'S 25-30 NM FROM DIA. A MORE THOROUGH PRE- BRIEF MAY HAVE HELPED. ALSO; I ONLY HAVE 4 HRS IN C310'S AND 60 HRS MULTI; 3 HRS IN THE C310 OVER 45 DAYS AGO.