Flight crew experience phantom TCASII and respond accordingly. Later learn the RA was self-generated...

Attributes
ACN	605855
Time
Date	200401
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : fra.vortac
State Reference	CA
Altitude	msl single value : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa.artcc
Operator	common carrier : air carrier
Make Model Name	B737-400
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	605855
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	605856
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude inflight encounter other maintenance problem : improper documentation non adherence : far non adherence : company policies
Independent Detector	other other : person 4
Resolutory Action	controller : issued advisory flight crew : took precautionary avoidance action none taken : anomaly accepted
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

At cruise altitude we received a pop-up traffic RA at a cruise altitude of FL350, near fra VOR en route from lax to sea. It was an immediate RA which commanded a 'descend now.' this conflict was at approximately 1200 ft at a very short distance. The captain followed the directed RA and descended immediately. As we descended, as I recall, I do not believe there was much (if any) change in the vertical separation with the target, which surprised me. I saw no traffic outside which also surprised me since we should have been close. I was somewhat concerned about this non diverging vertical separation. I called ATC to inform them we were leaving our assigned altitude on a TCASII RA and I also asked them if they could tell us about the traffic we were avoiding. At about the same time, after descending 500 ft (as I recall) the target disappeared. It was at this point that I presumed that what we had was a 'ghost target.' ATC verified that there was no other traffic at our altitude and the nearest target was 2000 ft below us at some distance away. The captain informed me that he would file the required company report for the RA altitude deviation and that was the last I thought of that. It never occurred to me to make a logbook entry. I later received a call from company maintenance informing me that this 'ghost traffic' was the result of an onboard transponder malfunction, that the ghost was a signal generated by one of the onboard xponders to the other transponder. As a transponder malfunction, our event required a logbook write-up. I had no idea this was a transponder/TCASII malfunction. I guess I didn't know enough about the system to realize that a 'ghost target' could be self-generated. I didn't think much about it at all. I just thought a target popped up and that's what happens sometimes on a TCASII system. It makes sense in retrospect that this is, in fact, a malfunction of the TCASII system. I will always write them up in the future.

Original NASA ASRS Text

Title: FLT CREW EXPERIENCE PHANTOM TCASII AND RESPOND ACCORDINGLY. LATER LEARN THE RA WAS SELF-GENERATED AND REQUIRED A LOGBOOK WRITE-UP WHICH THEY HAD NOT DONE.

Narrative: AT CRUISE ALT WE RECEIVED A POP-UP TFC RA AT A CRUISE ALT OF FL350, NEAR FRA VOR ENRTE FROM LAX TO SEA. IT WAS AN IMMEDIATE RA WHICH COMMANDED A 'DSND NOW.' THIS CONFLICT WAS AT APPROX 1200 FT AT A VERY SHORT DISTANCE. THE CAPT FOLLOWED THE DIRECTED RA AND DSNDED IMMEDIATELY. AS WE DSNDED, AS I RECALL, I DO NOT BELIEVE THERE WAS MUCH (IF ANY) CHANGE IN THE VERT SEPARATION WITH THE TARGET, WHICH SURPRISED ME. I SAW NO TFC OUTSIDE WHICH ALSO SURPRISED ME SINCE WE SHOULD HAVE BEEN CLOSE. I WAS SOMEWHAT CONCERNED ABOUT THIS NON DIVERGING VERT SEPARATION. I CALLED ATC TO INFORM THEM WE WERE LEAVING OUR ASSIGNED ALT ON A TCASII RA AND I ALSO ASKED THEM IF THEY COULD TELL US ABOUT THE TFC WE WERE AVOIDING. AT ABOUT THE SAME TIME, AFTER DSNDING 500 FT (AS I RECALL) THE TARGET DISAPPEARED. IT WAS AT THIS POINT THAT I PRESUMED THAT WHAT WE HAD WAS A 'GHOST TARGET.' ATC VERIFIED THAT THERE WAS NO OTHER TFC AT OUR ALT AND THE NEAREST TARGET WAS 2000 FT BELOW US AT SOME DISTANCE AWAY. THE CAPT INFORMED ME THAT HE WOULD FILE THE REQUIRED COMPANY RPT FOR THE RA ALTDEV AND THAT WAS THE LAST I THOUGHT OF THAT. IT NEVER OCCURRED TO ME TO MAKE A LOGBOOK ENTRY. I LATER RECEIVED A CALL FROM COMPANY MAINT INFORMING ME THAT THIS 'GHOST TFC' WAS THE RESULT OF AN ONBOARD XPONDER MALFUNCTION, THAT THE GHOST WAS A SIGNAL GENERATED BY ONE OF THE ONBOARD XPONDERS TO THE OTHER XPONDER. AS A XPONDER MALFUNCTION, OUR EVENT REQUIRED A LOGBOOK WRITE-UP. I HAD NO IDEA THIS WAS A XPONDER/TCASII MALFUNCTION. I GUESS I DIDN'T KNOW ENOUGH ABOUT THE SYS TO REALIZE THAT A 'GHOST TARGET' COULD BE SELF-GENERATED. I DIDN'T THINK MUCH ABOUT IT AT ALL. I JUST THOUGHT A TARGET POPPED UP AND THAT'S WHAT HAPPENS SOMETIMES ON A TCASII SYS. IT MAKES SENSE IN RETROSPECT THAT THIS IS, IN FACT, A MALFUNCTION OF THE TCASII SYS. I WILL ALWAYS WRITE THEM UP IN THE FUTURE.

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.

Flight crew experience phantom TCASII and respond accordingly. Later learn the RA was self-generated and required a logbook write-up which they had not done.