Narrative:

I was working local control 2 and had 2 in the pattern; a cessna and a helicopter. I cleared the cessna for the option and approved local control 1 to cross my runway at the approach end after my cessna with their airbus. My helicopter was number 2; and he was making short approaches. I asked him to get the airbus in sight; he did; so I told the helicopter to remain 200 ft south of the airbus and short approach approved and I cleared him for the option. Out of the window everything looked fine. The cessna landed and was exiting at the end (hotel). Meanwhile; the helicopter turned base in between taxiways kilo and juliet for his short approach. The asde-X alerted but the cessna was exiting at hotel and the helicopter was landing north of juliet. The asde-X however; did not depict what I a saw out of the window. Aov (air traffic safety oversight service) apparently looked at the asde-X because of the alert and saw the cessna exiting and the helicopter appearing on the runway; his base was south of kilo so he just appeared and on the asde-X you could see the helicopter slowly moving down the runway. The problem is from the asde-X you can't tell whether he is landing; departing or what; so while I look out the window and saw the cessna exiting and the helicopter landing north of juliet it was easy to paint a different picture from the asde-X. Aov didn't like it and I was called in by management to discuss the scenario. To add to this; the controller in charge/supervisor at the time wrote on the log that it was a false asde-X alert that the cessna was off of the runway when it went off and that they called it in to maintenance (soc). Regardless of this log entry; aov is still saying that this was either unsafe or improper practice. Even though both aircraft were cleared for the option; the helicopter was clearly not starting his follow through while the cessna was still on the runway. It is my understanding that the asde-X is not calibrated for separation purposes and this is a clear reason as to why it shouldn't be allowed to be used for separation. Just because I cleared someone for the option behind another aircraft a helicopter can still be on the runway at the same time as long as visual separation is applied and he is not starting his follow through while another aircraft is on the runway. Not only that; but the cessna was clearing when the helicopter turned onto the runway from his base leg. I recommend that a situation should never be judged based on asde-X alone.

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Original NASA ASRS Text

Title: SNA Local Controller described a traffic pattern event that was discovered after the fact by an AOV review of ASDE-X data; the Reporter noted the event had been identified to maintenance as a false target incident.

Narrative: I was working Local Control 2 and had 2 in the pattern; a Cessna and a helicopter. I cleared the Cessna for the option and approved Local Control 1 to cross my runway at the approach end after my Cessna with their Airbus. My helicopter was number 2; and he was making short approaches. I asked him to get the Airbus in sight; he did; so I told the helicopter to remain 200 FT south of the Airbus and short approach approved and I cleared him for the option. Out of the window everything looked fine. The Cessna landed and was exiting at the end (Hotel). Meanwhile; the helicopter turned base in between Taxiways Kilo and Juliet for his short approach. The ASDE-X alerted but the Cessna was exiting at Hotel and the helicopter was landing north of Juliet. The ASDE-X however; did not depict what I a saw out of the window. AOV (Air Traffic Safety Oversight Service) apparently looked at the ASDE-X because of the alert and saw the Cessna exiting and the helicopter appearing on the runway; his base was south of Kilo so he just appeared and on the ASDE-X you could see the helicopter slowly moving down the runway. The problem is from the ASDE-X you can't tell whether he is landing; departing or what; so while I look out the window and saw the Cessna exiting and the helicopter landing north of Juliet it was easy to paint a different picture from the ASDE-X. AOV didn't like it and I was called in by Management to discuss the scenario. To add to this; the CIC/Supervisor at the time wrote on the log that it was a FALSE ASDE-X alert that the Cessna was off of the runway when it went off and that they called it in to maintenance (SOC). Regardless of this log entry; AOV is still saying that this was either unsafe or improper practice. Even though both aircraft were cleared for the option; the helicopter was clearly not starting his follow through while the Cessna was still on the runway. It is my understanding that the ASDE-X is not calibrated for separation purposes and this is a clear reason as to why it shouldn't be allowed to be used for separation. Just because I cleared someone for the option behind another aircraft a helicopter can still be on the runway at the same time as long as visual separation is applied and he is not starting his follow through while another aircraft is on the runway. Not only that; but the Cessna was clearing when the helicopter turned onto the runway from his base leg. I recommend that a situation should never be judged based on ASDE-X alone.

Data retrieved from NASA's ASRS site as of April 2012 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.