Lax controller described a confused event when an automated departure run down to the TRACON failed ...

Narrative:

A B757-200 was departing runway 25R. The LC2 assist was unable to rundown the departure via automation; had to verbally run down aircraft. I investigated as to the cause of the 'dup id' message and found that there was another flight with same call sign; but as an arrival to lax. The 'T' affixed to the end of the callsign led me to believe the departure was a continuation of this arrival; and that the arrival had already landed and the radar track was in coast mode; not uncommon at lax. I removed the arrival control by using the ok function. The departure rundown was now completed via automation. Later I found out that the arrival was still inbound; and due to my actions; the data tag had been lost; but the radar track was still displayed. In removing the arrival control track; I created the same situation I was trying to prevent; i.e. Have an aircraft airborne with no associated data tag. In this case; an un-tagged departure would have been better than an un-tagged arrival. The first change would be to discuss with the company why they used a 'T' suffix on two callsigns; when the use of the suffix is to resolve confusion to same callsign events (like this one). A better fix would be to use two different suffixes; a 'Q' or 'J' etc.

Original NASA ASRS Text

Title: LAX Controller described a confused event when an automated departure run down to the TRACON failed because of an apparent 'dupe ID;' the reporter's corrective actions resulted in an arrival aircraft losing data block information.

Narrative: A B757-200 was departing Runway 25R. The LC2 assist was unable to rundown the departure via automation; had to verbally run down aircraft. I investigated as to the cause of the 'dup id' message and found that there was another flight with same call sign; but as an arrival to LAX. The 'T' affixed to the end of the callsign led me to believe the departure was a continuation of this arrival; and that the arrival had already landed and the RADAR track was in coast mode; not uncommon at LAX. I removed the arrival control by using the OK function. The departure rundown was now completed via automation. Later I found out that the arrival was still inbound; and due to my actions; the data tag had been lost; but the RADAR track was still displayed. In removing the arrival control track; I created the same situation I was trying to prevent; i.e. have an aircraft airborne with no associated data tag. In this case; an un-tagged departure would have been better than an un-tagged arrival. The first change would be to discuss with the company why they used a 'T' suffix on two callsigns; when the use of the suffix is to resolve confusion to same callsign events (like this one). A better fix would be to use two different suffixes; a 'Q' or 'J' etc.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1087590
Time
Date	201305
Local Time Of Day	1801-2400
Place
Locale Reference	LAX.Airport
State Reference	CA
Aircraft 1
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	Initial Climb
Flight Plan	IFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

Lax controller described a confused event when an automated departure run down to the TRACON failed because of an apparent 'dupe identification;' the reporter's corrective actions resulted in an arrival aircraft losing data block information.