P50 controller described a TCAS RA event between an IFR CE560 and a VFR PA28; apparently caused by a...

Narrative:

I had a PA28; VFR; southbound from biltmore airport on the east transition; at 3;500. Additional traffic was a C560 northbound on heading 350; level 4;000. When the two were still about 4 miles apart I saw the PA28's altitude fluctuating between non reporting; 3;500 and 3;700. I verified they were level 3;500 and they confirmed this; I advised them to reset transponder. I exchanged traffic between the two aircraft and the PA28 had the CE560 in sight when they were approximately 2 miles apart. The PA28's transponder appeared to be functioning properly after the reset for a sweep or two; but then when the two targets were just about to merge I got the resolution call from the CE560 indicating they were climbing. I called traffic again; advised the citation the cherokee had him in sight and was in fact at 3;500. I could have done a better job explaining it phraseology wise; instead of saying the cherokee's mode C was errant; I said something to the effect of; 'I do see the cherokee indicating 3;700 but have verified he's level 3;500. 'Thankfully there was no traffic in the 4;500 altitude for the transition as the citation climbed to 4;300. I advised the PA28 to have their transponder checked when they landed.

Original NASA ASRS Text

Title: P50 controller described a TCAS RA event between an IFR CE560 and a VFR PA28; apparently caused by a malfunctioning Mode C in the PA28; traffic was issued and sighted by the PA28 prior to the event.

Narrative: I had a PA28; VFR; Southbound from Biltmore airport on the East Transition; at 3;500. Additional traffic was a C560 Northbound on heading 350; level 4;000. When the two were still about 4 miles apart I saw the PA28's altitude fluctuating between non reporting; 3;500 and 3;700. I verified they were level 3;500 and they confirmed this; I advised them to reset transponder. I exchanged traffic between the two aircraft and the PA28 had the CE560 in sight when they were approximately 2 miles apart. The PA28's transponder appeared to be functioning properly after the reset for a sweep or two; but then when the two targets were just about to merge I got the resolution call from the CE560 indicating they were climbing. I called traffic again; advised the Citation the Cherokee had him in sight and was in fact at 3;500. I could have done a better job explaining it phraseology wise; instead of saying the Cherokee's Mode C was errant; I said something to the effect of; 'I do see the Cherokee indicating 3;700 but have verified he's level 3;500. 'Thankfully there was no traffic in the 4;500 altitude for the transition as the Citation climbed to 4;300. I advised the PA28 to have their transponder checked when they landed.

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	881487
Time
Date	201004
Local Time Of Day	1801-2400
Place
Locale Reference	P50.TRACON
State Reference	AZ
Aircraft 1
Make Model Name	Citation V/Ultra/Encore (C560)
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Vectors
Flight Plan	IFR
Aircraft 2
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Flight Plan	VFR
Person 1
Function	Approach Departure
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict

P50 controller described a TCAS RA event between an IFR CE560 and a VFR PA28; apparently caused by a malfunctioning mode C in the PA28; traffic was issued and sighted by the PA28 prior to the event.