Narrative:

I was vectoring several aircraft to airport, in particular, the DC10 was coming in from over the mountains to the east of anc. I had no reason to doubt the reliability of my radar presentation, so I descended the DC10 to 7000 ft as he appeared to be within that MVA area. I further vectored him to follow the DH8 that appeared to be on a dogleg final to runway 06R, using close to minimum separation. The DC10 never saw the DH8. When my colleague and I remarked that the headings of the aircraft looked wrong, we discovered that the azimuth of the radar -- not the map presentation -- was off by about 30 degrees! Maintenance was doing something to the radar and we knew nothing of it. For several mins I had no primary targets, no background video, and no way to ascertain where my aircraft actually were. At that point, I had 5 aircraft on the scope in my sector. I do not know if the DC10 was descended below the MVA, or if I had placed him in unsafe proximity to the DH8. When we got after maintenance, they corrected it, but they were trying to downplay the impact, saying that it should only have been one sweep! It was totally irresponsible for them to be tinkering with the radar without anything being said to ATC, and worse that they tried to cover it up. It is also important to note that DC10 was in the clouds for nearly all of his descent.

Related reports:

Google
 

Original NASA ASRS Text

Title: APCH CTLR VECTORING A DC10 INBOUND OVER THE MOUNTAINS AND DSNDING HIM TO THE MVA, TO FOLLOW A DCH8, DISCOVERED THAT THE AZIMUTH ON HIS RADAR WAS 30 DEGS OFF. MAINT WAS DOING SOME UNCOORD FINE TUNING OF THE RADAR AND RPTR CONCERNED HE MAY HAVE PUT THE DC10 TOO CLOSE TO TERRAIN AND THE DHC8.

Narrative: I WAS VECTORING SEVERAL ACFT TO ARPT, IN PARTICULAR, THE DC10 WAS COMING IN FROM OVER THE MOUNTAINS TO THE E OF ANC. I HAD NO REASON TO DOUBT THE RELIABILITY OF MY RADAR PRESENTATION, SO I DSNDED THE DC10 TO 7000 FT AS HE APPEARED TO BE WITHIN THAT MVA AREA. I FURTHER VECTORED HIM TO FOLLOW THE DH8 THAT APPEARED TO BE ON A DOGLEG FINAL TO RWY 06R, USING CLOSE TO MINIMUM SEPARATION. THE DC10 NEVER SAW THE DH8. WHEN MY COLLEAGUE AND I REMARKED THAT THE HEADINGS OF THE ACFT LOOKED WRONG, WE DISCOVERED THAT THE AZIMUTH OF THE RADAR -- NOT THE MAP PRESENTATION -- WAS OFF BY ABOUT 30 DEGS! MAINT WAS DOING SOMETHING TO THE RADAR AND WE KNEW NOTHING OF IT. FOR SEVERAL MINS I HAD NO PRIMARY TARGETS, NO BACKGROUND VIDEO, AND NO WAY TO ASCERTAIN WHERE MY ACFT ACTUALLY WERE. AT THAT POINT, I HAD 5 ACFT ON THE SCOPE IN MY SECTOR. I DO NOT KNOW IF THE DC10 WAS DSNDED BELOW THE MVA, OR IF I HAD PLACED HIM IN UNSAFE PROX TO THE DH8. WHEN WE GOT AFTER MAINT, THEY CORRECTED IT, BUT THEY WERE TRYING TO DOWNPLAY THE IMPACT, SAYING THAT IT SHOULD ONLY HAVE BEEN ONE SWEEP! IT WAS TOTALLY IRRESPONSIBLE FOR THEM TO BE TINKERING WITH THE RADAR WITHOUT ANYTHING BEING SAID TO ATC, AND WORSE THAT THEY TRIED TO COVER IT UP. IT IS ALSO IMPORTANT TO NOTE THAT DC10 WAS IN THE CLOUDS FOR NEARLY ALL OF HIS DSCNT.

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.