PLTDEV.

Narrative

THERE WAS AN UNAUTH DEP ATTRIBUTED TO ME AS PLT OF A PA- 28-201 WITH THE CALL SIGN NGA X. THE SIT WAS AS FOLLOWS: I RECEIVED CLRNC FOR A 'GOLDEN STATE DEP' VFR TO 4500 FT DIRECT TO CMA. GND CLRED ME TO TAXI TO RWY 8. AT AND BEFORE THE HOLD LINE I SWITCHED TO TWR FREQ. TWR INSTRUCTED ME TO 'HOLD SHORT OF RWY 8 FOR APCHING TFC LNDG RWY 8; AN EMBRAER COMMUTER. AFTER TOUCHDOWN TWR; INSTRUCTED ME 'POS AND HOLD RWY 8.' I CONFIRMED THE CLRNC AND MOVED INTO POS ON RWY 8. THEN I RECEIVED WHAT I HEARD TO BE 'NGA X CLRED FOR TKOF.' I THEN CONFIRMED RECEIPT OF THE CLRNC AS FOLLOWS: 'NGA X WE ARE ROLLING' AND PROCEEDED TO INITIATE MY TKOF ROLL AND ROTATION. AT ABOUT 700 FT AGL I HEARD 'THERE IS AN UNAUTH DEP AND 'NO PROB WE HAVE HIM ON TCASII. AS I INITIATED AND COMPLETED MY L TURN OVER THE NUMBERS OF RWY 15 TO A HDG OF 271 DEGS; I RECEIVED INSTRUCTIONS FROM TWR TO SWITCH TO DEP FREQ 120.4 AND TO CONTACT THE TWR UPON MY RETURN. I CONTACTED THE TWR AT WHICH TIME THEY EXPLAINED TO ME WHAT HAD OCCURRED FROM THEIR VANTAGE POINT. AS I WAS HOLDING IN POS ON RWY 8 THEY GAVE A DEP CLRNC TO CALL SIGN 'ACR Y' A SMALL COMMUTER; TO DEPART RWY 15. THE TWR'S TAPES SHOW (I HAVE NOT HEARD THESE TAPES) 'ACR Y' RECEIVING A CLRNC TO DEPART RWY 15. HOWEVER I HEARD A CLRNC FOR 'NGA X.' THEIR TAPES ALSO SHOW 2 CONCURRENT READBACKS OF DEP CLRNCS: MINE AND THAT OF 'ACR Y.' HOWEVER THERE WAS NO RESPONSE FROM THE TWR CLARIFYING THE CONCURRENT READBACKS; THAT WOULD HAVE AVOIDED THE CONCURRENT DEPS. THE TWR KEPT THE COMMUTER LOW AND I CLBED HIGH AND TURNED L BACK OVER THE NUMBERS ON RWY 15 AND PROCEEDED ON COURSE 271 DEGS. EVASIVE ACTION AND MANEUVERS WERE NOT REQUIRED. 1) WHEN INTERSECTING RWYS ARE IN USE SIMULTANEOUSLY; 'POS AND HOLD' SHOULD NOT BE ALLOWED UNTIL THE OPPOSING ACFT HAS RECEIVED CLRNC FOR TKOF ROLL. NO DELAY WOULD BE CREATED AND DUAL CLRNC ERRORS WOULD BE AVOIDED. 2) WHEN ACFT WITH SIMILAR CALL SIGNS ARE IN POTENTIALLY CONFLICTING SITS; ATC READBACKS SHOULD BE SLOW AND SUCCINCT; AND ALL TWR AND ACFT RADIO CONTACTS AND CONFIRMATIONS SHOULD BE RESPONDED TO AND/OR CLARIFIED.