Date: 201210
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Cessna 210 Centurion / Turbo Centurion 210C; 210D
Operator: FBO
Operating under: Part 91
Flight plan: IFR
Mission: Test Flight / Demonstration
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Engine
Component problem: Malfunctioning
Reporter organization: FBO
Reporter function: Captain; Pilot Flying; Single Pilot
Reporter qualification: Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Instrument
Human factors: Communication Breakdown; Situational Awareness; Workload; Training / Qualification; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action; Flight Crew Regained Aircraft Control
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Aircraft

Narrative: Purpose of the flight was to continue gathering flight test data at 20;000 FT (FL200) for an aircraft manufacturer testing program. Similar test conditions to the one that was about to take place today had already been accomplished at lower altitudes (below Class A airspace) without difficulty or the need of an additional flight crewmember. After receiving a weather briefing and completion of my preflight planning; which included a thorough review of the flight test profile; conditions to be tested and other risk mitigation tools used by my company during the course of flight testing; I filed an IFR flight plan using my company's ATC assigned call sign and my company specific test number. Due to the nature of the testing; I added the following comment to my flight plan: 'Need own NAV (navigation) (with) in altitude block 21;000 FT to 19;000 FT (FL210-190) for 40 minutes.' After a through preflight inspection; I started the engine; received the ATIS weather; contacted Clearance Delivery to pick up my IFR clearance and began the flight. The taxi; takeoff and subsequent climb to FL200 were uneventful as expected. In preparation for this series of high altitude flights; I had an additional oxygen bottle installed and had ordered a brand new double seal oxygen mask with built in microphone to ensure that during single pilot operations at high altitude; there would be nothing to distract me from accomplishing the flight test profile as planned. During the climb to altitude; I was assigned vectors by the Center Controller. After leveling off at FL200; I set up for the first test point which required a deceleration from VH (Maximum Airspeed at Maximum Continuous Power) to 1.3 Vs (130% of the flaps up stall speed) and then accelerate back to VH. Since this was supposed to be conducted as a level flight condition; I did not request a 'block altitude' prior to the initiation of the maneuver nor was a block altitude granted by ATC. With airplane's airspeed stable at VH; I reduced the throttle to an intermediate setting and allowed the airplane to decelerate toward 1.3 Vs while maintaining FL200. As the airspeed approached 1.3 Vs; I anticipated the need to increase power by 'leading' the power application so the airspeed did not decelerate below the target value of 1.3 Vs. As I advanced the throttle to increase power; the engine did not respond and there was no increase in engine power output. Since the airspeed was lower than that during 'normal' flight operations; the airplane was unable to maintain altitude and the only safe and prudent decision was to 'sacrifice' altitude to maintain airspeed. I attempted to contact Center Controller to make them aware of the situation but my transmission went without response. I can only speculate that the reason I did not receive a response was that it was probably blocked by transmissions from other aircraft on the same frequency. During the descent to maintain airspeed due to the lack of engine power output (the engine at no time stopped producing power but it did not respond to commanded power increases); I began troubleshooting by adjusting throttle and activating the electric fuel pump with no differing results in engine operations. I assumed I was having a potential fuel system malfunction (possibly fuel vapor) but my number one priority was to maintain positive aircraft control. As I was approaching 18;500 FT (FL185); I was contacted by the Center Controller stating in part that 'he had me at a different altitude than assigned.' I responded to his statement with 'I have no engine response and could not maintain altitude.' The Center Controller asked if I would like to declare an emergency; to which I declined and asked to continue the descent and that I might turn toward the North to make a landing at a nearby airport. After passing through 18;000 FT (FL180) I cancelled my IFR clearance and resumed my own navigation. Upon reaching 17;000 FT; the engine began operating normally and I informed the Center Controller that I would be turning back toward my destination and would be continuing the descent to 10;000 FT. The flight continued without event or further difficulty. In reviewing this event; I realize I should have immediately set my transponder to 7700 and declared an emergency when it was realized I could no longer maintain altitude. This action would have alerted the Center Controller of my situation as soon as possible and allowed him to move other traffic if required. I should have been a better steward of the airspace and made the Controller's job easier by not having him be surprised by an aircraft deviating from clearance. This event was likely a bootstrapping event since the aircraft was operating above the engine's critical altitude (the maximum altitude at which the turbocharger can maintain rated power) and the possibility of such an event should have been discussed before attempting the flight profile.




Synopsis: A C-210 pilot reported the engine did not fail; but ceased developing power while recovering from a minimum airspeed maneuver on a test flight at FL200. The aircraft descended to 17;000 FT where normal power was restored.