Narrative:

Cleared for ILS runway 36R (clt) from vector altitude of 2400 ft MSL. Autoplt and autothrottles coupled. Aircraft began descent at GS intercept point with last items of landing checklist being completed. Flight director indicated locked on to localizer and GS and on course on GS. At 1000 ft AGL xchk, noticed raw data GS indicator showed below GS and GS alerter and ATC call to check altitude and climb back up to 'raw data' GS commenced simultaneously. Lowest altitude approximately 900 ft AGL before climb. Ground and runway in sight. At that point, autoplt and autothrottles were disengaged to effect climb back to GS and visual approach completed to uneventful landing. Earlier in the day during an ILS approach to runway 36L at clt, the same ILS receiver/autoplt channel was used at which time the GS failed to capture/descend and approach was hand flown. This was thought to be anomaly and later coupled approach using first officer ilsecond officerther autoplt channel operated normally at a different airport. Captain's ILS/flight director/autoplt channel a was written up in aircraft's log for maintenance to troubleshoot/fix. Autoplt descent below GS was initially missed while completing final checklist items and since xhairs of flight director were showing on course/on GS and autoplt showed localizer and GS captured, all was thought to be ok until 'raw data' was compared. 'Below GS' alert (aircraft system) discrepancy with raw data at 1000 ft call and ATC altitude call all happened simultaneously. Glad to have multi pilot crew/ATC/aircraft alerter system to help monitor 'automatic' aircraft system! Supplemental information from acn 380912: we leveled off at 1800 ft realizing something wrong when ATC reported low altitude alert and asked us to check our altitude.

Google
 

Original NASA ASRS Text

Title: ACR B737-200 DSNDS PREMATURELY ON GS ON ILS APCH TO CLT'S RWY 36R. ACFT COMES WITHIN 1000 FT OF THE GND WHILE 8 NM FROM RWY. LOW ALT WARNING SYS FROM ATC ALERTED FLC WHO THEN CLBED BACK TO THE RAW DATA GS. GS CAPTURED APPROX 3 MI OUTSIDE OF FAF. CREW HAD FAILED TO NOTE RAW DATA ILS DISPLAY OR DME DISTANCE FROM RWY FOR XING ALT AT THE FINAL FIX.

Narrative: CLRED FOR ILS RWY 36R (CLT) FROM VECTOR ALT OF 2400 FT MSL. AUTOPLT AND AUTOTHROTTLES COUPLED. ACFT BEGAN DSCNT AT GS INTERCEPT POINT WITH LAST ITEMS OF LNDG CHKLIST BEING COMPLETED. FLT DIRECTOR INDICATED LOCKED ON TO LOC AND GS AND ON COURSE ON GS. AT 1000 FT AGL XCHK, NOTICED RAW DATA GS INDICATOR SHOWED BELOW GS AND GS ALERTER AND ATC CALL TO CHK ALT AND CLB BACK UP TO 'RAW DATA' GS COMMENCED SIMULTANEOUSLY. LOWEST ALT APPROX 900 FT AGL BEFORE CLB. GND AND RWY IN SIGHT. AT THAT POINT, AUTOPLT AND AUTOTHROTTLES WERE DISENGAGED TO EFFECT CLB BACK TO GS AND VISUAL APCH COMPLETED TO UNEVENTFUL LNDG. EARLIER IN THE DAY DURING AN ILS APCH TO RWY 36L AT CLT, THE SAME ILS RECEIVER/AUTOPLT CHANNEL WAS USED AT WHICH TIME THE GS FAILED TO CAPTURE/DSND AND APCH WAS HAND FLOWN. THIS WAS THOUGHT TO BE ANOMALY AND LATER COUPLED APCH USING FO ILS/OTHER AUTOPLT CHANNEL OPERATED NORMALLY AT A DIFFERENT ARPT. CAPT'S ILS/FLT DIRECTOR/AUTOPLT CHANNEL A WAS WRITTEN UP IN ACFT'S LOG FOR MAINT TO TROUBLESHOOT/FIX. AUTOPLT DSCNT BELOW GS WAS INITIALLY MISSED WHILE COMPLETING FINAL CHKLIST ITEMS AND SINCE XHAIRS OF FLT DIRECTOR WERE SHOWING ON COURSE/ON GS AND AUTOPLT SHOWED LOC AND GS CAPTURED, ALL WAS THOUGHT TO BE OK UNTIL 'RAW DATA' WAS COMPARED. 'BELOW GS' ALERT (ACFT SYS) DISCREPANCY WITH RAW DATA AT 1000 FT CALL AND ATC ALT CALL ALL HAPPENED SIMULTANEOUSLY. GLAD TO HAVE MULTI PLT CREW/ATC/ACFT ALERTER SYS TO HELP MONITOR 'AUTOMATIC' ACFT SYS! SUPPLEMENTAL INFO FROM ACN 380912: WE LEVELED OFF AT 1800 FT REALIZING SOMETHING WRONG WHEN ATC RPTED LOW ALT ALERT AND ASKED US TO CHK OUR ALT.

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.