Narrative:

We initially were in a holding pattern at willow intersection and then were radar-vectored for an approach ILS runway 26L at london gatwick airport. Since the WX was IMC we elected to execute a monitored approach. Autoplt 'a', autoplt 'B' and the 'land' system were engaged. Initially the localizer and the GS seemed to be captured. Shortly thereafter, however, we noticed that the airplane was drifting to the right of the localizer. I told the first officer to disconnect the autoplts, turn left and reintercept the localizer. We still had a right of course indication, so we executed a missed approach. This was done well above minimums. We executed the after takeoff and approach checklists. We retuned and reidented the ILS frequency and I decided to execute another coupled approach, this time using autoplt 'a' only since this was the higher grade autoplt. Again, the localizer seemed to be intercepted initially. We then noticed that the aircraft drifted to the right of course and this was confirmed by approach control, who suggested a heading of 265 degrees and maintain a 3:1 GS ratio. Having ground contact we saw that we were not lined up with the runway and executed a missed approach. Again, this was well above minimums. Suspecting autoplt tracking malfunction, I decided to execute a third and final approach, this time manual. We again retuned and reidented the ILS frequency. This was to be our final approach before proceeding to our alternate of prestwick. This time the localizer was captured normally, the approach completed and we landed without further incident. We landed with 65000 pounds of fuel. I wrote up the incident in our maintenance logbook.

Related reports:

Google
 

Original NASA ASRS Text

Title: WDB AUTOPLT FAILED TO TRACK ILS ON 2 CONSECUTIVE APCHS.

Narrative: WE INITIALLY WERE IN A HOLDING PATTERN AT WILLOW INTXN AND THEN WERE RADAR-VECTORED FOR AN APCH ILS RWY 26L AT LONDON GATWICK ARPT. SINCE THE WX WAS IMC WE ELECTED TO EXECUTE A MONITORED APCH. AUTOPLT 'A', AUTOPLT 'B' AND THE 'LAND' SYS WERE ENGAGED. INITIALLY THE LOC AND THE GS SEEMED TO BE CAPTURED. SHORTLY THEREAFTER, HOWEVER, WE NOTICED THAT THE AIRPLANE WAS DRIFTING TO THE R OF THE LOC. I TOLD THE FO TO DISCONNECT THE AUTOPLTS, TURN L AND REINTERCEPT THE LOC. WE STILL HAD A R OF COURSE INDICATION, SO WE EXECUTED A MISSED APCH. THIS WAS DONE WELL ABOVE MINIMUMS. WE EXECUTED THE AFTER TKOF AND APCH CHKLISTS. WE RETUNED AND REIDENTED THE ILS FREQ AND I DECIDED TO EXECUTE ANOTHER COUPLED APCH, THIS TIME USING AUTOPLT 'A' ONLY SINCE THIS WAS THE HIGHER GRADE AUTOPLT. AGAIN, THE LOC SEEMED TO BE INTERCEPTED INITIALLY. WE THEN NOTICED THAT THE ACFT DRIFTED TO THE R OF COURSE AND THIS WAS CONFIRMED BY APCH CTL, WHO SUGGESTED A HDG OF 265 DEGS AND MAINTAIN A 3:1 GS RATIO. HAVING GND CONTACT WE SAW THAT WE WERE NOT LINED UP WITH THE RWY AND EXECUTED A MISSED APCH. AGAIN, THIS WAS WELL ABOVE MINIMUMS. SUSPECTING AUTOPLT TRACKING MALFUNCTION, I DECIDED TO EXECUTE A THIRD AND FINAL APCH, THIS TIME MANUAL. WE AGAIN RETUNED AND REIDENTED THE ILS FREQ. THIS WAS TO BE OUR FINAL APCH BEFORE PROCEEDING TO OUR ALTERNATE OF PRESTWICK. THIS TIME THE LOC WAS CAPTURED NORMALLY, THE APCH COMPLETED AND WE LANDED WITHOUT FURTHER INCIDENT. WE LANDED WITH 65000 LBS OF FUEL. I WROTE UP THE INCIDENT IN OUR MAINT LOGBOOK.

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.