ACR LGT SPD DEV AND ALT DEV EXCURSION FROM CLRNC ALT ON VISUAL APCH TO DEN.

Narrative

LGT WAS ON A VFR APCH TO RWY 26R IN DENVER WHEN A )WX) BALLOON APPEARED DIRECTLY IN FRONT OF THE LGT. THE CAPT WAS FLYING AND HE TOOK EVASIVE ACTION (DOVE 300 FT) IN ORDER TO AVOID A COLLISION WITH THE BALLOON. ALTHOUGH THE BALLOON WAS THE CAUSE OF THE EVASIVE ACTION IT WAS NOT THE EVENT THAT WAS OF PRIMARY CONCERN TO THE CREW AT THE TIME. THE LGT WAS TURNED ONTO THE FINAL APCH COURSE 3 1/2 MI BEHIND AN HVT WHICH WAS MAKING A VISUAL APCH TO RWY 26L IN DENVER. THE 2 RWYS (26L AND 26R) ARE SEPARATED BY ABOUT 1000 FT. SINCE THE WIND AT THE ARPT WAS OUT OF THE S (190/11 KTS) THE CAPT FELT IT BEST TO INCREASE THE SPACING ON THE HVY ACFT AND THE LGT SLOWED TO APCH SPD. THE LGT RECEIVED A 'TFC; TFC' WARNING ON ITS TCASII SYS. THE RADAR SCREEN SHOWED THAT THE WARNING WAS TRIGGERED BY AN ACFT 1400 FT ABOVE THE LGT AND DIRECTLY BEHIND IT. THE LGT CREW ASKED THE DENVER TWR IF THERE WAS AN ACFT BEHIND IT AND WAS NOTIFIED THAT A COMMERCIAL MLG TYPE ACFT WAS BEHIND THE LGT MAKING A VISUAL APCH TO RWY 26L. THE LAST READING THE LGT CREW SAW OF THE ACFT FOLLOWING IT WAS 200 FT ABOVE AND DIRECTLY BEHIND. AS THE ACFT FOLLOWING THE LGT APCHED THE LGT'S POS; THE CAPT INCREASED THE LGT SPD IN ORDER TO AVOID BEING OVERRUN BY THE MLG. THE TCASII GAVE AN ORAL EMER NOTIFICATION 'CLB; CLB' AND CALLED FOR A CLB OF 3500 FPM TO AVOID A COLLISION WITH ANOTHER ACFT. A SILVER SPHERE APPEARED IN THE WINDSCREEN OF THE FLT (IT LOOKED LIKE A SINGLE ENG FIGHTER TYPE ACFT WITH A SINGLE TRAILING EXHAUST) AND THE CAPT INSTINCTIVELY DOVE THE LGT 300 FT TO AVOID A COLLISION. IN THE SPLIT SECOND AVAILABLE TO THE CAPT TO DECIDE WHAT TO DO IN THIS SITUATION HE FEELS THAT HIS VISUAL CLUES OVERPWRED THE TCASII WARNING. THE OBJECT TURNED OUT TO BE A LARGE (5 FT IN DIAMETER) SILVER BALLOON (WX?) WITH A TRAILING ROPE. SUBSEQUENT INQUIRY WITH DENVER TWR CONFIRMED THAT WX BALLOONS ARE RELEASED FROM THE DENVER ARPT BUT THE LGT CREW WAS UNABLE TO FIND OUT IF A BALLOON HAD BEEN RELEASED AROUND THE TIME OF THE INCIDENT. ALSO THE CREW WAS UNABLE TO FIND OUT IF WX BALLOONS ARE EQUIPPED WITH TRANSPONDERS. IF THEY ARE; PERHAPS THE BALLOON TRIGGERED THE TCASII WARNING. IF THIS CREW HAD NOT ENCOUNTERED THE BALLOON THIS RPT WOULD STILL BE A JUSTIFIABLE NASA RPT. ALLOWING SIMULTANEOUS VISUAL APCHS TO RWYS 26 AT THE DENVER ARPT SHOULD BE RECONSIDERED.