ZOB controller reported an aircraft had a loss of climb performance to FL410 resulting in the aircra...

Attributes
ACN	1401036
Time
Date	201611
Local Time Of Day	0601-1200
Place
Locale Reference	ZOB.ARTCC
State Reference	OH
Environment
Light	Daylight
Aircraft 1
Make Model Name	Small Transport Low Wing 2 Turbojet Eng
Flight Phase	Climb
Flight Plan	IFR
Aircraft 2
Make Model Name	Light Transport Low Wing 2 Turboprop Eng
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 30.5
Events
Anomaly	Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Inflight Event / Encounter Weather / Turbulence

Narrative:

Aircraft X was climbing to FL410 with a restriction to cross 10 miles north at FL410. Aircraft Y was level at FL400 20 west of psb. The restriction was given to aircraft X 50 miles from psb and reissued 30 miles from psb. When aircraft X reached FL407 he stated something about a temperature increase and that he might not make his restriction. I turned aircraft X 30 degrees left. After another radar hit I saw that aircraft X altitude showed FL404. I then turned aircraft Y 30 degrees left and descended him to FL390 not knowing what aircraft X was doing at the time. I told aircraft X to maintain his altitude and again he said something about temperature. Two hits later aircraft X showed level at FL390. Both aircraft passed a few miles apart and I asked aircraft X what he was doing. He stated that due to temperature inversion he was afraid the aircraft might stall and he descended to FL390 without a clearance. I read the pilot deviation phraseology to him. Don't descend without clearance and at least be more specific with details in this situation.

Original NASA ASRS Text

Title: ZOB Controller reported an aircraft had a loss of climb performance to FL410 resulting in the aircraft descending into conflict with an aircraft at FL400.

Narrative: Aircraft X was climbing to FL410 with a restriction to cross 10 miles North at FL410. Aircraft Y was level at FL400 20 west of PSB. The restriction was given to Aircraft X 50 miles from PSB and reissued 30 miles from PSB. When Aircraft X reached FL407 he stated something about a temperature increase and that he might not make his restriction. I turned Aircraft X 30 degrees left. After another radar hit I saw that Aircraft X altitude showed FL404. I then turned Aircraft Y 30 degrees left and descended him to FL390 not knowing what Aircraft X was doing at the time. I told Aircraft X to maintain his altitude and again he said something about temperature. Two hits later Aircraft X showed level at FL390. Both aircraft passed a few miles apart and I asked Aircraft X what he was doing. He stated that due to temperature inversion he was afraid the aircraft might stall and he descended to FL390 without a clearance. I read the pilot deviation phraseology to him. Don't descend without clearance and at least be more specific with details in this situation.

Data retrieved from NASA's ASRS site 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.

ZOB controller reported an aircraft had a loss of climb performance to FL410 resulting in the aircraft descending into conflict with an aircraft at FL400.