M20 PLT FLIES 1000 FT ABOVE INTENDED CRUISING ALT AND PENETRATES DFW CLASS B AIRSPACE DUE TO MISSET ALTIMETER.

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THE ACFT WAS PREPARED FOR DEP FROM AN UNCTLED FIELD WITH NO WX RPTING. THE ALTIMETER WAS THOUGHT TO BE SET TO FIELD ELEVATION OF 710 FT. SEVERAL CHKS OF THE ALTIMETER WERE MADE BEFORE DEP. PLT FELT THAT THE CLB TO THE DESIRED CRUISE ALT FO 2500 FT WAS TAKING LONGER THAN NORMAL; EVEN THOUGH THE RATE OF CLB WAS VERY GOOD. THE ACFT WAS LEVELED OFF IN CRUISE AT AN ALTIMETER READING OF 2500 FT. VISIBILITY WAS EXTREMELY GOOD AND THE PLT FELT LIKE HE WAS HIGHER THAN NORMAL AND CHKED THE DIGITAL READOUT ON THE XPONDER WHICH DISPLAYED A RPTING ALT OF 2600 FT WHICH THE PLT ACCEPTED AS CORRELATING WITH THE ALTIMETER. THE NORMAL COMMUTE FOR THIS PLT IS AT 2500 FT WHICH KEEPS THE ACFT BELOW THE FLOOR OF THE DFW CLASS B ENRTE TO THE DEST. THE ASOS FREQ WAS CHKED ABOUT 12 MI OUT AND A VERY MINOR ALTIMETER SETTING ADJUSTMENT WAS MADE IN ACCORDANCE WITH THAT RPT. THE PLT STARTED A DSCNT TO PATTERN ALT (1600 FT) ABOUT 9 NM SW OF THE ARPT AND RPTED THE ACFT'S POS AND INTENTION TO LAND ON RWY 34 VIA THE UNICOM FREQ. THE UNICOM OPERATOR RPTED WINDS AND ALTIMETER SETTINGS WHICH WERE ACKNOWLEDGED BY THE PLT. ONCE AGAIN; DURING DSCNT; THE PLT FELT THAT THE ACFT WAS HIGHER THAN NORMAL. CHKED ONCE AGAIN THE XPONDER READOUT. IT WASN'T UNTIL THE ACFT TURNED DOWNWIND THAT THE PLT FINALLY REALIZED THAT THE ACFT WAS 1000 FT HIGHER THAN THE ALTIMETER WAS INDICATING. A NORMAL LNDG WAS ACCOMPLISHED. AFTER SHUTDOWN; THE PLT REALIZED THAT THE ALTIMETER HAD BEEN INADVERTENTLY SET TO 29.70 INCHES INSTEAD OF 30.70 INCHES. THE LAST TIME THE ACFT WAS FLOWN WAS ABOUT 6 DAYS EARLIER WITH A LOW PRESSURE AREA IN PLACE. THIS FLT WAS IN AN EXTREME HIGH PRESSURE AREA. THE PLT; WHEN INITIALLY SETTING THE ALTIMETER IN THE DARKENED CONDITIONS; ONLY MOVED THE SETTING A SLIGHT AMOUNT TO GET THE NEEDLE TO INDICATE THE FIELD ELEVATION OF 710 FT; NEVER NOTICING THAT THE 1000 FT NEEDLE WAS BELOW 0 FT. THIS INCORRECT ALTIMETER SETTING RESULTED IN THE ACFT INADVERTENTLY ENTERING THE DFW CLASS B AIRSPACE AFTER XING THE 20 NM RING. THE 2600 FT ALT THE PLT FLEW THE PATTERN AT WAS BELOW THE FLOOR OF THAT SECTION BUT THERE WAS A 3-6 MIN PERIOD WHERE THE ACFT WAS INSIDE CLASS B AIRSPACE AT 3500 FT WITHOUT A CLRNC; WHERE THE FLOOR WAS 3000 FT MSL. THE MISCORRELATION OF THE XPONDER IS STILL A MYSTERY TO THE PLT AND WILL BE FURTHER STUDIED. IT WAS HIS UNDERSTANDING THAT THE READOUT SHOULD BE CORRECT FOR STANDARD SEA-LEVEL PRESSURES (29.92 INCHES) AND SHOULD HAVE ALERTED HIM; AT LEAST TO SOME DEGREE; OF THE SERIOUS ALT DIGRESSION. DARKNESS WAS A FACTOR; AS WAS THE VERY GOOD VISIBILITY WHICH ALLOWED THE PLT TO BELIEVE HE WAS LOWER THAN HE WAS. THE GPS ALT WAS NEVER CONSULTED AND MIGHT HAVE BEEN A CLUE TO THE PLT AS WELL. THE EXTREME BAROMETRIC PRESSURE DIFFERENCES BTWN THE 2 MOST RECENT FLTS CONTRIBUTED TO THE MISSETTING OF THE ALTIMETER. THIS COULD HAVE BEEN A MUCH MORE SERIOUS PROB IF THE ALTIMETER HAD BEEN MISSET IN THE OPPOSITE DIRECTION AS EXPECTED OBSTACLE WOULD HAVE BEEN COMPROMISED. THIS IS NOT LIKELY TO BE A MISTAKE THIS PLT WILL REPEAT!