Narrative:

The occurrence happened shortly after a position relief briefing was conducted for east radar at the local control position. Shortly after taking the position; a scheduled commuter arrival was observed on the northern fringes of the d-brite radar scope in handoff status. A twin turboprop departure approximately 5 mi north of the chattanooga airport was climbing northbound runway heading through 4500 ft to an assigned altitude of 10000 ft. Knowing that the commuter arrival was descending to 11000 ft; I amended the twin turboprop's altitude to 6000 ft for traffic; so that the commuter arrival could be descended safely into my airspace; which is 10000 ft and below; and subsequently vectored for a visual approach the commuter arrival was issued a descent to what I believed to be 7000 ft; and was then issued a vector for a left downwind arrival after observing the aircraft leaving 10000 ft. Believing that conflict to be resolved; my primary focus then shifted to other aircraft; which included more arrs to chattanooga; more departures from chattanooga; practice approachs; and VFR pop-ups requesting service. When I next observed the commuter arrival; I noticed that the altitude readout indicated 6900 ft. I quickly glanced at my active flight progress strips to confirm the altitude I had issued; and my strip marking indicated 7000 ft. When I observed the commuter arrival descending further; I instructed the aircraft to maintain 7000 ft. The commuter arrival was instructed again to maintain 7000 ft; and was issued a turn wbound further away from the departure corridor. The twin turboprop reported the commuter arrival in sight; and was instructed to maintain visual separation. I judged the closest distance between the 2 aircraft to be 3 NM horizontally; and approximately 500-700 ft vertically. No automated 'conflict alert' was generated; nor did either aircraft report a TCAS conflict/resolution. Contributing factors: having to work approach and departure radar from the tower cabin attendant d-brite scope is cumbersome at best. Typically; the radar is decombined to the TRACON downstairs; staffing permitting; during normal operations; and combined upstairs during periods of low activity; such as opening; and just before closing. On this particular day; the radar was not decombined so that employee briefings could be accommodated. When the decision to keep the radar upstairs was made; traffic was light; and the supervisor left word to call down if we got busy. Our IFR departures are assigned 5000 ft initially. It is common practice to restrict any aircraft descending that you feel may come in close proximity to the departure corridor to 6000 ft or above. Because the twin turboprop departure was passing 4500 ft; and already climbing to 10000 ft as assigned by the previous controller; I did not want to abruptly stop the aircraft's climb at 5000 ft; so I issued 6000 ft to the departure. Out of habit; I believe I issued 6000 ft to the commuter arrival; although I was thinking 7000 ft the whole time. When I took the handoff on the commuter arrival; I did not place the flight progress strip in front of me as I normally do when I am downstairs; nor did I write down the altitude as I was issuing it to the aircraft.

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Original NASA ASRS Text

Title: APCH CTL RESPONSIBILITIES WERE BEING PERFORMED IN THE TWR CAB USING THE D-BRITE RADAR WHEN A NEAR CONFLICT OCCURRED BTWN 2 IFR ACFT. SEPARATION WAS MAINTAINED.

Narrative: THE OCCURRENCE HAPPENED SHORTLY AFTER A POS RELIEF BRIEFING WAS CONDUCTED FOR E RADAR AT THE LCL CTL POS. SHORTLY AFTER TAKING THE POS; A SCHEDULED COMMUTER ARR WAS OBSERVED ON THE NORTHERN FRINGES OF THE D-BRITE RADAR SCOPE IN HDOF STATUS. A TWIN TURBOPROP DEP APPROX 5 MI N OF THE CHATTANOOGA ARPT WAS CLBING NBOUND RWY HDG THROUGH 4500 FT TO AN ASSIGNED ALT OF 10000 FT. KNOWING THAT THE COMMUTER ARR WAS DSNDING TO 11000 FT; I AMENDED THE TWIN TURBOPROP'S ALT TO 6000 FT FOR TFC; SO THAT THE COMMUTER ARR COULD BE DSNDED SAFELY INTO MY AIRSPACE; WHICH IS 10000 FT AND BELOW; AND SUBSEQUENTLY VECTORED FOR A VISUAL APCH THE COMMUTER ARR WAS ISSUED A DSCNT TO WHAT I BELIEVED TO BE 7000 FT; AND WAS THEN ISSUED A VECTOR FOR A L DOWNWIND ARR AFTER OBSERVING THE ACFT LEAVING 10000 FT. BELIEVING THAT CONFLICT TO BE RESOLVED; MY PRIMARY FOCUS THEN SHIFTED TO OTHER ACFT; WHICH INCLUDED MORE ARRS TO CHATTANOOGA; MORE DEPS FROM CHATTANOOGA; PRACTICE APCHS; AND VFR POP-UPS REQUESTING SVC. WHEN I NEXT OBSERVED THE COMMUTER ARR; I NOTICED THAT THE ALT READOUT INDICATED 6900 FT. I QUICKLY GLANCED AT MY ACTIVE FLT PROGRESS STRIPS TO CONFIRM THE ALT I HAD ISSUED; AND MY STRIP MARKING INDICATED 7000 FT. WHEN I OBSERVED THE COMMUTER ARR DSNDING FURTHER; I INSTRUCTED THE ACFT TO MAINTAIN 7000 FT. THE COMMUTER ARR WAS INSTRUCTED AGAIN TO MAINTAIN 7000 FT; AND WAS ISSUED A TURN WBOUND FURTHER AWAY FROM THE DEP CORRIDOR. THE TWIN TURBOPROP RPTED THE COMMUTER ARR IN SIGHT; AND WAS INSTRUCTED TO MAINTAIN VISUAL SEPARATION. I JUDGED THE CLOSEST DISTANCE BTWN THE 2 ACFT TO BE 3 NM HORIZONTALLY; AND APPROX 500-700 FT VERTICALLY. NO AUTOMATED 'CONFLICT ALERT' WAS GENERATED; NOR DID EITHER ACFT RPT A TCAS CONFLICT/RESOLUTION. CONTRIBUTING FACTORS: HAVING TO WORK APCH AND DEP RADAR FROM THE TWR CAB D-BRITE SCOPE IS CUMBERSOME AT BEST. TYPICALLY; THE RADAR IS DECOMBINED TO THE TRACON DOWNSTAIRS; STAFFING PERMITTING; DURING NORMAL OPS; AND COMBINED UPSTAIRS DURING PERIODS OF LOW ACTIVITY; SUCH AS OPENING; AND JUST BEFORE CLOSING. ON THIS PARTICULAR DAY; THE RADAR WAS NOT DECOMBINED SO THAT EMPLOYEE BRIEFINGS COULD BE ACCOMMODATED. WHEN THE DECISION TO KEEP THE RADAR UPSTAIRS WAS MADE; TFC WAS LIGHT; AND THE SUPVR LEFT WORD TO CALL DOWN IF WE GOT BUSY. OUR IFR DEPS ARE ASSIGNED 5000 FT INITIALLY. IT IS COMMON PRACTICE TO RESTRICT ANY ACFT DSNDING THAT YOU FEEL MAY COME IN CLOSE PROX TO THE DEP CORRIDOR TO 6000 FT OR ABOVE. BECAUSE THE TWIN TURBOPROP DEP WAS PASSING 4500 FT; AND ALREADY CLBING TO 10000 FT AS ASSIGNED BY THE PREVIOUS CTLR; I DID NOT WANT TO ABRUPTLY STOP THE ACFT'S CLB AT 5000 FT; SO I ISSUED 6000 FT TO THE DEP. OUT OF HABIT; I BELIEVE I ISSUED 6000 FT TO THE COMMUTER ARR; ALTHOUGH I WAS THINKING 7000 FT THE WHOLE TIME. WHEN I TOOK THE HDOF ON THE COMMUTER ARR; I DID NOT PLACE THE FLT PROGRESS STRIP IN FRONT OF ME AS I NORMALLY DO WHEN I AM DOWNSTAIRS; NOR DID I WRITE DOWN THE ALT AS I WAS ISSUING IT TO THE ACFT.

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