Air carrier came in close proximity to an small aircraft after departure.

Attributes
ACN	121821
Time
Date	198909
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : aus
State Reference	TX
Altitude	msl bound lower : 600 msl bound upper : 4000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : aus
Operator	general aviation : personal
Make Model Name	Small Aircraft, High Wing, 1 Eng, Fixed Gear
Flight Phase	climbout : intermediate altitude
Route In Use	departure other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	climbout : initial
Route In Use	departure other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller non radar : 1 controller radar : 4
ASRS Report	121821
Person 2
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe conflict : airborne less severe non adherence : required legal separation other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Miss Distance	horizontal : 0 vertical : 400
Supplementary
Air Traffic Incident	Operational Error

Narrative:

I was working radar east, arrival and departure. Small aircraft X departed on an IFR flight eastbound, climbing to 4000' initially. The tower called, saying that they had air carrier Y ready to go northbound. The departure would cross the path of small aircraft X. When I observed the mode C of small aircraft X indicate 3500' climbing, I called the tower and said to let air carrier Y go, but to restrict him to 3000'. A minute later I observed small aircraft X's altitude at 4200', and asked him to verify that he would stay at 4000'. He replied, 'yes, out of 3000' for 4000'.' mode C was wrong, although verified on departure roll on the radar using field elevation. Tower had already switched air carrier Y to me, but he did not call until 4 mi later when the targets had merged and passed. Evidently, the crew was busy. No solution to this. Later, looking at the computer strip, the remarks said 'mode C intermittent,' to me meaning 'on' or 'off.' plus, the remarks section can be overlooked (I had several other aircraft on frequency). I believe I'll look at mode C more closely from now on.

Original NASA ASRS Text

Title: ACR CAME IN CLOSE PROX TO AN SMA AFTER DEP.

Narrative: I WAS WORKING RADAR E, ARR AND DEP. SMA X DEPARTED ON AN IFR FLT EBND, CLBING TO 4000' INITIALLY. THE TWR CALLED, SAYING THAT THEY HAD ACR Y READY TO GO NBND. THE DEP WOULD CROSS THE PATH OF SMA X. WHEN I OBSERVED THE MODE C OF SMA X INDICATE 3500' CLBING, I CALLED THE TWR AND SAID TO LET ACR Y GO, BUT TO RESTRICT HIM TO 3000'. A MINUTE LATER I OBSERVED SMA X'S ALT AT 4200', AND ASKED HIM TO VERIFY THAT HE WOULD STAY AT 4000'. HE REPLIED, 'YES, OUT OF 3000' FOR 4000'.' MODE C WAS WRONG, ALTHOUGH VERIFIED ON DEP ROLL ON THE RADAR USING FIELD ELEVATION. TWR HAD ALREADY SWITCHED ACR Y TO ME, BUT HE DID NOT CALL UNTIL 4 MI LATER WHEN THE TARGETS HAD MERGED AND PASSED. EVIDENTLY, THE CREW WAS BUSY. NO SOLUTION TO THIS. LATER, LOOKING AT THE COMPUTER STRIP, THE REMARKS SAID 'MODE C INTERMITTENT,' TO ME MEANING 'ON' OR 'OFF.' PLUS, THE REMARKS SECTION CAN BE OVERLOOKED (I HAD SEVERAL OTHER ACFT ON FREQ). I BELIEVE I'LL LOOK AT MODE C MORE CLOSELY FROM NOW ON.

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