Iad controller described amass alert received for aircraft on landing flare; no go around issued; kn...

Attributes
ACN	718915
Time
Date	200612
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : iad.airport
State Reference	VA
Altitude	agl single value : 100
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tower : iad.tower
Operator	common carrier : air carrier
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	descent : approach other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : local
Qualification	controller : non radar
Experience	controller radar : 22
ASRS Report	718915
Person 2
Affiliation	company : air carrier
Function	flight crew : captain
Events
Anomaly	other anomaly other
Independent Detector	atc equipment other atc equipment : amass other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	faa : investigated
Supplementary
Problem Areas	Aircraft FAA
Primary Problem	Ambiguous
Situations
ATC Facility	radar equipment : iad.tower

Narrative:

Air carrier X over the threshold for runway 19R at iad caused an amass alert due to a duplicate beacon that the ARTS computer picked up beacon code XXX amass safety logic software issued a go around thinking that there were 2 aircraft attempting to land at the same time. The aircraft was beyond the threshold and in the flare and was not sent around. Pilot confirmed that he was squawking the assigned code of XXX. Our own technical operations personnel said there was another target at FL300 that may have caused a ghost target that was captured by the interface between ASR and surface radar. Query to FAA surface radar system team revealed that a certain transponder; the rockwell-collins tpr-901; will cause a duplicate beacon of XXX. The company knows this and a modification to fix the problem is available. If there are aircraft out there flying around with this particular transponder; a safe situation can be made unsafe if a controller reacts incorrectly to what our engineers call a nuisance alert (vice false alert). An airworthiness inspector has the information and hopefully will attempt to contact the company to see if they are operating with that model transponder.

Original NASA ASRS Text

Title: IAD CTLR DESCRIBED AMASS ALERT RECEIVED FOR ACFT ON LNDG FLARE; NO GAR ISSUED; KNOWN XPONDER ANOMALY LISTED AS PROBABLE CAUSE.

Narrative: ACR X OVER THE THRESHOLD FOR RWY 19R AT IAD CAUSED AN AMASS ALERT DUE TO A DUPLICATE BEACON THAT THE ARTS COMPUTER PICKED UP BEACON CODE XXX AMASS SAFETY LOGIC SOFTWARE ISSUED A GAR THINKING THAT THERE WERE 2 ACFT ATTEMPTING TO LAND AT THE SAME TIME. THE ACFT WAS BEYOND THE THRESHOLD AND IN THE FLARE AND WAS NOT SENT AROUND. PLT CONFIRMED THAT HE WAS SQUAWKING THE ASSIGNED CODE OF XXX. OUR OWN TECHNICAL OPS PERSONNEL SAID THERE WAS ANOTHER TARGET AT FL300 THAT MAY HAVE CAUSED A GHOST TARGET THAT WAS CAPTURED BY THE INTERFACE BTWN ASR AND SURFACE RADAR. QUERY TO FAA SURFACE RADAR SYS TEAM REVEALED THAT A CERTAIN XPONDER; THE ROCKWELL-COLLINS TPR-901; WILL CAUSE A DUPLICATE BEACON OF XXX. THE COMPANY KNOWS THIS AND A MODIFICATION TO FIX THE PROB IS AVAILABLE. IF THERE ARE ACFT OUT THERE FLYING AROUND WITH THIS PARTICULAR XPONDER; A SAFE SITUATION CAN BE MADE UNSAFE IF A CTLR REACTS INCORRECTLY TO WHAT OUR ENGINEERS CALL A NUISANCE ALERT (VICE FALSE ALERT). AN AIRWORTHINESS INSPECTOR HAS THE INFO AND HOPEFULLY WILL ATTEMPT TO CONTACT THE COMPANY TO SEE IF THEY ARE OPERATING WITH THAT MODEL XPONDER.

Data retrieved from NASA's ASRS site as of January 2009 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.

Iad controller described amass alert received for aircraft on landing flare; no go around issued; known transponder anomaly listed as probable cause.