Narrative:

There was an unauthorized departure attributed to me as pilot of a PA- 28-201 with the call sign nga X. The situation was as follows: I received clearance for a 'golden state departure' VFR to 4500 ft direct to cma. Ground cleared me to taxi to runway 8. At and before the hold line I switched to tower frequency. Tower instructed me to 'hold short of runway 8 for approaching traffic landing runway 8, an embraer commuter. After touchdown tower, instructed me 'position and hold runway 8.' I confirmed the clearance and moved into position on runway 8. Then I received what I heard to be 'nga X cleared for takeoff.' I then confirmed receipt of the clearance as follows: 'nga X we are rolling' and proceeded to initiate my takeoff roll and rotation. At about 700 ft AGL I heard 'there is an unauthorized departure and 'no problem we have him on TCASII. As I initiated and completed my left turn over the numbers of runway 15 to a heading of 271 degrees, I received instructions from tower to switch to departure frequency 120.4 and to contact the tower upon my return. I contacted the tower at which time they explained to me what had occurred from their vantage point. As I was holding in position on runway 8 they gave a departure clearance to call sign 'air carrier Y' a small commuter, to depart runway 15. The tower's tapes show (I have not heard these tapes) 'air carrier Y' receiving a clearance to depart runway 15. However I heard a clearance for 'nga X.' their tapes also show 2 concurrent readbacks of departure clrncs: mine and that of 'air carrier Y.' however there was no response from the tower clarifying the concurrent readbacks, that would have avoided the concurrent departures. The tower kept the commuter low and I climbed high and turned left back over the numbers on runway 15 and proceeded on course 271 degrees. Evasive action and maneuvers were not required. 1) when intersecting runways are in use simultaneously, 'position and hold' should not be allowed until the opposing aircraft has received clearance for takeoff roll. No delay would be created and dual clearance errors would be avoided. 2) when aircraft with similar call signs are in potentially conflicting sits, ATC readbacks should be slow and succinct, and all tower and aircraft radio contacts and confirmations should be responded to and/or clarified.

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Original NASA ASRS Text

Title: 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.

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.