AWOS ALTIMETER RPTING OFF BY PT 21 AFTER OPS WERE INTERRUPTED BY A LIGHTNING STRIKE NEAR THE TWR. A B767 DEPARTED WITH THE ERRONEOUS ALTIMETER SETTING TO AN ALT OF 1790 FT INSTEAD OF 2000 FT. THE ACFT WAS BELOW THE MVA ALT. AWOS ALTIMETER IS CHKED ONCE PER DAY WITH ANEROID BAROMETER.

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ON JUN/TUE/96; AT APPROX XX15Z THE PWR FOR THE AWOS-3 WAS TEMPORARILY INTERRUPTED BY A LIGHTNING STRIKE NEAR THE TWR. THE SYS AUTOMATICALLY REINITIALIZED ITSELF AND PERFORMED A SELF TEST. AFTER THE SELF TEST THE AWOS-3 APPEARED TO BE WORKING NORMALLY. AT APPROX XX30Z A HVY B767 WAS ISSUED THE AWOS INFO AND DEPARTED. THE ACR WAS CLRED TO CLB TO AN ALT OF 2000 FT MSL INITIALLY AFTER TKOF. AT APPROX XX40Z A DATA SPECIALIST FROM ORLANDO INTL CALLED AND ASKED IF WE COULD CHK OUR ALTIMETER BECAUSE IT DIFFERS FROM THEIRS AND ORLANDO EXECUTIVE'S BY .21. THE ANEROID BAROMETER INDICATED AN ALTIMETER SETTING OF 30.04 AND THE AWOS-3 WAS INDICATING AN ALTIMETER SETTING OF 30.25. AT NO TIME DID THE AWOS-3 SHOW ANY INDICATION OF ANY ERROR. I IMMEDIATELY LOGGED THE AWOS-3 OTS DUE TO AN ERROR IN ONE OF ITS CRITICAL ELEMENTS. I MONITORED THE AWOS-3 FOR THE NEXT HR AND OBSERVED THE ALTIMETER AS HIGH AS 30.45. THIS SIT WAS UNSAFE BECAUSE: THE HVY B767 CLBED TO AN ALT OF 1790 FT MSL INSTEAD OF 2000 FT MSL BECAUSE OF THE ERRONEOUS ALTIMETER SETTING OF 30.25. AN ALT OF 1790 FT IS BELOW THE MINIMUM VECTORING ALT FOR THE AREA. AT APPROX XY55Z AN ACFT WAS INBOUND TO THE ARPT ON AN ILS APCH. WE VERIFIED THE CORRECT ALTIMETER OF 30.04 WITH THE PLT BECAUSE THE AWOS-3 WAS NOW INDICATING 30.45 AND ALTHOUGH ORLANDO APCH KNEW OF THE PROB WITH THE ALTIMETER; THEY WERE STILL GETTING THE ERRONEOUS INFO RELAYED TO THEM THROUGH THE WX MESSAGE SWITCHING SVC. THE WX WAS IFR AND AT TIMES MINIMUMS DUE TO SEVERE TSTMS MOVING THROUGH THE AREA. AFTER THE ACFT LANDED WITHOUT INCIDENT I ASKED THE PLT TO SET HIS ALTIMETER SETTING TO 30.45 AND TO TELL ME HIS ALT. THE PLT SAID HIS ALTIMETER INDICATED 450 FT MSL. THE SURFACE ELEVATION AT THE SANFORD ARPT IS 55 FT MSL!! TO UNDERSTAND THE SERIOUSNESS OF THIS INCIDENT YOU NEED TO REALIZE THAT AN ACFT WILL DSND AT SFB TO A DECISION HT OF 255 FT MSL ON AN ILS APCH AND DSND TO A MINIMUM DSCNT ALT OF 440 FT MSL ON A LOC APCH. IF THE ACFT HAD RECEIVED THE ERRONEOUS ALTIMETER SETTING THAT WAS BEING XMITTED BY THE AWOS-3 AND WITH THE WX DOWN TO APCH MINIMUMS THE ACFT COULD HAVE IMPACTED THE GND ANYWHERE BTWN THE OM AND THE RWY. THE ACFT'S ALTIMETER WHEN PULLED FROM THE WRECKAGE WOULD OF INDICATED 450 FT MSL AND THE VERT SPD INDICATOR WOULD OF INDICATED A SLOW DSCNT (TOWARDS DECISION HT OR MDA). RELYING ON THE AWOS-3 FOR ACCURATE INFO IS UNSATISFACTORY. WE CHK THE AWOS-3'S ALTIMETER TO THE ANEROID BAROMETER ONLY ONCE A DAY AND ANY ERRORS AFTER THE CHK MAY GO UNNOTICED. WHEN THE TWR IS CLOSED THERE IS NO WAY TO VERIFY THE ALTIMETER SETTING. THE AWOS-3 IS BROADCASTED 24 HRS A DAY AND RELAYED TO OTHER CTLING FACILITIES AND FLT SVC STATIONS THROUGH THE MODEM.