X landed on occupied runway over the top of Y holding in position. System error.

Attributes
ACN	267031
Time
Date	199403
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : tmb
State Reference	FL
Altitude	agl bound lower : 0 agl bound upper : 150
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : tmb
Operator	general aviation : personal
Make Model Name	Experimental
Operating Under FAR Part	Part 91
Flight Phase	descent : approach landing other
Flight Plan	None
Aircraft 2
Operator	general aviation : personal
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	ground : holding
Flight Plan	None
Person 1
Affiliation	government : faa
Function	controller : local
Qualification	controller : non radar
Experience	controller non radar : 10
ASRS Report	267031
Person 2
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : private
Events
Anomaly	conflict : ground critical conflict : nmac incursion : runway non adherence : published procedure non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Miss Distance	horizontal : 0 vertical : 150
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I was working local control at the VFR tower. Both of our local control position was combined at 1 local position/sector. WX was VFR. Tower equipment available: BRITE radar display. The traffic was moderate to heavy. Experimental aircraft X in question was given instructions to make straight-in runway 9L. Later I put a cessna C-172 Y into position and hold on runway 9L. I put that C-172 into position and gave the experimental a clearance to land based on a target I saw on a 2 mi final as displayed on the BRITE radar. The aircraft on the 2 mi final turned out not to be the experimental. Instead, the experimental was crossing over the threshold runway 9L about 150 ft over the c- 172. I don't know how far to the left or right the experimental went around the C-172. No accident just a possible loss of separation. Contributing factors: traffic load and sectors combined too much for controller and most important -- BRITE radar display unreliable by not always displaying all targets in the airspace. (This is a recurring problem that has been brough to management's attention many times.) I never saw a radar target displayed for the small experimental and mistook another aircraft's target while being very busy.

Original NASA ASRS Text

Title: X LANDED ON OCCUPIED RWY OVER THE TOP OF Y HOLDING IN POS. SYS ERROR.

Narrative: I WAS WORKING LCL CTL AT THE VFR TWR. BOTH OF OUR LCL CTL POS WAS COMBINED AT 1 LCL POS/SECTOR. WX WAS VFR. TWR EQUIP AVAILABLE: BRITE RADAR DISPLAY. THE TFC WAS MODERATE TO HVY. EXPERIMENTAL ACFT X IN QUESTION WAS GIVEN INSTRUCTIONS TO MAKE STRAIGHT-IN RWY 9L. LATER I PUT A CESSNA C-172 Y INTO POS AND HOLD ON RWY 9L. I PUT THAT C-172 INTO POS AND GAVE THE EXPERIMENTAL A CLRNC TO LAND BASED ON A TARGET I SAW ON A 2 MI FINAL AS DISPLAYED ON THE BRITE RADAR. THE ACFT ON THE 2 MI FINAL TURNED OUT NOT TO BE THE EXPERIMENTAL. INSTEAD, THE EXPERIMENTAL WAS XING OVER THE THRESHOLD RWY 9L ABOUT 150 FT OVER THE C- 172. I DON'T KNOW HOW FAR TO THE L OR R THE EXPERIMENTAL WENT AROUND THE C-172. NO ACCIDENT JUST A POSSIBLE LOSS OF SEPARATION. CONTRIBUTING FACTORS: TFC LOAD AND SECTORS COMBINED TOO MUCH FOR CTLR AND MOST IMPORTANT -- BRITE RADAR DISPLAY UNRELIABLE BY NOT ALWAYS DISPLAYING ALL TARGETS IN THE AIRSPACE. (THIS IS A RECURRING PROB THAT HAS BEEN BROUGH TO MGMNT'S ATTN MANY TIMES.) I NEVER SAW A RADAR TARGET DISPLAYED FOR THE SMALL EXPERIMENTAL AND MISTOOK ANOTHER ACFT'S TARGET WHILE BEING VERY BUSY.

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.