Narrative:

I was working the north radar aircraft X; a PA34/I; was handed off to me approximately 15 NM southwest of cmh at 7000 ft. South radar advised that the aircraft's transponder had been intermittent. The aircraft's route was direct gvq; which put it on a northeasterly track that would take it nearly directly over cmh. Upon initial contact the pilot inquired as to how his transponder appeared. I advised that there was neither mode C altitude nor a transponder beacon return displayed. All other tracked aircraft on our stars displays; which utilizes our cmh airport surveillance radar (ASR-9...mode south); appeared just fine. However; having experienced intermittent xponders in the past; I always make it a policy (now that we have the capability with our stars equipment) to switch to the london ohio air route surveillance radar (qwo ARSR) to compare. If both show a primary target only; I can state with absolute certainty that the aircraft's transponder is inoperative. However; often I observe a disparity; such as I describe here. When I switched to qwo; air carrier X mode C altitude and beacon return were displayed just fine. Upon switching back to our ASR-9; the aircraft was a 'skin paint' target only. For the next 40 mi or so; I noted that aircraft X transponder appeared to work very intermittently when using our ASR-9; yet every time I observed it using qwo; the transponder reply appeared rock solid. Back in the mid 90's; when we first switched to mode south; we experienced this problem with a high percentage of GA aircraft.

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Original NASA ASRS Text

Title: CMH CTLR DESCRIBED SUSPECTED ACFT XPONDER FAILURE DUE TO LACK OF BEACON TARGET BUT CONFIRMED OTHER LCL RADAR WAS DISPLAYING TARGET.

Narrative: I WAS WORKING THE N RADAR ACFT X; A PA34/I; WAS HANDED OFF TO ME APPROX 15 NM SW OF CMH AT 7000 FT. S RADAR ADVISED THAT THE ACFT'S XPONDER HAD BEEN INTERMITTENT. THE ACFT'S RTE WAS DIRECT GVQ; WHICH PUT IT ON A NORTHEASTERLY TRACK THAT WOULD TAKE IT NEARLY DIRECTLY OVER CMH. UPON INITIAL CONTACT THE PLT INQUIRED AS TO HOW HIS XPONDER APPEARED. I ADVISED THAT THERE WAS NEITHER MODE C ALT NOR A XPONDER BEACON RETURN DISPLAYED. ALL OTHER TRACKED ACFT ON OUR STARS DISPLAYS; WHICH UTILIZES OUR CMH ARPT SURVEILLANCE RADAR (ASR-9...MODE S); APPEARED JUST FINE. HOWEVER; HAVING EXPERIENCED INTERMITTENT XPONDERS IN THE PAST; I ALWAYS MAKE IT A POLICY (NOW THAT WE HAVE THE CAPABILITY WITH OUR STARS EQUIP) TO SWITCH TO THE LONDON OHIO AIR RTE SURVEILLANCE RADAR (QWO ARSR) TO COMPARE. IF BOTH SHOW A PRIMARY TARGET ONLY; I CAN STATE WITH ABSOLUTE CERTAINTY THAT THE ACFT'S XPONDER IS INOP. HOWEVER; OFTEN I OBSERVE A DISPARITY; SUCH AS I DESCRIBE HERE. WHEN I SWITCHED TO QWO; ACR X MODE C ALT AND BEACON RETURN WERE DISPLAYED JUST FINE. UPON SWITCHING BACK TO OUR ASR-9; THE ACFT WAS A 'SKIN PAINT' TARGET ONLY. FOR THE NEXT 40 MI OR SO; I NOTED THAT ACFT X XPONDER APPEARED TO WORK VERY INTERMITTENTLY WHEN USING OUR ASR-9; YET EVERY TIME I OBSERVED IT USING QWO; THE XPONDER REPLY APPEARED ROCK SOLID. BACK IN THE MID 90'S; WHEN WE FIRST SWITCHED TO MODE S; WE EXPERIENCED THIS PROB WITH A HIGH PERCENTAGE OF GA ACFT.

Data retrieved from NASA's ASRS site as of May 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.