AN INSTRUCTOR AND PRIVATE PLT; PRIOR TO TKOF; INCORRECTLY SET THE ALTIMETER OF THE BE35. THEY WERE FLYING 1000 FT HIGH. THE TRUE ALT WAS 1000 FT LOWER THAN INDICATED ON THE ALTIMETER.

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AT THE TIME OF THE OCCURRENCE; I WAS ACCOMPANYING A FORMER STUDENT; NOW A PVT PLT; WHO HAD NOT BEEN VERY ACTIVE RECENTLY; AND WANTED ME TO RIDE ALONG WITH HER ON THE FLT FOR REASSURANCE AND TO ESTABLISH CURRENCY. THE PLAN CALLED FOR A VFR DEP FROM OUR HOME ARPT (N40) AT 560 FT ELEVATION; CLB TO 8500 FT TO OVERFLY THE PHL CLASS B AIRSPACE AND CONTINUE TO OUR DEST OF KENMORR AIRPARK IN MD (3W3). WE DEPARTED THE N40 ARPT AND CLBED TO AN INDICATED ALT OF 7500 FT WHERE WE LEVELED OFF TEMPORARILY TO AVOID SOME SCATTERED CLOUDS ABOVE US. AT ABOUT XA35; WE WOULD HAVE CROSSED THE LATERAL BOUNDARY; ABOVE THE PHL CLASS B AT A POINT ABOUT 5 NM W OF THE WILLOW GROVE NAVAL AIR STATION (KNXX). AFTER ABOUT 10 MINS ON OUR 245 DEG HDG; NOW ABOUT 12 NM NE OF ILG; WE CONTINUED OUR CLB; NOW ABLE TO AVOID THE CLOUDS; TO AN INDICATED ALT OF 8500 FT. AT THIS POINT WE DECIDED TO CHK THE ATIS AT BWI TO SEE WHAT THE SURFACE WINDS NEAR OUR DEST WERE DOING. WHEN WE DID SO; WE REALIZED THAT OUR ALTIMETER SETTING WAS HIGH BY 1000 FT. DURING THE PRE-TKOF CHK; NOT HAVING A LCL ALTIMETER SETTING; THE ALTIMETER IS USUALLY SET TO OUR FIELD ELEVATION OF 560 FT. IT WAS INSTEAD ERRONEOUSLY SET 1000 FT HIGH TO 1560 FT. WHEN WE RECEIVED THE BWI ALTIMETER SETTING WE REALIZED WE HAD UNKNOWINGLY PENETRATED THE PHL CLASS B FOR APPROX 10 MINS OR 25 NM. THE ERROR IN SETTING THE ALTIMETER CAME PRIMARILY FROM THE PF WHO WAS NOT CURRENT IN ACFT AND HAD MISTAKENLY SET IT. I MISSED IT AS WELL. SCRUTINIZING THE INSTRUMENT SETTING ON THE GND WOULD HAVE AVOIDED THE ERROR. PARTICIPATING IN VFR FLT FOLLOWING FROM ATC; CHKING ATIS SOONER; OR CONDUCTING THE FLT UNDER IFR; WOULD HAVE NO DOUBT BROUGHT THE ERROR TO OUR ATTN SOONER. NEXT TIME WE'LL BOTH BE MORE DILIGENT TO AVOID THIS TYPE OF ERROR AND READ THE INSTRUMENT CAREFULLY ON THE GND.