Narrative:

Aircraft #1 (X) was at FL390. ZMP climbed aircraft to FL430 prior to handoff to ZDV. Aircraft #1 made what I believed to be a slow climb to FL430 which prompted me to hold on to communications of aircraft #2 (Y) about to enter ZMP airspace until conflict was resolved. I accepted handoff on aircraft #1, which now showed level at FL430 and subsequently changed aircraft #2 to ZMP frequency. Aircraft #1 checked on my frequency level at FL430 and stated they 'needed lower.' I acknowledged request but stated I was unable due to the traffic at FL410 opposite direction. Aircraft #1 'rogered.' moments later aircraft #1 requested vector for descent. I turned aircraft #1 25 degrees left and advised they could expect lower when I observed target established on the heading. Moments later aircraft #1 showed altitude 500 ft low with transmission they could not hold altitude, then the target disappeared from my radar scope. I advised aircraft #1 to expedite the turn, restated that they were not cleared lower yet and to resquawk the assigned beacon code. What seemed like a very long time later, target reacquired 500-600 ft low, clear of aircraft #2 and I assigned aircraft #1 FL390. No explanation was given my airways facility or management on the pilot as to why the target disappeared. It was determined then radar data before and after target loss, and some interpretation, that the vector was issued in sufficient time to maintain standard separation. Even within the vector the aircraft would not have collided, but loss of standard separation was possible. The pilot of the aircraft #1 stated several times that he had TCASII on and did not observe any other aircraft within 10 NM. It was estimated aircraft missed by 6 1/2 mi. After pilot landed, he called ZDV and discussed the incident with an operations manager. The pilot said that according to the book, the aircraft should have been able to maintain FL430. However, upon getting to the assigned altitude, with a slight temperature change, etc, the aircraft hit the stall limit and the envelope was just too small to operate, hence the descent. Pilots, as well as controllers, should add in a little 'extra' cushion instead of pushing the limit all the time and these incidents would be reduced.

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Original NASA ASRS Text

Title: 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.

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.