Narrative:

During taxi out at mdpp, we noticed what appeared to be helicopter traffic over the runway at 1500 ft on the TCASII. The WX was broken ceiling, so we were unable to verify visually. Since the traffic appeared to be almost stationary, we concluded that it was a helicopter. Prior to beginning our takeoff roll, we verified that there was no conflicting traffic either visually or on TCASII. After I selected wheels up for the flying first officer, I looked down at the TCASII screen and saw the traffic that had disappeared previously -- once again in our overhead position at 1500 ft. We began a push-over commencing at around 700 ft and passed 400 ft below the traffic. The TCASII gave us no TA's due to the built-in inhibits. We never visually acquired the traffic due to the cloud deck, and after we cleared him, we saw the top of the cloud deck at 1400 ft and bases at around 1000 ft. We queried the tower about the traffic, and he had no knowledge of his presence. It was only due to a re-check of the TCASII screen and taking immediate action that we avoided what I am sure would have been a mid-air collision. The tower at mdpp said that they would investigate what the traffic was there for and try to locate him. My first officer did a superb job in controling the aircraft in a dangerous and spontaneous situation. There were no injuries nor were the passenger at any time aware of the incident.

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Original NASA ASRS Text

Title: NMAC BTWN A DEP B757 AND UNIDENTED TFC, SUPPOSEDLY A HELI, OFF THE DEP RWY AT 1500 FT AT MDPP, FO.

Narrative: DURING TAXI OUT AT MDPP, WE NOTICED WHAT APPEARED TO BE HELI TFC OVER THE RWY AT 1500 FT ON THE TCASII. THE WX WAS BROKEN CEILING, SO WE WERE UNABLE TO VERIFY VISUALLY. SINCE THE TFC APPEARED TO BE ALMOST STATIONARY, WE CONCLUDED THAT IT WAS A HELI. PRIOR TO BEGINNING OUR TKOF ROLL, WE VERIFIED THAT THERE WAS NO CONFLICTING TFC EITHER VISUALLY OR ON TCASII. AFTER I SELECTED WHEELS UP FOR THE FLYING FO, I LOOKED DOWN AT THE TCASII SCREEN AND SAW THE TFC THAT HAD DISAPPEARED PREVIOUSLY -- ONCE AGAIN IN OUR OVERHEAD POS AT 1500 FT. WE BEGAN A PUSH-OVER COMMENCING AT AROUND 700 FT AND PASSED 400 FT BELOW THE TFC. THE TCASII GAVE US NO TA'S DUE TO THE BUILT-IN INHIBITS. WE NEVER VISUALLY ACQUIRED THE TFC DUE TO THE CLOUD DECK, AND AFTER WE CLRED HIM, WE SAW THE TOP OF THE CLOUD DECK AT 1400 FT AND BASES AT AROUND 1000 FT. WE QUERIED THE TWR ABOUT THE TFC, AND HE HAD NO KNOWLEDGE OF HIS PRESENCE. IT WAS ONLY DUE TO A RE-CHK OF THE TCASII SCREEN AND TAKING IMMEDIATE ACTION THAT WE AVOIDED WHAT I AM SURE WOULD HAVE BEEN A MID-AIR COLLISION. THE TWR AT MDPP SAID THAT THEY WOULD INVESTIGATE WHAT THE TFC WAS THERE FOR AND TRY TO LOCATE HIM. MY FO DID A SUPERB JOB IN CTLING THE ACFT IN A DANGEROUS AND SPONTANEOUS SIT. THERE WERE NO INJURIES NOR WERE THE PAX AT ANY TIME AWARE OF THE INCIDENT.

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.