Narrative:

We were using the backup radar; as the primary was released to maintenance. An A320 was on final for runway 19R; and a B737 was luaw on runway 25R. I cleared the B737 for take off when the A320 was 3 miles out; which is a normal operation. The A320 was at 190 KTS indicated on the display; and appeared to be ok for landing. The B737 started rolling; and then I noticed that the A320 disappeared from the radar; both primary and beacon slash. The next update had a beacon slash a mile and a half closer than the previous hit; only 10 seconds earlier. I looked at the B737; who was nose wheel up; and then at the A320; who announced that he was going around. I issued traffic to the A320; who did not see him; then turned the A320 30 degrees left to avoid B737. I then went to the B737 to issue traffic; and he reported him in sight; while saying 'that was nice'. I was not looking at the arrival when he actually initiated the go around; so am not sure at what exact point he started. I am wondering; however; if the back up radar is not as accurate; and the three mile line that I used to judge a safe distance was not good enough. Maybe the A320 was closed than the radar indicated; and that is why; on the go around; the A320 ended up much closer than I would have liked to the runway 25 departure. Give refresher training on the use of the backup radars. This is probably the first time I have used it in over a decade; and did not realize that the normal spacing used for judgments may not be sufficient; due to a less accurate display.

Google
 

Original NASA ASRS Text

Title: LAS Controller described a go around event while utilizing a back up RADAR system; claiming that the lack of familiarity and training deficiencies were both casual factors.

Narrative: We were using the backup RADAR; as the primary was released to maintenance. An A320 was on final for Runway 19R; and a B737 was LUAW on Runway 25R. I cleared the B737 for take off when the A320 was 3 miles out; which is a normal operation. The A320 was at 190 KTS indicated on the display; and appeared to be OK for landing. The B737 started rolling; and then I noticed that the A320 disappeared from the RADAR; both primary and beacon slash. The next update had a beacon slash a mile and a half closer than the previous hit; only 10 seconds earlier. I looked at the B737; who was nose wheel up; and then at the A320; who announced that he was going around. I issued traffic to the A320; who did not see him; then turned the A320 30 degrees left to avoid B737. I then went to the B737 to issue traffic; and he reported him in sight; while saying 'That was nice'. I was not looking at the arrival when he actually initiated the go around; so am not sure at what exact point he started. I am wondering; however; if the back up RADAR is not as accurate; and the three mile line that I used to judge a safe distance was not good enough. Maybe the A320 was closed than the RADAR indicated; and that is why; on the go around; the A320 ended up much closer than I would have liked to the Runway 25 departure. Give refresher training on the use of the backup RADARs. This is probably the first time I have used it in over a decade; and did not realize that the normal spacing used for judgments may not be sufficient; due to a less accurate display.

Data retrieved from NASA's ASRS site as of July 2013 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.