Sfo controller experienced amass alert on landing aircraft but did not issue go around.

Attributes
ACN	627835
Time
Date	200408
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : sfo.airport
State Reference	CA
Altitude	agl single value : 100
Environment
Flight Conditions	Marginal
Light	Dawn
Aircraft 1
Controlling Facilities	tower : sfo.tower
Operator	common carrier : air carrier
Make Model Name	Regional Jet CL65, Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : local
Qualification	controller : radar
Experience	controller limited radar : 12 controller non radar : 4 controller time certified in position1 : 11 controller time certified in position2 : 7
ASRS Report	627835
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	other anomaly other aircraft equipment problem : critical
Independent Detector	atc equipment other atc equipment : amass other controllera
Resolutory Action	controller : issued new clearance
Supplementary
Problem Areas	FAA Navigational Facility ATC Human Performance
Primary Problem	Navigational Facility

Narrative:

Runway 28R/10L closed, ILS runway 28L approach in use. Air carrier X checked in tagged for runway 28L on ILS runway 28L approach. I cleared air carrier X to land runway 28L. I had air carrier X in sight at approximately 4 mi final and he appeared to be lined up for runway 28L all the way down. When air carrier X was over the lights for runway 28L I scanned the runways and the asde. I was looking right at air carrier X as he approached his flare after overflying the runway 28L numbers when amass alerted, 'warning, go around, runway 28R landing on a closed runway.' I re-scanned runway 28L and it appeared clear. Air carrier X was still airborne but now in the flare over runway 28L at taxiway north or P. I looked at the amass. It was alerting on air carrier X which it clearly showed on runway 28L. It was alerting 'go around, runway 28R, closed runway.' runway 28R was indicated closed by the large 'X's at each end. I decided to let air carrier X land on runway 28L because it appeared to me to be safe. I felt it was far more dangerous to send air carrier X around at the time of the alert.

Original NASA ASRS Text

Title: SFO CTLR EXPERIENCED AMASS ALERT ON LNDG ACFT BUT DID NOT ISSUE GAR.

Narrative: RWY 28R/10L CLOSED, ILS RWY 28L APCH IN USE. ACR X CHKED IN TAGGED FOR RWY 28L ON ILS RWY 28L APCH. I CLRED ACR X TO LAND RWY 28L. I HAD ACR X IN SIGHT AT APPROX 4 MI FINAL AND HE APPEARED TO BE LINED UP FOR RWY 28L ALL THE WAY DOWN. WHEN ACR X WAS OVER THE LIGHTS FOR RWY 28L I SCANNED THE RWYS AND THE ASDE. I WAS LOOKING RIGHT AT ACR X AS HE APCHED HIS FLARE AFTER OVERFLYING THE RWY 28L NUMBERS WHEN AMASS ALERTED, 'WARNING, GO AROUND, RWY 28R LNDG ON A CLOSED RWY.' I RE-SCANNED RWY 28L AND IT APPEARED CLR. ACR X WAS STILL AIRBORNE BUT NOW IN THE FLARE OVER RWY 28L AT TXWY N OR P. I LOOKED AT THE AMASS. IT WAS ALERTING ON ACR X WHICH IT CLRLY SHOWED ON RWY 28L. IT WAS ALERTING 'GO AROUND, RWY 28R, CLOSED RWY.' RWY 28R WAS INDICATED CLOSED BY THE LARGE 'X'S AT EACH END. I DECIDED TO LET ACR X LAND ON RWY 28L BECAUSE IT APPEARED TO ME TO BE SAFE. I FELT IT WAS FAR MORE DANGEROUS TO SEND ACR X AROUND AT THE TIME OF THE ALERT.

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.