BE40 FLC UNABLE TO MAINTAIN ALT WHEN IN TRANSITION FROM ZDV TO ZMP.

Narrative

ACFT #1 (X) WAS AT FL390. ZMP CLBED ACFT TO FL430 PRIOR TO HDOF TO ZDV. ACFT #1 MADE WHAT I BELIEVED TO BE A SLOW CLB TO FL430 WHICH PROMPTED ME TO HOLD ON TO COMS OF ACFT #2 (Y) ABOUT TO ENTER ZMP AIRSPACE UNTIL CONFLICT WAS RESOLVED. I ACCEPTED HDOF ON ACFT #1; WHICH NOW SHOWED LEVEL AT FL430 AND SUBSEQUENTLY CHANGED ACFT #2 TO ZMP FREQ. ACFT #1 CHKED ON MY FREQ LEVEL AT FL430 AND STATED THEY 'NEEDED LOWER.' I ACKNOWLEDGED REQUEST BUT STATED I WAS UNABLE DUE TO THE TFC AT FL410 OPPOSITE DIRECTION. ACFT #1 'ROGERED.' MOMENTS LATER ACFT #1 REQUESTED VECTOR FOR DSCNT. I TURNED ACFT #1 25 DEGS L AND ADVISED THEY COULD EXPECT LOWER WHEN I OBSERVED TARGET ESTABLISHED ON THE HEADING. MOMENTS LATER ACFT #1 SHOWED ALT 500 FT LOW WITH XMISSION THEY COULD NOT HOLD ALT; THEN THE TARGET DISAPPEARED FROM MY RADAR SCOPE. I ADVISED ACFT #1 TO EXPEDITE THE TURN; RESTATED THAT THEY WERE NOT CLRED LOWER YET AND TO RESQUAWK THE ASSIGNED BEACON CODE. WHAT SEEMED LIKE A VERY LONG TIME LATER; TARGET REACQUIRED 500-600 FT LOW; CLR OF ACFT #2 AND I ASSIGNED ACFT #1 FL390. NO EXPLANATION WAS GIVEN MY AIRWAYS FACILITY OR MGMNT ON THE PLT AS TO WHY THE TARGET DISAPPEARED. IT WAS DETERMINED THEN RADAR DATA BEFORE AND AFTER TARGET LOSS; AND SOME INTERP; THAT THE VECTOR WAS ISSUED IN SUFFICIENT TIME TO MAINTAIN STANDARD SEPARATION. EVEN WITHIN THE VECTOR THE ACFT WOULD NOT HAVE COLLIDED; BUT LOSS OF STANDARD SEPARATION WAS POSSIBLE. THE PLT OF THE ACFT #1 STATED SEVERAL TIMES THAT HE HAD TCASII ON AND DID NOT OBSERVE ANY OTHER ACFT WITHIN 10 NM. IT WAS ESTIMATED ACFT MISSED BY 6 1/2 MI. AFTER PLT LANDED; HE CALLED ZDV AND DISCUSSED THE INCIDENT WITH AN OPS MGR. THE PLT SAID THAT ACCORDING TO THE BOOK; THE ACFT SHOULD HAVE BEEN ABLE TO MAINTAIN FL430. HOWEVER; UPON GETTING TO THE ASSIGNED ALT; WITH A SLIGHT TEMP CHANGE; ETC; THE ACFT HIT THE STALL LIMIT AND THE ENVELOPE WAS JUST TOO SMALL TO OPERATE; HENCE THE DSCNT. PLTS; AS WELL AS CTLRS; SHOULD ADD IN A LITTLE 'EXTRA' CUSHION INSTEAD OF PUSHING THE LIMIT ALL THE TIME AND THESE INCIDENTS WOULD BE REDUCED.