T34B PLT ON NIGHT TRAINING FLT USES GPS TO KEEP CLR OF CLASS B AIRSPACE BUT VERY CLOSE TO THE 7 MI CIRCLE. HE ENTERS THE AIRSPACE AND ALSO DISCOVERS HE HAS CLBED ABOVE THE FLOOR OF THE AIRSPACE.

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THE FLT WAS A ROUND ROBIN ANP-MTN-ANP TO MAINTAIN BOTH NIGHT AND MAKE/MODEL CURRENCY. I FILED A VFR FLT PLAN WITH ALTOONA FSS AND OPENED WITH LEESBURG. AFTER LNDG AT MTN; I WAS RETURNING TO ANP AT 1400 FT MSL AND; IT BEING A BEAUTIFUL NIGHT; DECIDED TO FLY OVER FOR A CLOSER LOOK AT THE KEY BRIDGE; NE OF BWI. AT THIS POINT; I WAS NO LONGER TALKING TO MTN TWR BUT WAS STILL MONITORING THAT FREQ. APCHING THE BRIDGE; I MONITORED MY DISTANCE GPS FROM BWI TO STAY OUTSIDE THE 7 NM CLASS B RING. AT 7.2 NM SHOWING; I MADE A STEEP 180 DEG TURN TO THE L. THE ALTIMETER IN THIS T34B IS THE ORIGINAL NAVY INST; CIRCA 1957; WITH THE 1000'S SHOWN IN A SMALL WINDOW AND THE 100'S SHOWN BY THE NEEDLE. IN THIS ACFT; THE RED PANEL ILLUMINATION IS VERY UNEVEN; ESPECIALLY ON THE ALTIMETER; AND AS I ROLLED OUT OF MY TURN I NOTICED THE ALT WAS READING APPROX 2100 FT. THE FLOOR OF THE CLASS B THERE IS 1500 FT. ANOTHER ASPECT; WHICH OCCURRED TO ME LATER; WAS THAT I WAS DETERMINING GPS DISTANCE FROM BWI; NOT BAL; AND THIS MAY HAVE RESULTED IN MY BEING SLIGHTLY WITHIN THE 7 NM RING. CONTRIBUTING FACTORS WERE THUS: 1) PLT DECISION TO FLY CLOSE TO INNER RING OF CLASS B; 2) PLT SETTING GPS TO MEASURE DISTANCE FROM ARPT CTR INSTEAD OF VOR; 3) PLT INATTN TO ALT; ABETTED BY 4) POOR INST PANEL LIGHTING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT SINCE THE GPS UNIT IS NOT TSO'ED FOR IFR HE ALWAYS CHKS ON HIS RTE AND DOES PREFLT PLANNING WITHOUT REF TO THE GPS. IT IS USED PRIMARILY AS CONFIRMATION ENRTE; THUS HE DOES NOT UPDATE REGULARLY. HE NEVER USES THE GPS FOR IFR APCHS BUT DOES VERIFY DISTANCES WHEN NO DME IS AVAILABLE. HE BELIEVES THE UNIT IS QUITE USER FRIENDLY AND IS NOT DIFFICULT TO USE OR UNDERSTAND. RPTR WOULD LIKE TO SEE A DISPLAY ON THE NAV PAGE WITH INFO REGARDING THE NUMBER OF SATELLITES WHICH ARE UP AND TRACKING SIMILAR TO WHAT APPEARS WHEN THE UNIT IS WARMING UP. ONE CAN GO BACK TO THIS DISPLAY BUT IF THE INFO WERE ON THE NAV PAGE ONE WOULD HAVE A WARNING WHEN PROPER TRACKING MAY BE A PROB.