Narrative:

I established aircraft X on the ILS localizer/GS to runway 28L and was cleared for the straight-in to runway 28L by pdx ATC (126.9 MHZ). Shortly thereafter, I was handed off to pdx tower on 123.77 MHZ who cleared me to land behind aircraft Z on short final. (I reported that aircraft in sight as requested.) aircraft Y was south of the airport complex entering on the left downwind for runway 28L. The pdx tower controller first instructed aircraft Y to turn base (I think they were attempting to put him between myself and the BE99 over the numbers). As aircraft Y started the turn, the pdx tower controller instructed aircraft Y to disregard the previous instruction, turn back to the downwind, and plan to turn base just east of rocky butte (would result in an approximately 2 mi base turn). As I descended through 1000 ft and about 3 mi final, the pdx tower controller again instructed aircraft Y to turn base. Aircraft Y was already closer in than usual due to the previous maneuvers and above me when he started the turn. 2 things became apparent: 1) the pdx tower controller made an error in issuing a base turn instruction to aircraft Y at that time. 2) the pilot of aircraft Y had lost sight of my aircraft on final (probably under his nose structure), and/or assumed that I was further down the final approach than I actually was. My reaction was to call this to the attention of the pdx tower controller as soon as the radio was clear by saying 'portland tower, this isn't going to work, aircraft X.' moments later (as aircraft X was turning final and paralleling my aircraft a few hundred ft above and to the south), the pdx tower controller issued instructions to aircraft Y to maintain altitude and turn left to re-enter the downwind for runway 28L, which he promptly did. The closest proximity between the 2 aircraft was approximately 300 ft vertically and 500 ft horizontally. The relative motion of the 2 aircraft did not indicate an impending collision.

Related reports:

Google
 

Original NASA ASRS Text

Title: NMAC BTWN AN SMT TRAINING FLT ON FINAL AND ANOTHER ACFT TURNED BASE TO FINAL BY TWR CTLR TOO EARLY FOR SEPARATION WITH RPTR ACFT X.

Narrative: I ESTABLISHED ACFT X ON THE ILS LOC/GS TO RWY 28L AND WAS CLRED FOR THE STRAIGHT-IN TO RWY 28L BY PDX ATC (126.9 MHZ). SHORTLY THEREAFTER, I WAS HANDED OFF TO PDX TWR ON 123.77 MHZ WHO CLRED ME TO LAND BEHIND ACFT Z ON SHORT FINAL. (I RPTED THAT ACFT IN SIGHT AS REQUESTED.) ACFT Y WAS S OF THE ARPT COMPLEX ENTERING ON THE L DOWNWIND FOR RWY 28L. THE PDX TWR CTLR FIRST INSTRUCTED ACFT Y TO TURN BASE (I THINK THEY WERE ATTEMPTING TO PUT HIM BTWN MYSELF AND THE BE99 OVER THE NUMBERS). AS ACFT Y STARTED THE TURN, THE PDX TWR CTLR INSTRUCTED ACFT Y TO DISREGARD THE PREVIOUS INSTRUCTION, TURN BACK TO THE DOWNWIND, AND PLAN TO TURN BASE JUST E OF ROCKY BUTTE (WOULD RESULT IN AN APPROX 2 MI BASE TURN). AS I DSNDED THROUGH 1000 FT AND ABOUT 3 MI FINAL, THE PDX TWR CTLR AGAIN INSTRUCTED ACFT Y TO TURN BASE. ACFT Y WAS ALREADY CLOSER IN THAN USUAL DUE TO THE PREVIOUS MANEUVERS AND ABOVE ME WHEN HE STARTED THE TURN. 2 THINGS BECAME APPARENT: 1) THE PDX TWR CTLR MADE AN ERROR IN ISSUING A BASE TURN INSTRUCTION TO ACFT Y AT THAT TIME. 2) THE PLT OF ACFT Y HAD LOST SIGHT OF MY ACFT ON FINAL (PROBABLY UNDER HIS NOSE STRUCTURE), AND/OR ASSUMED THAT I WAS FURTHER DOWN THE FINAL APCH THAN I ACTUALLY WAS. MY REACTION WAS TO CALL THIS TO THE ATTN OF THE PDX TWR CTLR AS SOON AS THE RADIO WAS CLR BY SAYING 'PORTLAND TWR, THIS ISN'T GOING TO WORK, ACFT X.' MOMENTS LATER (AS ACFT X WAS TURNING FINAL AND PARALLELING MY ACFT A FEW HUNDRED FT ABOVE AND TO THE S), THE PDX TWR CTLR ISSUED INSTRUCTIONS TO ACFT Y TO MAINTAIN ALT AND TURN L TO RE-ENTER THE DOWNWIND FOR RWY 28L, WHICH HE PROMPTLY DID. THE CLOSEST PROX BTWN THE 2 ACFT WAS APPROX 300 FT VERTLY AND 500 FT HORIZLY. THE RELATIVE MOTION OF THE 2 ACFT DID NOT INDICATE AN IMPENDING COLLISION.

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.