Date: 201204
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: 20
Light: Daylight
Aircraft: Global Express (BD700)
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Test Flight / Demonstration
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Personal
Reporter function: Pilot Flying; Captain
Reporter qualification: Air Traffic Control Fully Certified; Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Training / Qualification; Time Pressure; Communication Breakdown; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Other / Unknown
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Procedure; Aircraft; Weather
Primary problem: Ambiguous

Narrative: I would like to elaborate on a loss of separation event that occurred. The events below are to the best of my recollection at this time. I was the Pilot in Command of a Global Express aircraft on an IFR flight plan. In the remarks section of the flight plan was 'training flight; HMU check.' The aircraft had just been returned to service after maintenance and the event occurred off an airway in VMC conditions at FL450. Late morning; the crew was flying at FL430. The crew decided to climb to a higher altitude and referred to approved performance data (FMS and cruise charts.) After establishing that the aircraft performance was within the acceptable range for FL450 we requested a climb. The Controller cleared us to FL450 and we commenced a slow climb. After leveling off; the aircraft would not accelerate to cruise speed. After approximately four minutes our first request to ATC was for a lower altitude. We stated 'We would like a lower altitude if possible.' The Controller said 'We have your request; expect lower in nine minutes' After approximately four minutes our second request was for a lower altitude with a turn if necessary. The Controller responded 'Turn left 15 degrees-lower in three minutes. The crew set maximum continuous thrust to maintain airspeed and altitude; but the aircraft performance continued to degrade. After two minutes the aircraft safety parameters were in jeopardy and the crew took evasive action and informed ATC 'We need an immediate descent!!' ATC cleared us to FL400 with a turn to 180 degrees. We observed an aircraft visually and on our TCAS. After passing FL430 we were re-assigned maintain FL430 if able. We told ATC we were unable and continued to FL400 and received a request to contact ATC Sector Supervisor via telephone upon landing. I believe some contributing factors associated with this occurrence were: The possibility of a temperature inversion or unusual warmer than expected air at FL450. Turbulence associated with the assigned altitude and the lack of phraseology to convey immediacy of situation to ATC. [This was] an aircraft returning to service after a heavy inspection.




Synopsis: A BD-700 climbed to FL450 on a test flight but aircraft performance deteriorated to an unsafe level. After several requests; FL400 was approved but traffic separation was lost.