Narrative:

I was flying level at FL230. My transponder put out a couple of signals that I was at FL240. An airliner nearby at FL240 got a TCASII alert and descended toward me. Seeing him descend, I banked sharply away so there was never really a question of us hitting each other. However, it was disconcerting to have an airliner descend toward me in response to an RA. The backgnd to the situation is that I had been having problems with my altitude encoder. This was the first flight after the encoder had been repaired the third time in 3 months. I thought it was working. Unfortunately the repair shop had not returned to me on time my tcad as they were supposed to. That meant that I had no in- cockpit readout of the output of my altitude encoder (I have a transponder without an encoder readout). At the time of the incident I had been level at FL230 for some time. I assume that I was pumping out that altitude. A little beyond palmdale I saw an airliner descending toward me. He had been cleared to FL240. He called out to the controller that he was responding to an RA and descending to FL230. He was easy to see at night, and, though I wasn't sure he was the plane making the call, I banked sharply away. I don't know if he could see me, but he kept coming at me. The controller said that I was at FL230. The pilot replied curtly, 'responding to an RA, descending to FL230.' as we passed each other, the controller asked me my altitude. I said 'FL230.' he replied that he had shown me at FL240. I was initially incredulous. He clarified that I had only been a couple of hits. There are several lessons to be learned here. The most consequential is that it had never occurred to me that in this TCAS era, having a malfunctioning altitude encoder is a very serious problem indeed. I figured that as in the past, if the encoder malfunctioned, it would simply be an inconvenience for ATC as they asked me to stop squawking altitude. It had never occurred to me that in certain circumstances, ATC is no longer in control even in positive control airspace. Second lesson: having an in-cockpit readout of what the encoder is pumping out is very valuable. In the past, when the tcad was in my plane, then I knew -- even before the controller realized it -- that the encoder was on the blink. I could then alert them, rather than vice versa. Third lesson: when in doubt, stop squawking altitude. I should have been more on the ball in interrogating controllers about whether the encoder was working properly. A previous controller had made a mention that my altitude had dropped out momentarily, and this should have been a signal to me (given my past history with the encoder) to monitor the encoder carefully. Fourth lesson: the common practice of airlines turning on their lights when they are called as traffic is very helpful, even in positive controled airspace. This airliner was easy to see (I don't know if he turned his lights on in response to a call, but they were on). As a result, what could have been quite harrowing was just annoying. This courteous airliner practice should be reinforced. Fifth lesson (and this may be sour grapes): repair stations should take encoder repair seriously. This was the third time I had brought the encoder in for repair. Avionics in fresno had done a slipshod job 3 times, since the problem kept recurring on every flight. If he had understood that lives could have been threatened by his lackadaisical attitude, maybe he would have paid more attention. Supplemental information from acn 294419: air carrier Y, while executing the civet arrival into lax, and while level at FL240 45 mi east of pdz VOR, and assigned 250 KIAS, our aircraft received a TCASII RA directing us to descend immediately, without having first received a TA, nor a pointout of traffic from ATC. I disconnected the autoplt and immediately complied with the RA. My first officer advised ATC that we were complying with an RA, and that we would be descending straight ahead. We were in total IMC, rain, moderate icing, moderate chop to light turbulence conditions. At about FL235 we received an urgent call from ATC, advising us that the traffic we were descending to was at 12 O'clock, level at FL230. In other words, the TCASII directed us to descend into another aircraft, which was never a conflict in the first place. Had we continued to follow the TCASII it would have directed a collision with the aircraft at our 12 O'clock position. I subsequently directed a call to lax approach upon landing. Calling TRACON supervisor who advised me that the aircraft was avoided only because we discontinued our descent. That there was a defective transponder on the aircraft ahead of us, and that it was reporting its altitude as FL240, when in fact it was at FL230, as assigned.

Related reports:

Google
 

Original NASA ASRS Text

Title: SMA X ACFT EQUIP PROB XPONDER MODE C CAUSED LTSS FROM ACR Y RESPONDING TO TCASII RA EVASIVE ACTION DSND.

Narrative: I WAS FLYING LEVEL AT FL230. MY XPONDER PUT OUT A COUPLE OF SIGNALS THAT I WAS AT FL240. AN AIRLINER NEARBY AT FL240 GOT A TCASII ALERT AND DSNDED TOWARD ME. SEEING HIM DSND, I BANKED SHARPLY AWAY SO THERE WAS NEVER REALLY A QUESTION OF US HITTING EACH OTHER. HOWEVER, IT WAS DISCONCERTING TO HAVE AN AIRLINER DSND TOWARD ME IN RESPONSE TO AN RA. THE BACKGND TO THE SIT IS THAT I HAD BEEN HAVING PROBS WITH MY ALT ENCODER. THIS WAS THE FIRST FLT AFTER THE ENCODER HAD BEEN REPAIRED THE THIRD TIME IN 3 MONTHS. I THOUGHT IT WAS WORKING. UNFORTUNATELY THE REPAIR SHOP HAD NOT RETURNED TO ME ON TIME MY TCAD AS THEY WERE SUPPOSED TO. THAT MEANT THAT I HAD NO IN- COCKPIT READOUT OF THE OUTPUT OF MY ALT ENCODER (I HAVE A XPONDER WITHOUT AN ENCODER READOUT). AT THE TIME OF THE INCIDENT I HAD BEEN LEVEL AT FL230 FOR SOME TIME. I ASSUME THAT I WAS PUMPING OUT THAT ALT. A LITTLE BEYOND PALMDALE I SAW AN AIRLINER DSNDING TOWARD ME. HE HAD BEEN CLRED TO FL240. HE CALLED OUT TO THE CTLR THAT HE WAS RESPONDING TO AN RA AND DSNDING TO FL230. HE WAS EASY TO SEE AT NIGHT, AND, THOUGH I WASN'T SURE HE WAS THE PLANE MAKING THE CALL, I BANKED SHARPLY AWAY. I DON'T KNOW IF HE COULD SEE ME, BUT HE KEPT COMING AT ME. THE CTLR SAID THAT I WAS AT FL230. THE PLT REPLIED CURTLY, 'RESPONDING TO AN RA, DSNDING TO FL230.' AS WE PASSED EACH OTHER, THE CTLR ASKED ME MY ALT. I SAID 'FL230.' HE REPLIED THAT HE HAD SHOWN ME AT FL240. I WAS INITIALLY INCREDULOUS. HE CLARIFIED THAT I HAD ONLY BEEN A COUPLE OF HITS. THERE ARE SEVERAL LESSONS TO BE LEARNED HERE. THE MOST CONSEQUENTIAL IS THAT IT HAD NEVER OCCURRED TO ME THAT IN THIS TCAS ERA, HAVING A MALFUNCTIONING ALT ENCODER IS A VERY SERIOUS PROB INDEED. I FIGURED THAT AS IN THE PAST, IF THE ENCODER MALFUNCTIONED, IT WOULD SIMPLY BE AN INCONVENIENCE FOR ATC AS THEY ASKED ME TO STOP SQUAWKING ALT. IT HAD NEVER OCCURRED TO ME THAT IN CERTAIN CIRCUMSTANCES, ATC IS NO LONGER IN CTL EVEN IN POSITIVE CTL AIRSPACE. SECOND LESSON: HAVING AN IN-COCKPIT READOUT OF WHAT THE ENCODER IS PUMPING OUT IS VERY VALUABLE. IN THE PAST, WHEN THE TCAD WAS IN MY PLANE, THEN I KNEW -- EVEN BEFORE THE CTLR REALIZED IT -- THAT THE ENCODER WAS ON THE BLINK. I COULD THEN ALERT THEM, RATHER THAN VICE VERSA. THIRD LESSON: WHEN IN DOUBT, STOP SQUAWKING ALT. I SHOULD HAVE BEEN MORE ON THE BALL IN INTERROGATING CTLRS ABOUT WHETHER THE ENCODER WAS WORKING PROPERLY. A PREVIOUS CTLR HAD MADE A MENTION THAT MY ALT HAD DROPPED OUT MOMENTARILY, AND THIS SHOULD HAVE BEEN A SIGNAL TO ME (GIVEN MY PAST HISTORY WITH THE ENCODER) TO MONITOR THE ENCODER CAREFULLY. FOURTH LESSON: THE COMMON PRACTICE OF AIRLINES TURNING ON THEIR LIGHTS WHEN THEY ARE CALLED AS TFC IS VERY HELPFUL, EVEN IN POSITIVE CTLED AIRSPACE. THIS AIRLINER WAS EASY TO SEE (I DON'T KNOW IF HE TURNED HIS LIGHTS ON IN RESPONSE TO A CALL, BUT THEY WERE ON). AS A RESULT, WHAT COULD HAVE BEEN QUITE HARROWING WAS JUST ANNOYING. THIS COURTEOUS AIRLINER PRACTICE SHOULD BE REINFORCED. FIFTH LESSON (AND THIS MAY BE SOUR GRAPES): REPAIR STATIONS SHOULD TAKE ENCODER REPAIR SERIOUSLY. THIS WAS THE THIRD TIME I HAD BROUGHT THE ENCODER IN FOR REPAIR. AVIONICS IN FRESNO HAD DONE A SLIPSHOD JOB 3 TIMES, SINCE THE PROB KEPT RECURRING ON EVERY FLT. IF HE HAD UNDERSTOOD THAT LIVES COULD HAVE BEEN THREATENED BY HIS LACKADAISICAL ATTITUDE, MAYBE HE WOULD HAVE PAID MORE ATTN. SUPPLEMENTAL INFO FROM ACN 294419: ACR Y, WHILE EXECUTING THE CIVET ARR INTO LAX, AND WHILE LEVEL AT FL240 45 MI E OF PDZ VOR, AND ASSIGNED 250 KIAS, OUR ACFT RECEIVED A TCASII RA DIRECTING US TO DSND IMMEDIATELY, WITHOUT HAVING FIRST RECEIVED A TA, NOR A POINTOUT OF TFC FROM ATC. I DISCONNECTED THE AUTOPLT AND IMMEDIATELY COMPLIED WITH THE RA. MY FO ADVISED ATC THAT WE WERE COMPLYING WITH AN RA, AND THAT WE WOULD BE DSNDING STRAIGHT AHEAD. WE WERE IN TOTAL IMC, RAIN, MODERATE ICING, MODERATE CHOP TO LIGHT TURB CONDITIONS. AT ABOUT FL235 WE RECEIVED AN URGENT CALL FROM ATC, ADVISING US THAT THE TFC WE WERE DSNDING TO WAS AT 12 O'CLOCK, LEVEL AT FL230. IN OTHER WORDS, THE TCASII DIRECTED US TO DSND INTO ANOTHER ACFT, WHICH WAS NEVER A CONFLICT IN THE FIRST PLACE. HAD WE CONTINUED TO FOLLOW THE TCASII IT WOULD HAVE DIRECTED A COLLISION WITH THE ACFT AT OUR 12 O'CLOCK POS. I SUBSEQUENTLY DIRECTED A CALL TO LAX APCH UPON LNDG. CALLING TRACON SUPVR WHO ADVISED ME THAT THE ACFT WAS AVOIDED ONLY BECAUSE WE DISCONTINUED OUR DSCNT. THAT THERE WAS A DEFECTIVE XPONDER ON THE ACFT AHEAD OF US, AND THAT IT WAS RPTING ITS ALT AS FL240, WHEN IN FACT IT WAS AT FL230, AS ASSIGNED.

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.