AN ACR IS CLRED FOR TAKE OFF WHEN A GENERAL AVIATION ACFT HAD BEEN CLRED TO LAND ON AN INTERSECTING RWY. THE GENERAL AVIATION ACFT IS ABLE TO STOP SHORT OF THE CROSSING RWY EVEN THOUGH A GAR CLRNC WAS ISSUED TOO LATE TO BE EFFECTIVE.

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CLEARED TFC TO LAND RWY 4. CLEARED B737 FOR TAKE OFF RWY 1; FORGOT ABOUT LNDG ACFT ON RWY 4; REALIZED THIS AND ATTEMPTED TO SEND HIM AROUND WITHOUT KNOWING WHERE ACFT WAS; I THEN SIGHTED ACFT ON RWY 4. HE HAD ALREADY LANDED AND WAS TAXIING OUT. I THEN INSTRUCTED HIM TO TURN SHORT OF RWY 1. I DETERMINED THAT THE DEP ON RWY 1 WAS NOT A FACTOR ANYMORE AND MADE NO ATTEMPT TO STOP HIS DEP. FACTORS: RADAR TAG DROPPED ON RWY 4 ARR 2 NM FROM ARPT. VERY BUSY AT THE TIME - 20 ACFT IN 10 MINS; HELI POS COMBINED WITH LOCAL. SUPVR IN BACK OF TWR WORKING CLRNC. ASSISTANT LOCAL DOING TFC MGMNT FUNCTION AT THE TIME OF OE. NO TMU IN TWR. NOTES: I NORMALLY HAVE ACFT RPT ON FINAL FOR RWY BECAUSE THEY GET BELOW THE HORIZON AND ARE VERY HARD TO PICK OUT FROM GROUND CLUTTER - I DID NOT DO THIS. MY SCAN FAILED TO PICK UP THIS ACFT. RADAR TAG DROP DID NOT HELP THIS. WHEN THIS TYPE OF ERROR OCCURS USUALLY SOMEONE IN THE TWR CATCHES IT; NOBODY SAW THIS UNTIL AFTER IT WAS OVER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: DURING CALLBACK THE RPTR ESTIMATED THAT THE RADAR TAG ON THE GENERAL AVIATION ACFT DROPPED OUT AT APPROX 2.5 MILES WHICH IS EARLY. THIS SYSTEM IS BEING INVESTIGATED. WITHOUT THE TAG HE FORGOT THE INBOUND AIRCRAFT. THE STAFFING AT THE TWR; ACCORDING TO THE RPTR; IS BELOW WHAT IT SHOULD BE. EACH POS IS DOUBLED UP. HIS ASSISTANT WAS OCCUPIED WITH TFC MGMNT AND THE SUPVR WAS WORKING CLRNC. THE RPTR WAS WORKING LOCAL AND ANOTHER FREQ; THE 'HILO' POS WHICH CTLS TFC RPTRS AND HELI TFC. THE BRITE RADAR IN THE TWR CAB IS POSITIONED OVERHEAD AND THE RPTR FINDS IT DIFFICULT TO USE IN THIS POS. THE VISUAL SCAN FOR TFC IS AFFECTED BY THE HEIGHT OF THE TWR. WHEN YOU ARE LOOKING DOWN FOR TFC ON FINAL AGAINST THE RAIL YARDS AND BUILDINGS THEY ARE DIFFICULT TO SEE. SUPPLEMENTAL INFO FROM ACN 511254: OUR FLT; ACFT X; WAS CLRED INTO POS AND HOLD ON RWY 1 AT DCA. WE WERE THEN CLRED FOR TKOF. DURING THE TKOF ROLL; ACFT Y LANDED ON RWY 4; AN INTERSECTING RWY. WE HEARD THE TWR TELL THE ACFT Y TO GAR. ACFT Y STOPPED ON THE RWY PRIOR TO THE INTERSECTION. HAD THE PLT OF ACFT Y GAR; THE POINT WHEN THE TWR GAVE THOSE INSTRUCTIONS; IT COULD HAVE CAUSED AN NMAC. I MENTIONED TO THE CAPT THAT I HEARD AN ACFT HAD BEEN CLRED TO LAND ON RWY 4 (AS WE WERE CLRED FOR TKOF). THE CAPT SAW THE ACFT AS WE PASSED THE INTERSECTION. ACFT Y STOPPED PRIOR TO THE INTERSECTION.