AN INADVERTENT ENTRY INTO THE CLASS D AIRSPACE OF MRB.

Narrative

I WAS FLYING THE PLANE WITH THE AT19 CHK PLT WORKING THE NAV CHARTS. OUR LCL CHK PLT ASKED ME TO PREPARE A VFR FLT PLAN FROM ZZZ TO CHO. I CALLED FSS FOR A WX BRIEF ABOUT 30 MINS BEFORE TKOF. I PLANNED THE FLT FROM ZZZ VIA FDK VOR AND MRB VOR DIRECT CHO. I THOUGHT THIS WOULD AVOID ALL PROHIBITED AND RESTR AREAS. AFTER TKOF; THE CHK PLT WANTED TO FLY A MORE DIRECT RTE TO SAVE TIME. KNOWING HE HAD PRIOR ATC EXPERIENCE; I GUESS I BLINDLY FOLLOWED HIS INSTRUCTION. I DIDN'T CLOSELY WATCH THE NAV CHART. WE SAW THAT WE WOULD PASS ABOUT 5 MI NW OF JYO ARPT. I CALLED JYO CTAF AND SAID WE WERE PASSING ABOUT 5 MI NW AT 1400 FT ENRTE TO CHO. THEN I HEARD A STERN VOICE ASKING IF WE WERE AT 1400 FT AND HE STATED WE WERE CLOSER THAN MY 5 MI ESTIMATE. HE SAID SOMETHING LIKE 'RPT CLR OF THE CLASS D AIRSPACE.' THIS SURPRISED ME. I CALLED BACK A LITTLE LATER AND RPTED CLR. WHEN WE CONTACTED CHO APCH CTL; WE WERE GIVEN A PHONE NUMBER TO CALL POTOMAC APCH CTL CONCERNING A POSSIBLE VIOLATION. AFTER LNDG; THE CHK PLT CALLED. I SHOULD HAVE FILED A VFR FLT PLAN; BUT AFTER GETTING THE WX BRIEF FROM FSS; I GOT IN MY MIND WE WERE READY TO GO. DISCUSSING THE RTE WITH FSS MIGHT HAVE ALERTED US TO AIRSPACE RESTR IN THE WASHINGTON DC AREA. ACCORDING TO MY INTERP OF THE INFO ON THE WASHINGTON SECTIONAL BORDER CONCERNING SFAR 94; I ASSUMED WE WERE OK ON OUR RTE.