Narrative:

On jun/tue/96, at approximately XX15Z the power for the AWOS-3 was temporarily interrupted by a lightning strike near the tower. The system automatically reinitialized itself and performed a self test. After the self test the AWOS-3 appeared to be working normally. At approximately XX30Z a heavy B767 was issued the AWOS information and departed. The air carrier was cleared to climb to an altitude of 2000 ft MSL initially after takeoff. At approximately XX40Z a data specialist from orlando international called and asked if we could check 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 hour and observed the altimeter as high as 30.45. This situation was unsafe because: the heavy B767 climbed to an altitude of 1790 ft MSL instead of 2000 ft MSL because of the erroneous altimeter setting of 30.25. An altitude of 1790 ft is below the minimum vectoring altitude for the area. At approximately XY55Z an aircraft was inbound to the airport on an ILS approach. We verified the correct altimeter of 30.04 with the pilot because the AWOS-3 was now indicating 30.45 and although orlando approach knew of the problem with the altimeter, they were still getting the erroneous information relayed to them through the WX message switching service. The WX was IFR and at times minimums due to severe thunderstorms moving through the area. After the aircraft landed without incident I asked the pilot to set his altimeter setting to 30.45 and to tell me his altitude. The pilot said his altimeter indicated 450 ft MSL. The surface elevation at the sanford airport is 55 ft MSL!! To understand the seriousness of this incident you need to realize that an aircraft will descend at sfb to a decision ht of 255 ft MSL on an ILS approach and descend to a minimum descent altitude of 440 ft MSL on a localizer approach. If the aircraft had received the erroneous altimeter setting that was being transmitted by the AWOS-3 and with the WX down to approach minimums the aircraft could have impacted the ground anywhere between the OM and the runway. The aircraft's altimeter when pulled from the wreckage would of indicated 450 ft MSL and the vertical speed indicator would of indicated a slow descent (towards decision ht or MDA). Relying on the AWOS-3 for accurate information is unsatisfactory. We check the AWOS-3's altimeter to the aneroid barometer only once a day and any errors after the check may go unnoticed. When the tower is closed there is no way to verify the altimeter setting. The AWOS-3 is broadcasted 24 hours a day and relayed to other controling facilities and flight service stations through the modem.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.