Narrative:

Daytime VFR -- moderate traffic. PA28 3 mi northeast of chs heading 210 degrees at 3000 ft for right downwind entry for visual approach to runway 3. E145 departs runway 3 on runway heading climbing to 2000 ft. E145 did not have transponder on. I radar idented E145 and told him to recycle transponder, then issued traffic on PA28 (at E145's 2 O'clock position, and 2 mi at 3000 ft, swbound). Immediately after, E145's transponder activated with an altitude of 1200 ft. I again called traffic on PA28 (now 3 O'clock position, 1 1/2 mi, level 3000 ft). E145 replies negative traffic. PA28 reports E145 in sight. I instruct PA28 to maintain visual separation with E145 and then instruct E145 to climb to 10000 ft, and that PA28, now at 5 O'clock position, and 2 mi, had him in sight and was no factor. E145 then states that he's responding to an RA on traffic and is descending. I advise E145 that the only traffic within 10 mi of E145 was the PA28, now 6 O'clock position, and 4 mi. E145 continues to descend below MVA (1600 ft) and again states he's responding to RA. I again state that there was no traffic within 10 mi of E145 (other than PA28). E145 then said RA was over and that he was continuing climb. I believe the cause of this was that the E145 did not have transponder on initially, in that TCASII relies on baseline altitude information from the planes own mode C as reported by transponder to analyze potential threats from other transponder-equipped aircraft. For those few seconds while processing information from the E145 on mode C after initialization, the TCASII issued the RA on the PA28 based on incomplete data. Had the E145 activated the transponder at commencement of takeoff roll, there would not have been an incident. While I realize that presently, pilots are mandated to follow an RA, in this case, had the RA came sooner, it would have put the E145 in dangerous proximity to the ground. There were no other aircraft near the E145, and the pilot was informed of this, but still he descended below the MVA to comply with the RA. How safe is that? My recommendation would be to alter the TCASII logic so that in a case similar to this, a TA would be issued until mode C altitude information is stabilized and verified prior to issuing an RA. I realize that a TA without altitude information puts an increased workload on pilots, but in this case, that TA, along with my traffic information, would have enabled the pilots to realize that the traffic was not a factor.

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Original NASA ASRS Text

Title: A DSCNT BELOW MVA IS PERFORMED BY A FLT CREW OF A DEPARTING E145 IN RESPONSE TO A TCASII RA FROM A PA28 WHO HAD ALREADY PASSED THE FLT WITH MORE THAN ADEQUATE CLRNC 2 NM NE OF CHS, SC.

Narrative: DAYTIME VFR -- MODERATE TFC. PA28 3 MI NE OF CHS HEADING 210 DEGS AT 3000 FT FOR R DOWNWIND ENTRY FOR VISUAL APCH TO RWY 3. E145 DEPARTS RWY 3 ON RWY HEADING CLBING TO 2000 FT. E145 DID NOT HAVE XPONDER ON. I RADAR IDENTED E145 AND TOLD HIM TO RECYCLE XPONDER, THEN ISSUED TFC ON PA28 (AT E145'S 2 O'CLOCK POS, AND 2 MI AT 3000 FT, SWBOUND). IMMEDIATELY AFTER, E145'S XPONDER ACTIVATED WITH AN ALT OF 1200 FT. I AGAIN CALLED TFC ON PA28 (NOW 3 O'CLOCK POS, 1 1/2 MI, LEVEL 3000 FT). E145 REPLIES NEGATIVE TFC. PA28 RPTS E145 IN SIGHT. I INSTRUCT PA28 TO MAINTAIN VISUAL SEPARATION WITH E145 AND THEN INSTRUCT E145 TO CLB TO 10000 FT, AND THAT PA28, NOW AT 5 O'CLOCK POS, AND 2 MI, HAD HIM IN SIGHT AND WAS NO FACTOR. E145 THEN STATES THAT HE'S RESPONDING TO AN RA ON TFC AND IS DSNDING. I ADVISE E145 THAT THE ONLY TFC WITHIN 10 MI OF E145 WAS THE PA28, NOW 6 O'CLOCK POS, AND 4 MI. E145 CONTINUES TO DSND BELOW MVA (1600 FT) AND AGAIN STATES HE'S RESPONDING TO RA. I AGAIN STATE THAT THERE WAS NO TFC WITHIN 10 MI OF E145 (OTHER THAN PA28). E145 THEN SAID RA WAS OVER AND THAT HE WAS CONTINUING CLB. I BELIEVE THE CAUSE OF THIS WAS THAT THE E145 DID NOT HAVE XPONDER ON INITIALLY, IN THAT TCASII RELIES ON BASELINE ALT INFO FROM THE PLANES OWN MODE C AS RPTED BY XPONDER TO ANALYZE POTENTIAL THREATS FROM OTHER XPONDER-EQUIPPED ACFT. FOR THOSE FEW SECONDS WHILE PROCESSING INFO FROM THE E145 ON MODE C AFTER INITIALIZATION, THE TCASII ISSUED THE RA ON THE PA28 BASED ON INCOMPLETE DATA. HAD THE E145 ACTIVATED THE XPONDER AT COMMENCEMENT OF TKOF ROLL, THERE WOULD NOT HAVE BEEN AN INCIDENT. WHILE I REALIZE THAT PRESENTLY, PLTS ARE MANDATED TO FOLLOW AN RA, IN THIS CASE, HAD THE RA CAME SOONER, IT WOULD HAVE PUT THE E145 IN DANGEROUS PROX TO THE GND. THERE WERE NO OTHER ACFT NEAR THE E145, AND THE PLT WAS INFORMED OF THIS, BUT STILL HE DSNDED BELOW THE MVA TO COMPLY WITH THE RA. HOW SAFE IS THAT? MY RECOMMENDATION WOULD BE TO ALTER THE TCASII LOGIC SO THAT IN A CASE SIMILAR TO THIS, A TA WOULD BE ISSUED UNTIL MODE C ALT INFO IS STABILIZED AND VERIFIED PRIOR TO ISSUING AN RA. I REALIZE THAT A TA WITHOUT ALT INFO PUTS AN INCREASED WORKLOAD ON PLTS, BUT IN THIS CASE, THAT TA, ALONG WITH MY TFC INFO, WOULD HAVE ENABLED THE PLTS TO REALIZE THAT THE TFC WAS NOT A FACTOR.

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.